THE JOURNAL OF CHEMICAL PHYSICS 139, 174314 (2013)
Site-dependent Si KL23 L23 resonant Auger electron spectra following inner-shell excitation of Cl3 SiSi(CH3 )3 Isao H. Suzuki,1,a) Hikari Endo,2 Kanae Nagai,2 Osamu Takahashi,3 Yusuke Tamenori,4 and Shin-ichi Nagaoka2 1
Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba 305-0801, Japan and Advanced Institute of Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba 305-8568, Japan 2 Department of Chemistry, Faculty of Science, Ehime University, Matsuyama 790-8577, Japan 3 Institute for Sustainable Science and Development, Hiroshima University, Higashi-Hiroshima 739-8526, Japan 4 Synchrotron Radiation Research Institute/SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun 679-5198, Japan
(Received 3 July 2013; accepted 17 October 2013; published online 6 November 2013) Spectator resonant Auger electron spectra with the Si 1s photoexcitation of Cl3 SiSi(CH3 )3 have been measured using an electron spectroscopic technique combined with undulator radiation. The transition with the highest intensity in the total ion yield (TIY) spectrum, coming from excitation of a Si 1s electron on the Cl-side into a vacant valence orbital, generates the resonant Auger decay in which the excited electron remains in this valence orbital. Photoexcitation of 1s electrons into some Rydberg orbitals induces Auger shake-down transitions, because higher-lying Rydberg orbitals in the two Si atoms closely positioned hold spatially overlapping considerably. A broad TIY peak slightly above the 1s ionization thresholds appreciably yields resonant Auger decays in which a slow photoelectron is re-captured into a higher-lying Rydberg orbital. The normal Auger peak shape at this photon energy is distorted due to a post-collision interaction effect. These findings provide a clear understanding on properties of the excited orbitals which are ambiguous in the measurement of the TIY only. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4827860] I. INTRODUCTION
Core level ionization energy shows a large chemical shift in the binding energy of molecules and thus electronic structures of the molecules have been discussed on the chemical shift measured using a photoelectron spectroscopic technique in a number of instances.1 Molecules containing Si atoms draw attention of many researchers to studies on site-specific dissociation because Si material is often used for fabrication of electronic devices. Basic studies were attempted for clarification of the site-specific effect using monochromatized synchrotron radiation. Yields of some fragment ions were discovered to strongly depend on the site of core-ionized Si atoms within a molecule.2–4 This site-specificity depends on the distance between the relevant Si atoms in molecules, and this dependence is connected with easiness or difficulty of electron flow between the Si atoms. Element-specific fragmentations were also investigated by several research groups, which showed state-selectivity of core-excited states of some elements within different molecules in the process of decomposition into ionic fragments.5, 6 Shallow core-level photoabsorption spectra of small molecules clearly provide properties of vacant molecular orbitals, while upon transitions of deep core electrons, photoabsorption peaks usually exhibit complex structures due to an a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0021-9606/2013/139(17)/174314/7/$30.00
increase in the lifetime width.7–11 Si K-shell excitation spectra of Si-containing molecules were measured using monochromatized synchrotron radiation by Bodeur and co-workers.9, 10 They assigned a peak below the ionization threshold to the excitation of a Si 1s electron into a vacant molecular orbital on the basis of the calculation using the ab initio configuration interaction technique. The multiple scattering Xα method was frequently utilized and this method nicely reproduced characteristics of shape resonance above the threshold.12 When an inner-shell electron is emitted from an atom or molecule, the created core hole is filled through Auger electron emission because of the low probability of radiative processes in the soft X-ray region. Auger electrons give us information on energy values of doubly charged states and on spatial distributions of relevant orbitals. In photoexcitation of an inner-shell electron into a vacant orbital, resonant Auger electrons emerge, which are classified into two types: spectator Auger process and participator one.13–19 The final state in the former process is represented with a spectator electron and a doubly charged ion core which is assumed to be the same as the state in the normal Auger (NA) decay. In this spectator process of rare gas atoms the excited electron was found to remain in the initially excited orbital with the same principal quantum number, or to be shaken up to an orbital with a different quantum number.13, 14 Since a molecule has vibrational modes, spectator Auger electron spectra usually exhibit complex structures, making it difficult to precisely analyze
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the final states.15, 16 Resonant Auger decays of the participator type, which result in the same state as the conventional photoelectron emission, were studied for several molecules, and in those studies variation in photoelectron yields into particular channels and ultra-fast dissociation processes were clarified.17, 18 In the photoexcitation into an anti-bonding valence molecular orbital, some molecules decompose prior to Auger decay and a decomposed fragment emits an Auger electron which has a kinetic energy different from that of the molecule. Chlorine L-shell resonant Auger electron spectra of HCl were precisely analyzed, which demonstrated that an interference effect between the continuum of the molecular ion and that of the dissociated fragment ion takes place during the participator Auger decay.18 In regard to resonant X-ray emission spectra in the Cl K-shell, the spectra indicated the occurrence of the interference effect between the Cl 1s−1 and 2p−1 core holes.19 Normal KLL Auger electron spectra of Si-containing molecules and vaporized Si atoms were investigated by use of an electron beam technique.20, 21 The spectra of those molecules show relatively simple characteristics in the KL23 L23 decays owing to a pseudo-atomic process of Auger decays between the inner-shells. The analyzed spectra provided energies of doubly charged molecular ions having two Si 2p holes and indicated that a state of 1 D2 in the atomic representation had the dominant intensity.20 Since simple spectra were measured for resonant KLL Auger transitions, an energy transformation of photoexcited states into electron kinetic energies was clearly elucidated in spectator resonant KLL Auger electrons of SiF4 and similar molecules.22–24 The molecule Cl3 SiSi(CH3 )3 , represented as MCDS in the present study, includes two Si atoms, which are positioned at different chemical environments; one bonding to three Cl atoms, represented here with Si[Cl], and the other bonding to three methyl substituents, Si[Me]. Studies on the site-specific fragmentation following Si inner-shell excitation were carried out using this molecule and its related molecules.2–4 Then the Si atoms of this molecule gave essentially no vibrational structure in the inner-shell photoelectron spectra, coming from little movement of Si atoms surrounded by some substituents in this molecule.25 These conditions are very efficient as a research on the measurement of KLL resonant Auger electron spectra of the MCDS, in which electron emission is expected to proceed in the pseudo-atomic phenomenon. It is interesting to find different or same behavior in resonant Auger decays of two Si atoms; i.e., whether a sitespecific phenomenon occurs or not. Further it is important to compare photoabsorption spectrum of this molecule in the 1s ionization region with those of SiCl4 and Si(CH3 )4 in order to clarify the property of Si 1s-excited states of this molecule. In the present study, resonant KL23 L23 Auger electrons of spectator type following Si 1s excitation of the MCDS have been measured using monochromatized undulator radiation and a hemispherical electron energy analyzer. The measured electron kinetic energy spectra show the final state configurations consisting of two 2p orbital holes and of a Rydberg electron or a valence electron. Rydberg electrons are found to be shaken-up and shaken-down, while the excited valence electrons dominantly behave as a spectator.
J. Chem. Phys. 139, 174314 (2013)
II. EXPERIMENTAL
Measurements were performed on the c branch of the soft X-ray photochemistry beamline BL27SU at the SPring8 facility.26 The energy of the monochromator was calibrated using the Ne 1s photoelectrons.27 The photon bandwidth employed was about 0.93 eV in most instances and the uncertainty of the photon energy was ±0.3 eV. The intensity of the monochromatized incident photon beam during the measurements was monitored by collecting the drain current of the post-focusing mirror in the beamline. An electron spectrometer used consists of a hemispherical electron spectrometer (Gammadata Scienta, SES-2002) fitted to a gas cell (GC-50) by way of a multi-element lens in a differentially pumped chamber.14 Sample gas of MCDS was supplied into the gas cell and its vapor pressure observed in the chamber during the measurements was 2 × 10−4 Pa. The energy bandwidth in the electron spectra was estimated to be 0.47 eV. The electron energy was calibrated using the peak energies for the Auger decay from the Ne K-shell hole state28 and the uncertainty of the electron energy was ±0.2 eV. The total ion yield (TIY) spectrum was observed using an ion detector with a chevron type MCP assembly at the midpoint between the post-focusing mirror and the electron spectrometer. This spectrum approximates the photoabsorption spectrum in the soft X-ray region.
III. CALCULATION
The X-ray photoabsorption (XA) spectrum in the Si 1s excitation region of MCDS was calculated using the density functional theory (DFT) as described previously.11 Geometry optimization of MCDS was carried out using the GAUSSIAN 03 program at the MP2/cc-pVTZ level of approximation.29 The XA spectrum calculations were performed using the StoBe-deMon program.30 The theoretical XA spectra were generated by the transition potential (DFT-TP) method.31 The orbitals for the molecules were determined with a halfoccupied core orbital at the ionization site using the IGLOIII basis of Kutzelnigg et al.32 for core-hole Si atoms, the (311/211/1) basis set for non-core-hole Si atoms, the (41/31/1) basis set for Cl atoms, the (321/311/1) basis set for C atoms, and the (311/1) basis set for H atoms. The augment basis sets on Si atoms were used for Rydberg orbitals: ζ s = 0.017 and 0.0056667, ζ p = 0.014 and 0.0046667, and ζ d = 0.015 and 0.003. The non-core-excited Si, Cl, and C atoms were described by effective core potentials. The gradientcorrected exchange (PD86) and correlation functional (PD91) established by Perdew and Wang were applied.33 To estimate correct description for low-lying resonant excited states, fully relaxed Kohn-Sham calculations were performed.34 The relativistic correction added to the ionization potential was 2.0 eV for the Si K-edge. Functional correction of 4.7 eV was estimated by adjusting a large resonant peak, and the whole spectrum was shifted. Finally, the spectrum was generated by a Gaussian convolution of the discrete lines by varying the broadenings. The computed spectrum was broadened using Gaussian functions with a full width at half maximum of 1.0 eV below 1844 eV, and then linearly increasing up to 6.0 eV at 1849 eV.
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IV. RESULTS AND DISCUSSION
Figure 1 shows the experimental TIY spectrum as well as calculated photoabsorption (XA) spectra of MCDS in the Si 1s transition region. Fig. 1(a) exhibits the measured TIY spectrum; Fig. 1(b) does the calculated XA spectrum of the MCDS; Fig. 1(c) does that for the Si[Cl] excitation in the MCDS; and Fig. 1(d) does the spectrum for the Si[Me] excitation. The spectrum at Fig. 1(b) corresponds to the summation of those at Figs. 1(c) and 1(d). The bars with hatching in Fig. 1(a) denote the 1s ionization energies for two Si atoms, Si[Cl] and Si[Me], respectively.25 Arrows denote peaks or structures with intensity increases, and electron energy spectra were measured at these photon energies (see Fig. 2). It is found that the TIY spectrum consists of a sharp peak, some structures with high intensity and a shoulder near the peak. The calculated XA spectrum for the Si[Cl] excitation is very close to the calculated one for the SiCl4 molecule,35 while the calculated spectrum for the Si[Me] excitation is different from the corresponding spectrum for the Si(CH3 )4 molecule. The latter spectrum exhibits only one peak near the 1s ionization threshold and is almost the same as that experimentally observed.10, 11, 24 However, the XA spectrum calculated for the MCDS is very close to that of the TIY. The highest peak in the TIY shows up at 1844.4 eV, which corresponds to the excitation of the 1s electron of the Si[Cl] into an unoccupied valence orbital, as described below. Table I lists energies for the peaks and some structures in the TIY spectrum and the assignments of these structures, which have been obtained from the comparison with the DFT calculation. This table includes those for two molecules related to the present MCDS, i.e., SiCl4 and Si(CH3 )4 .11, 24, 35
FIG. 2. Resonant and normal KL23 L23 Auger electron spectra following Si 1s photoexcitation at several photon energies in Cl3 SiSi(CH3 )3 : (a) 1842.7 eV, (b) 1844.4 eV, (c) 1846.9 eV, (d) 1851.3 eV, (e) 1860.2 eV, and (f) 1951.8 eV. See the text for labels added to some structures like V, V , and so forth. Bars at the left end denote background fluctuations.
TABLE I. Spectral structures at Si 1s photoabsorption of Si-containing molecules. Uncertainty in excitation energy for the MCDS is ±0.3 eV. I.E.: ionization energy. Excitation energy (eV) Cl3 SiSi(CH3 )3
1842.7 1844.4 1846.5 1846.9 1849.2 1851.3
1860.2
Assignment
Term value (eV)
Si[Me]: 1s−1 u1 4.2 4.8 Si[Cl]: 1s−1 u2 Si[Cl]: high Rydberg 2.7 (or Si[Me]: high Rydberg) 0.4 Si[Me]: I.E. Si[Cl]: I.E. Doubly excited [or shape Si[Cl]: −2.1 resonance] Si[Me]: −4.4 Shape resonance Si[Cl]: −11 Si[Me]: −13.3
Si(CH3 )4 a 1843.3 1846.3
6t2 (and 4p) I.E.
3.0
1844.8 1846.2 1850.7 1851.3 1857.7
8a1 9t2 I.E. High Rydberg Doubly excited
5.9 4.5
SiCl4 b FIG. 1. Total ion yield (TIY) spectrum of Cl3 SiSi(CH3 )3 (MCDS) in the Si 1s excitation region, as well as calculated photoabsorption (XA) spectra. (a) Experimental TIY spectrum. (b) Calculated XA spectrum of MCDS. (c) Calculated XA spectrum for Si[Cl] excitation in the MCDS. (d) Calculated XA spectrum for Si[Me] excitation. Bars with hatching denote the ionization energies, 1846.9 eV for Si[Me] and 1849.2 eV for Si[Cl]. Arrows denote peak structures or intensity increases.
a b
Data cited from Ref. 24. Data cited from Ref. 35.
−0.6 −7.0
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The shoulder at 1842.7 eV comes from the excitation of the Si[Me] 1s electron into the vacant valence orbital (u1 ). The highest peak at 1844.4 eV mainly originates from the excitation of the Si[Cl] 1s electron into the vacant valence orbital (u2 ) and small contribution to this excitation probably comes from the Si[Me] 1s transition to a Rydberg orbital. These orbitals, u1 and u2 , are assigned to the valence orbitals largely composed of p-type orbitals of Si[Me] atom and Si[Cl] atom, respectively. The structure at 1846.5 eV probably corresponds to the excitation of the Si[Cl] 1s electron into a Rydberg orbital of a principal quantum number (n) of 4 or so (see below). This structure partly includes a contribution from the excitation of the Si[Me] 1s electron into a higher-lying Rydberg orbital, 7p or so. The structure around 1851.3 eV is presumed to come from a shape resonance effect or from excitation of two electrons into vacant orbitals; that is, excitation to doubly excited states, 1s−1 v−1 w1 w2 , where v indicates a valence orbital, and w1 and w2 denote vacant valence orbitals. It seems more probable that the doubly excited states exist there, because the present calculation holding one-electron character does not show clear increase around 1851 eV. A small intensity increase around 1860 eV probably corresponds to a shape resonance. The term values for the excited states are listed in Table I. A comparison of the term values of the vacant valence orbitals among the three molecules suggests an interesting point. The term value for the valence orbital for the Si[Me] of the MCDS, 4.2 eV, is higher than that for Si(CH3 )4 , 3.0 eV, and that for Si[Cl] of the MCDS, 4.8 eV, is lower than that for SiCl4 , 5.9 eV. This finding is the same as the characteristics of the relation among these molecules for the 1s ionization energy. These results reflect that the electronic structure near the Si atoms in the present molecule consists of a convolution from the electronic structures of the Si(CH3 )4 and SiCl4 . Figure 2 shows electron kinetic energy spectra of Si KL23 L23 resonant Auger decay, together with the normal Auger electron spectrum. Figs. 2(a)–2(e) exhibit the spectra measured at photon energies of 1842.7 eV, 1844.4 eV, 1846.5 eV, 1851.3 eV, and 1860.2 eV, respectively. Fig. 2(f) depicts the normal Auger spectrum measured at 1951.8 eV. The clear peaks for the normal Auger electrons in Fig. 2(f) indicate the energy values of 1602.6 eV and 1604.2 eV, which correspond to the formation of the doubly charged state of 1 D2 of Si[Cl] and that of Si[Me], respectively. Here the atomic representation on the Si atom state is utilized because the Auger decay usually takes place as pseudo-atomic process. These processes are represented with the following: M + hv → M+ (Si 1s−1 ) + e → M+2 (Si 2p−2 :1 D2 ) + 2e. (1) Here M denotes the MCDS molecule. The states of 1 S0 of Si atoms are presumed to correspond to small structures appearing around 1597 eV and 1600 eV marginally. In order to examine electron kinetic energy spectra closely, we have measured these spectra as a function of the incident photon energy near the Si 1s ionization thresholds. The resultant two dimensional (2D) map is exhibited in Fig. 3, which includes the TIY spectrum at the right end. In this figure, a long vertical line around the electron kinetic en-
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FIG. 3. Two-dimensional map of electron emission spectrum in Si KL23 L23 Auger decays as a parameter of photon energy. The total ion yield spectrum is displayed at the right end. See the text for island structures named as V, V , and R.
ergy of 1604 eV is found, together with a thinner line around 1603 eV. This long thicker line comes from the normal Auger decay generating the Si[Me] 1 D2 core, and the thinner line corresponds to the normal Auger decay to the Si[Cl] 1 D2 core. It is found that the thicker line becomes slightly bent near the ionization threshold because of the post-collision interaction, while the thinner one does not provide a clear bent feature owing to low signal yields. The island (V) with the highest intensity is seen around the electron kinetic energy of 1608 eV and the photon energy of 1844 eV. This island comes from the spectator resonant Auger decay of the Si[Cl] 1s−1 u2 state. This decay is expressed as follows: M∗ (Si[Cl]1s −1 u2 ) → M+ (Si[Cl]2p−2 :1 D2 u2 ) + e.
(2)
The island (V ) on the higher electron energy side and on the lower photon energy side originates from the similar Auger decay. It is found that there are an island (R) around the electron energy of 1608 eV and some structures around 1605 eV at the photon energy of 1846.5 eV. These structures correspond to the resonant Auger decays accompanied by shake effects (see below). Let us explain assignments of electron signals using the spectra in Fig. 2. The spectrum in Fig. 2(a), which has been measured at the photon energy of 1842.7 eV, shows the highest peak at 1610.2 eV (V ). This peak originates from a resonant Auger decay of spectator type. In this decay the valence electron initially excited from the Si[Me] 1s orbital behaves as a spectator and then remains in this excited orbital during the Auger decay. This decay is expressed in the following manner: M + hv → M∗ (Si[Me]1s −1 u1 ) → M+ (Si[Me]2p−2 :1 D2 u1 ) + e.
(3)
The small peak at 1607.7 eV (V) comes from the spectator resonant Auger decay, Si[Cl] 1s−1 u2 to Si[Cl]2p−2 : 1 D2 u2 . The transition from the Si[Cl] 1s orbital becomes possible through overlapping of the wide energy width of the deep core holes. These spectating phenomena of the valence electrons have been found to occur in the instances of the resonant
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TABLE II. Characteristic energies related to Si KL23 L23 Auger decays in Cl3 Si-Si(Me)3 (in units of eV). Note that the term of 2p−2 is of 1 D2 except for indication of 1 S0 . See text for several labels such as T, n∗ , and so forth. Values in the parentheses denote those estimated on the assumption that the relevant orbitals are of Rydberg-type. Photon energy
Tneu
n∗
n
δ
Electron energy
Tion
n∗
n
δ
1842.7
4.2
(1.8
3
1.2)
Si[Me]1s−1 u1
1610.2 1607.7 1603.0
10.2 11.6 3.0 (or 7.5)
(2.3 (2.2 4.3 (2.7
4 4 6 4
1.7) 1.8) 1.7 1.3)
Si[Me], u1 Si[Cl], u2 Si[Me], R1 (or Si[Me]:1 S0 u1 )
1844.4
4.8
(1.7
3
1.3)
Si[Cl]1s−1 u2
1609.4 1602.9
11.6 5.1 (or 10.3)
(2.2 3.3 (2.3
4 5 4
1.8) 1.7 1.7)
Si[Cl], u2 Si[Cl], R2 (or Si[Cl]:1 S0 u2 )
1846.5
2.7 0.4
2.2 5.8
4 7
1.8 1.2
Si[Cl]1s−1 R (or Si[Me]1s−1 R)
1608.4
8.5 (or 4.6 2.2 (or 6.1
2.5 3.4 5.0 3.0
4 5 6 4
1.5 1.6) 1.0 1.0)
Si[Cl], R3 (or Si[Me], R3 ) Si[Me], R4 (or Si[Cl], R4 )
Assignment
1606 1851.3
Doubly excited (or shape res.)
1604.6 1603.2 1610.0 1608 1597
1860.2
Shape res.
1951.8
KLL Auger decay of other Si-containing molecules like SiCl4 and Si(CH3 )4 .22–24, 35 In particular, the electron signals for this type of the decay have provided very high yield. The small structure around 1603 eV probably corresponds to a shake-up transition, as represented with the following: ∗
M + hv → M (Si[Me]1s +
−1
u1 )
−2 1
→ M (Si[Me]2p : D2 R1 ) + e,
(4)
where the valence electron, initially excited from the Si[Me] 1s orbital, is shaken up to a Rydberg orbital, R1 . Branching ratio for these processes, V : V: R1 , is approximately estimated to be 7:2:1. At this point, it is convenient to introduce a term value for a resonant Auger final state. The term value is calculated using the following equation: Tion = (EK − EA ) − (EPR − ERA ).
(5)
Here EK , EA , EPR , and ERA denote the ionization energy of the Si 1s electron, the normal Auger electron energy of KL23 L23 forming the 1 D2 core holes, the photon energy yielding the resonance excitation, and the electron energy of the resonant Auger decay, respectively. Table II lists the characteristics of the peaks and structures exhibited in Fig. 2 as well as characteristic energies for resonant and normal Auger electron spectra measured at several photon energies. Tneu denotes a term value for the Si 1s photoexcited state. Effective quantum numbers, n∗ (= n – δ), and quantum defects (δ) are estimated for Rydberg type orbitals in Si 1s photoexcited states and reso-
5.3 (or 1.4 3.3
3.2 6.2 4.1
5 8 6
1.8 1.8) 1.9
Assignment (Si 2p−2 :1 D2 )
NA, Si[Me] NA, Si[Cl] Re-capture, Si[Cl], R5 (or re-capture, Si[Me], R5 ) Re-capture, Si[Cl], R6 NA, Si[Cl]: 1 S0
1604.6 1603.2 1597
NA, Si[Me] NA, Si[Cl]) NA, Si[Cl]: 1 S0
1604.2 1602.6 1600 1597
NA, Si[Me] NA, Si[Cl] NA, Si[Me]:1 S0 NA, Si[Cl]: 1 S0
nant Auger final states. The effective quantum numbers are calculated using a simple Rydberg equation, Tion or Tneu =Zc2 Ry/(n∗ )2 .
(6)
In this equation, Ry denotes the Rydberg energy, 13.6 eV, and Zc indicates the core charge. Since the quantum number is an integer (n > 3), a couple of integer values are assumed for n after the calculation of n∗ and the corresponding δ values are calculated at several photon energies. On account of reasonability for excited core states of Si molecules, the dataset for n and δ values has been decided and listed in Table II. The resonant Auger final state corresponding to the small peak at 1603 eV has a quantum number of 6 if it is yielded through the shake-up process. In Fig. 2(b), the sharp peak at 1609.4 eV comes from a spectator resonant Auger decay, whose final state is represented with Si[Cl]2p−2 :1 D2 u2 . The orbital u2 in this state is different from the u1 in process (3) and thus the term values are different from each other: 11.6 eV for this u2 orbital and 10.2 eV for the u1 in process (3), as indicated in Table II. The small peak around 1602.9 eV probably corresponds to a shake-up transition of the initially excited electron into a Rydberg orbital, Si[Cl]2p−2 2 D1 R2 (branching ratio of V:R2 is 9: 1). Another possibility is suggested that this structure partly comes from a spectator resonant Auger decay in which the core hole state is 1 S0 and the spectator electron is the excited valence electron (u2 ). This suggestion is based on the finding that the energy difference between 1609.4 eV and 1602.9 eV, 6.5 eV, is not so different from that between the normal
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Auger electron energies, 5.2 eV calculated from 1602.6 eV for Si[Cl]2p−2 :1 D2 and 1597.4 eV for Si[Cl]2p−2 :1 S0 . The spectrum in Fig. 2(c) exhibits a profile different from the spectra in Figs. 2(a) and 2(b) discussed above. The highest peak at 1608.4 eV probably comes from the resonant Auger decay into the state of Si[Cl]2p−2 :1 D2 R3 (R3 :4p or so), as expressed with the following: M + hv → M∗ (Si[Cl]1s −1 R3 ) → M+ (Si[Cl]2p−2 :1 D2 R3 ) + e,
(7)
where the excited Rydberg electron remains in the same orbital. Further, it is possible to expect that the 1s excited state is of Si[Me]1s−1 R3 and that the final state is of Si[Me]2p−2 :1 D2 R3 . Here R3 and R3 are presumed to be 7p and 5p or so, respectively. The broad peak structure on the lower energy side, about 1606 eV, is presumed to correspond to the resonant decay into the state of Si[Me]2p−2 :1 D2 R4 (R4 :6p or so). In this decay the Rydberg electron initially excited is shaken down from 7p into 6p or so, as denoted in Table II. Here the approximate branching ratio of R3 :R4 is 6:4. Faint structures around 1601 eV on the lower energy side seems to correspond to the resonant Auger decay of the shake-up or no change in the principal quantum number. These findings seem peculiar in consideration of shake phenomena of the excited Rydberg electron in other Si-containing molecules.22–24 Many shake-up Auger decays of the Rydberg electrons had been observed in the resonant KLL Auger transitions and the quantum number of the Rydberg electron was promoted by 1, i.e., from n into n + 1. This promotion was in agreement with the expectation based on the hydrogenic model, where the n value in the singly charged core turns into n in the doubly charged core, n = n[(Zc + 1)/Zc ]1/2 .13 The peculiar shake effect of the present molecule seems to result from the spatial overlapping of higher-lying Rydberg orbitals with large orbital radii in the two Si atoms, which have direct bonding with each other. The bond distance between the two Si atoms is estimated to be 2.37 Å.25 Thus, it is natural that high Rydberg orbitals of the Si atoms are closely distributed and effectively coupled. Another possibility seems to be connected with mixing between Rydberg orbitals and valence orbitals. It cannot be concluded at present which effect plays a major role on the shake phenomena of this molecule. In the spectrum in Fig. 2(d), measured at 1851.3 eV, a twin peak with tailing into higher energy is found to have a considerable yield at 1603.2 eV and 1604.6 eV. This twin peak originates from the normal Auger decay into states of Si[Cl]2p−2 :1 D2 and Si[Me]2p−2 :1 D2 and the tailing results from the post-collision interaction with a photoelectron having low kinetic energy.22, 35 Some structures around 1608 eV and 1610 eV seem to correspond to resonant Auger decay and then these come from higher-lying Rydberg states which are generated through a re-capture of the slow photoelectron (branching ratio of NA:R5 + R6 is 7:3). In other words, the photoelectron is re-trapped by the 2p−2 :1 D2 core of the Si atom during the Auger transition. It is not possible, at present, to clarify which Si atom on the two sites plays a significant role in this trapping phenomenon. As mentioned above, the slight intensity-increase at this photon energy in the TIY prob-
FIG. 4. Schematic drawing of KL23 L23 Auger decay pathways from Si 1s holes states. Bars with labels denote Si 2p−2 : 1 D2 u (or R) states. Bars with labels and hatchings indicate doubly charged states of 1 D2 core holes and 1s−1 energy levels. Arrows denote decay pathways and arrows with small circles indicate those of electron emission following photoelectron re-capture.
ably comes from the transition into the doubly excited states. If the two excited electrons remain in the excited orbitals, the spectator Auger decay induces emission of electrons with higher kinetic energy than the NA electrons. These electrons seem to correspond to the structures around 1610 eV in Fig. 2(d). The quantum number of the electrons in the final state is calculated to be 5 or so, on the assumption using the Rydberg formula. Thus, the possibility cannot be excluded that the structures around 1610 eV slightly come from the spectator process of the two electrons. The faint structures around 1597 eV are presumed to correspond to the normal Auger decay of another core state, 2p−2 :1 S0 . At the photon energy of 1860.2 eV (Fig. 2(e)), two peaks with considerable tailing are found at 1603 eV and 1604.6 eV in the spectrum. These peaks come from the normal Auger decay into 1 D2 core holes on both Si-sites. Faint structure is seen around 1598 eV, probably corresponding to the 1 S0 core holes. Schematic diagram of the resonant KL23 L23 Auger decays from Si 1s holes states is illustrated in Fig. 4. Bars with some labels denote the Si 2p−2 :1 D2 u (or R) states with a single charge, together with the Si 1s holes states. Bars with labels and hatchings indicate the doubly charged states of 1 D2 core holes as well as Si 1s−1 energy levels. The arrows depict the decay pathways with substantial yields from the Si 1s holes states into the Si 2p−2 :1 D2 core holes states. Arrows with small circles denote the decay pathways of electron emission following photoelectron re-capture. The photoexcitation into vacant valence orbitals mainly induces the spectator Auger decay to the 1 D2 u states and partly generates the Auger shake-up transitions. The excitation into the Rydberg states yields the Auger shake-down transition. The photoionization slightly above the ionization thresholds induces the photoelectron re-capture accompanied by the resonant Auger transitions with appreciable yields, together with the normal Auger decay. It is important to compare the present results of the MCDS to those of F3 SiCH2 CH2 Si(CH3 )3 (FSMSE) in the resonant KL23 L23 Auger decay.22 In the previous instance, the
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FSMSE indicated a clear site-selectivity in the Auger decay following the excitation into the Rydberg orbital, where the shake-down transition into the valence orbital takes place as well as the shake-up one into the higher-lying Rydberg orbital on the F-site while only shake-up transitions occur on the CH3 -site. In the present instance of the MCDS, the Rydberg excitation induces apparently peculiar effects in Auger processes, in which a high Rydberg electron seems to shakedown to a lower-lying Rydberg orbital on the CH3 -site while a Rydberg electron remains in the same orbital on the Cl-site. The present result shows a clear contrast to the previous findings. This contrast probably originates from the significant spatial-overlapping between wave functions of the Rydberg electrons of the two Si atoms directly bonding each other in the MCDS.
V. SUMMARY
Resonant Auger electron spectra of Si KL23 L23 type in Cl3 SiSi(CH3 )3 have been measured using the electron spectrometer combined with monochromatized undulator radiation. A spectator electron excited through initial photoabsorption remains dominantly at the excited valence orbital, initially populated, during the Si KL23 L23 Auger decay at the photon energy showing the highest photoabsorption yield, 1844.4 eV. Photoexcitation of Si 1s electron into some Rydberg orbitals induces Auger shake-down transitions, which probably originate from the mixing effect between Rydberg orbitals of the two Si atoms on the CH3 -side and the Cl-side. A resonant Auger decay from the state formed via the re-capture of a slow photoelectron has been observed at the photon energy slightly above the Si 1s ionization thresholds. The present results provide that the Si-site dependent decay behavior takes place in the resonant Auger decay to some extent and that this dependence is lost partly. These findings clarified dynamics of excited electrons in resonant KL23 L23 Auger decays, giving important information on electronelectron interaction in molecules and on energy levels of core-excited molecular ions. The present results are expected to stimulate theoretical studies on photoexcitation and decay processes associated with core excited states of molecules.
ACKNOWLEDGMENTS
The authors wish to express sincere thanks to the members of the research team for soft X-ray photochemistry for their fruitful comments, and to the SPring-8 facility staff for their assistance during the course of the experiments. The authors are indebted to Professor Joji Ohshita of Hiroshima University providing us with MCDS. I.H.S. expresses an appreciation for support by Professor Kenji Ito of Photon Factory during this study. O.T. was supported by a Grant-in-Aid for Scientific Research (C) (Contract No. 23540476) of the Ministry of Education, Culture, Sports, Science, and Technology, Japan. This research was carried out with the approval of the SPring-8 Program Advisory Committee (Proposal No. 2012A1067).
J. Chem. Phys. 139, 174314 (2013) 1 K.
Siegbahn, C. Nordling, G. Johansson, J. Hedman, P. F. Hedén, K. Hamrin, U. Gelius, T. Bergmark, L. O. Werme, R. Manne, and Y. Baer, ESCA Applied to Free Molecules (North-Holland, Amsterdam, 1969), pp. 156– 163. 2 S. Nagaoka, G. Prümper, H. Fukuzawa, M. Hino, M. Takemoto, Y. Tamenori, J. Harries, I. H. Suzuki, O. Takahashi, K. Okada, K. Tabayashi, X.-J. Liu, T. Lischke, and K. Ueda, Phys. Rev. A 75, 020502(R) (2007). 3 S. Nagaoka, M. Takemoto, G. Prümper, H. Fukuzawa, Y. Tamenori, I. H. Suzuki, and K. Ueda, J. Chem. Phys. 129, 204309 (2008). 4 S. Nagaoka, H. Fukuzawa, G. Prümper, M. Takemoto, O. Takahashi, K. Yamaguchi, T. Kakiuchi, K. Tabayashi, I. H. Suzuki, J. R. Harries, Y. Tamenori, and K. Ueda, J. Phys. Chem. A 115, 8822 (2011). 5 D. Ceolin, C. Miron, K. Le Guen, R. Guillemin, P. Morin, E. Shigemasa, P. Millié, M. Ahmad, P. Lablanquie, F. Penent, and M. Simon, J. Chem. Phys. 123, 234303 (2005). 6 R. Guillemin, W. C. Stolte, and D. W. Lindle, J. Phys. B 42, 125101 (2009). 7 M. Kato, Y. Morishita, M. Oura, H. Yamaoka, Y. Tamenori, K. Okada, T. Matsudo, T. Gejo, I. H. Suzuki, and N. Saito, J. Electron Spectrosc. Relat. Phenom. 160, 39 (2007). 8 J. D. Bozek, K. H. Tan, G. M. Bancroft, and J. S. Tse, Chem. Phys. Lett. 138, 33 (1987). 9 S. Bodeur, I. Nenner, and P. Millie, Phys. Rev. A 34, 2986 (1986). 10 S. Bodeur, P. Millié, and I. Nenner, Phys. Rev. A 41, 252 (1990). 11 O. Takahashi, I. H. Suzuki, Y. Kono, A. Ikeda, T. Ouchi, K. Ueda, I. Higuchi, Y. Tamenori, and S. Nagaoka, J. Phys.: Conf. Ser. 235, 012018 (2010). 12 J. S. Tse, Z. F. Liu, J. D. Bozek, and G. M. Bancroft, Phys. Rev. A 39, 1791 (1989). 13 G. B. Armen, J. Phys. B 29, 677 (1996). 14 K. Okada, M. Kosugi, A. Fujii, S. Nagaoka, T. Ibuki, S. Samori, Y. Tamenori, H. Ohashi, I. H. Suzuki, and K. Ohno, J. Phys. B 38, 421 (2005). 15 S. Aksela, O.-P. Sairanen, H. Aksela, G. M. Bancroft, and K. H. Tan, Phys. Rev. A 37, 2934 (1988). 16 G. G. B. de Souza, P. Morin, and I. Nenner, J. Chem. Phys. 90, 7071 (1989). 17 O. Kugeler, G. Prümper, R. Hentge, J. Viefhaus, D. Rolles, U. Becker, S. Marburger, and U. Hergenhahn, Phys. Rev. Lett. 93, 033002 (2004). 18 E. Sokell, A. A. Wills, M. Wiedenhoeft, X. Feng, D. Rolles, and N. Berrah, J. Phys. B 38, 1535 (2005). 19 M. Kavˇ ciˇc, M. Žitnik, K. Buˇcar, A. Miheliˇc, S. Carniato, L. Journel, R. Guillemin, and M. Simon, Phys. Rev. Lett. 105, 113004 (2010). 20 P. Kelfve, B. Blomster, H. Siegbahn, K. Siegbahn, E. Sanhueza, and O. Goscinski, Phys. Scr. 21, 75 (1980). 21 T. Kantia, S. Aksela, P. Turunen, L. Partanen, and H. Aksela, J. Phys. B 43, 205002 (2010). 22 I. H. Suzuki, A. Nitta, H. Fukuzawa, K. Ueda, O. Takahashi, Y. Tamenori, and S. Nagaoka, J. Chem. Phys. 131, 164309 (2009). 23 I. H. Suzuki, Y. Kono, A. Ikeda, T. Ouchi, K. Ueda, O. Takahashi, I. Higuchi, Y. Tamenori, and S. Nagaoka, Phys. Rev. A 82, 045401 (2010). 24 I. H. Suzuki, Y. Kono, A. Ikeda, T. Ouchi, K. Ueda, O. Takahashi, I. Higuchi, Y. Tamenori, and S. Nagaoka, J. Chem. Phys. 134, 084312 (2011). 25 S. Nagaoka, H. Endo, K. Nagai, Y. Tamenori, O. Takahashi, and I. H. Suzuki, “Photoelectron spectra of several orbitals and L23 VV Auger electron spectra in Cl3 SiSi(CH3 )3 ” (unpublished). 26 H. Ohashi, E. Ishiguro, Y. Tamenori, H. Kishimoto, M. Tanaka, M. Irie, T. Tanaka, and T. Ishikawa, Nucl. Instrum. Methods Phys. Res. A 467–468, 529 (2001). 27 Y. Tamenori, H. Ohashi, E. Ishiguro, and T. Ishikawa, Rev. Sci. Instrum. 73, 1588 (2002). 28 V. Schmidt, Electron Spectrometry of Atoms using Synchrotron Radiation (Cambridge University Press, Cambridge, 1997), Chap. 6. 29 M. J. Frisch, G. W. Trucks, H. B. Schlegel et al., Gaussian 03, Revision C.02, Gaussian, Inc., Wallingford CT, 2004. 30 K. Hermann, StoBe-deMon, version 3.1, Stockholm University, Stockholm, 2011. 31 L. Triguero, L. G. M. Pettersson, and H. Ågren, Phys. Rev. B 58, 8097 (1998). 32 W. Kutzelnigg, U. Fleischer, and M. Schindler, NMR-Basic Principles and Progress (Springer-Verlag, Heidelberg, 1990). 33 J. P. Perdew and Y. Wang, Phys. Rev. B 33, 8800 (1986). 34 L. G. M. Pettersson, U. Wahlgren, and O. Gropen, Chem. Phys. 80, 7 (1983). 35 I. H. Suzuki, Y. Kono, K. Sakai, M. Kimura, K. Ueda, Y. Tamenori, O. Takahashi, and S. Nagaoka, J. Phys. B 46, 075101 (2013).
The Journal of Chemical Physics is copyrighted by the American Institute of Physics (AIP). Redistribution of journal material is subject to the AIP online journal license and/or AIP copyright. For more information, see http://ojps.aip.org/jcpo/jcpcr/jsp
Site-dependent Si KL23 L23 resonant Auger electron spectra following inner-shell excitation of Cl3 SiSi(CH3 )3 Isao H. Suzuki,1,a) Hikari Endo,2 Kanae Nagai,2 Osamu Takahashi,3 Yusuke Tamenori,4 and Shin-ichi Nagaoka2 1
Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba 305-0801, Japan and Advanced Institute of Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba 305-8568, Japan 2 Department of Chemistry, Faculty of Science, Ehime University, Matsuyama 790-8577, Japan 3 Institute for Sustainable Science and Development, Hiroshima University, Higashi-Hiroshima 739-8526, Japan 4 Synchrotron Radiation Research Institute/SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun 679-5198, Japan
(Received 3 July 2013; accepted 17 October 2013; published online 6 November 2013) Spectator resonant Auger electron spectra with the Si 1s photoexcitation of Cl3 SiSi(CH3 )3 have been measured using an electron spectroscopic technique combined with undulator radiation. The transition with the highest intensity in the total ion yield (TIY) spectrum, coming from excitation of a Si 1s electron on the Cl-side into a vacant valence orbital, generates the resonant Auger decay in which the excited electron remains in this valence orbital. Photoexcitation of 1s electrons into some Rydberg orbitals induces Auger shake-down transitions, because higher-lying Rydberg orbitals in the two Si atoms closely positioned hold spatially overlapping considerably. A broad TIY peak slightly above the 1s ionization thresholds appreciably yields resonant Auger decays in which a slow photoelectron is re-captured into a higher-lying Rydberg orbital. The normal Auger peak shape at this photon energy is distorted due to a post-collision interaction effect. These findings provide a clear understanding on properties of the excited orbitals which are ambiguous in the measurement of the TIY only. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4827860] I. INTRODUCTION
Core level ionization energy shows a large chemical shift in the binding energy of molecules and thus electronic structures of the molecules have been discussed on the chemical shift measured using a photoelectron spectroscopic technique in a number of instances.1 Molecules containing Si atoms draw attention of many researchers to studies on site-specific dissociation because Si material is often used for fabrication of electronic devices. Basic studies were attempted for clarification of the site-specific effect using monochromatized synchrotron radiation. Yields of some fragment ions were discovered to strongly depend on the site of core-ionized Si atoms within a molecule.2–4 This site-specificity depends on the distance between the relevant Si atoms in molecules, and this dependence is connected with easiness or difficulty of electron flow between the Si atoms. Element-specific fragmentations were also investigated by several research groups, which showed state-selectivity of core-excited states of some elements within different molecules in the process of decomposition into ionic fragments.5, 6 Shallow core-level photoabsorption spectra of small molecules clearly provide properties of vacant molecular orbitals, while upon transitions of deep core electrons, photoabsorption peaks usually exhibit complex structures due to an a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]
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increase in the lifetime width.7–11 Si K-shell excitation spectra of Si-containing molecules were measured using monochromatized synchrotron radiation by Bodeur and co-workers.9, 10 They assigned a peak below the ionization threshold to the excitation of a Si 1s electron into a vacant molecular orbital on the basis of the calculation using the ab initio configuration interaction technique. The multiple scattering Xα method was frequently utilized and this method nicely reproduced characteristics of shape resonance above the threshold.12 When an inner-shell electron is emitted from an atom or molecule, the created core hole is filled through Auger electron emission because of the low probability of radiative processes in the soft X-ray region. Auger electrons give us information on energy values of doubly charged states and on spatial distributions of relevant orbitals. In photoexcitation of an inner-shell electron into a vacant orbital, resonant Auger electrons emerge, which are classified into two types: spectator Auger process and participator one.13–19 The final state in the former process is represented with a spectator electron and a doubly charged ion core which is assumed to be the same as the state in the normal Auger (NA) decay. In this spectator process of rare gas atoms the excited electron was found to remain in the initially excited orbital with the same principal quantum number, or to be shaken up to an orbital with a different quantum number.13, 14 Since a molecule has vibrational modes, spectator Auger electron spectra usually exhibit complex structures, making it difficult to precisely analyze
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the final states.15, 16 Resonant Auger decays of the participator type, which result in the same state as the conventional photoelectron emission, were studied for several molecules, and in those studies variation in photoelectron yields into particular channels and ultra-fast dissociation processes were clarified.17, 18 In the photoexcitation into an anti-bonding valence molecular orbital, some molecules decompose prior to Auger decay and a decomposed fragment emits an Auger electron which has a kinetic energy different from that of the molecule. Chlorine L-shell resonant Auger electron spectra of HCl were precisely analyzed, which demonstrated that an interference effect between the continuum of the molecular ion and that of the dissociated fragment ion takes place during the participator Auger decay.18 In regard to resonant X-ray emission spectra in the Cl K-shell, the spectra indicated the occurrence of the interference effect between the Cl 1s−1 and 2p−1 core holes.19 Normal KLL Auger electron spectra of Si-containing molecules and vaporized Si atoms were investigated by use of an electron beam technique.20, 21 The spectra of those molecules show relatively simple characteristics in the KL23 L23 decays owing to a pseudo-atomic process of Auger decays between the inner-shells. The analyzed spectra provided energies of doubly charged molecular ions having two Si 2p holes and indicated that a state of 1 D2 in the atomic representation had the dominant intensity.20 Since simple spectra were measured for resonant KLL Auger transitions, an energy transformation of photoexcited states into electron kinetic energies was clearly elucidated in spectator resonant KLL Auger electrons of SiF4 and similar molecules.22–24 The molecule Cl3 SiSi(CH3 )3 , represented as MCDS in the present study, includes two Si atoms, which are positioned at different chemical environments; one bonding to three Cl atoms, represented here with Si[Cl], and the other bonding to three methyl substituents, Si[Me]. Studies on the site-specific fragmentation following Si inner-shell excitation were carried out using this molecule and its related molecules.2–4 Then the Si atoms of this molecule gave essentially no vibrational structure in the inner-shell photoelectron spectra, coming from little movement of Si atoms surrounded by some substituents in this molecule.25 These conditions are very efficient as a research on the measurement of KLL resonant Auger electron spectra of the MCDS, in which electron emission is expected to proceed in the pseudo-atomic phenomenon. It is interesting to find different or same behavior in resonant Auger decays of two Si atoms; i.e., whether a sitespecific phenomenon occurs or not. Further it is important to compare photoabsorption spectrum of this molecule in the 1s ionization region with those of SiCl4 and Si(CH3 )4 in order to clarify the property of Si 1s-excited states of this molecule. In the present study, resonant KL23 L23 Auger electrons of spectator type following Si 1s excitation of the MCDS have been measured using monochromatized undulator radiation and a hemispherical electron energy analyzer. The measured electron kinetic energy spectra show the final state configurations consisting of two 2p orbital holes and of a Rydberg electron or a valence electron. Rydberg electrons are found to be shaken-up and shaken-down, while the excited valence electrons dominantly behave as a spectator.
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II. EXPERIMENTAL
Measurements were performed on the c branch of the soft X-ray photochemistry beamline BL27SU at the SPring8 facility.26 The energy of the monochromator was calibrated using the Ne 1s photoelectrons.27 The photon bandwidth employed was about 0.93 eV in most instances and the uncertainty of the photon energy was ±0.3 eV. The intensity of the monochromatized incident photon beam during the measurements was monitored by collecting the drain current of the post-focusing mirror in the beamline. An electron spectrometer used consists of a hemispherical electron spectrometer (Gammadata Scienta, SES-2002) fitted to a gas cell (GC-50) by way of a multi-element lens in a differentially pumped chamber.14 Sample gas of MCDS was supplied into the gas cell and its vapor pressure observed in the chamber during the measurements was 2 × 10−4 Pa. The energy bandwidth in the electron spectra was estimated to be 0.47 eV. The electron energy was calibrated using the peak energies for the Auger decay from the Ne K-shell hole state28 and the uncertainty of the electron energy was ±0.2 eV. The total ion yield (TIY) spectrum was observed using an ion detector with a chevron type MCP assembly at the midpoint between the post-focusing mirror and the electron spectrometer. This spectrum approximates the photoabsorption spectrum in the soft X-ray region.
III. CALCULATION
The X-ray photoabsorption (XA) spectrum in the Si 1s excitation region of MCDS was calculated using the density functional theory (DFT) as described previously.11 Geometry optimization of MCDS was carried out using the GAUSSIAN 03 program at the MP2/cc-pVTZ level of approximation.29 The XA spectrum calculations were performed using the StoBe-deMon program.30 The theoretical XA spectra were generated by the transition potential (DFT-TP) method.31 The orbitals for the molecules were determined with a halfoccupied core orbital at the ionization site using the IGLOIII basis of Kutzelnigg et al.32 for core-hole Si atoms, the (311/211/1) basis set for non-core-hole Si atoms, the (41/31/1) basis set for Cl atoms, the (321/311/1) basis set for C atoms, and the (311/1) basis set for H atoms. The augment basis sets on Si atoms were used for Rydberg orbitals: ζ s = 0.017 and 0.0056667, ζ p = 0.014 and 0.0046667, and ζ d = 0.015 and 0.003. The non-core-excited Si, Cl, and C atoms were described by effective core potentials. The gradientcorrected exchange (PD86) and correlation functional (PD91) established by Perdew and Wang were applied.33 To estimate correct description for low-lying resonant excited states, fully relaxed Kohn-Sham calculations were performed.34 The relativistic correction added to the ionization potential was 2.0 eV for the Si K-edge. Functional correction of 4.7 eV was estimated by adjusting a large resonant peak, and the whole spectrum was shifted. Finally, the spectrum was generated by a Gaussian convolution of the discrete lines by varying the broadenings. The computed spectrum was broadened using Gaussian functions with a full width at half maximum of 1.0 eV below 1844 eV, and then linearly increasing up to 6.0 eV at 1849 eV.
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IV. RESULTS AND DISCUSSION
Figure 1 shows the experimental TIY spectrum as well as calculated photoabsorption (XA) spectra of MCDS in the Si 1s transition region. Fig. 1(a) exhibits the measured TIY spectrum; Fig. 1(b) does the calculated XA spectrum of the MCDS; Fig. 1(c) does that for the Si[Cl] excitation in the MCDS; and Fig. 1(d) does the spectrum for the Si[Me] excitation. The spectrum at Fig. 1(b) corresponds to the summation of those at Figs. 1(c) and 1(d). The bars with hatching in Fig. 1(a) denote the 1s ionization energies for two Si atoms, Si[Cl] and Si[Me], respectively.25 Arrows denote peaks or structures with intensity increases, and electron energy spectra were measured at these photon energies (see Fig. 2). It is found that the TIY spectrum consists of a sharp peak, some structures with high intensity and a shoulder near the peak. The calculated XA spectrum for the Si[Cl] excitation is very close to the calculated one for the SiCl4 molecule,35 while the calculated spectrum for the Si[Me] excitation is different from the corresponding spectrum for the Si(CH3 )4 molecule. The latter spectrum exhibits only one peak near the 1s ionization threshold and is almost the same as that experimentally observed.10, 11, 24 However, the XA spectrum calculated for the MCDS is very close to that of the TIY. The highest peak in the TIY shows up at 1844.4 eV, which corresponds to the excitation of the 1s electron of the Si[Cl] into an unoccupied valence orbital, as described below. Table I lists energies for the peaks and some structures in the TIY spectrum and the assignments of these structures, which have been obtained from the comparison with the DFT calculation. This table includes those for two molecules related to the present MCDS, i.e., SiCl4 and Si(CH3 )4 .11, 24, 35
FIG. 2. Resonant and normal KL23 L23 Auger electron spectra following Si 1s photoexcitation at several photon energies in Cl3 SiSi(CH3 )3 : (a) 1842.7 eV, (b) 1844.4 eV, (c) 1846.9 eV, (d) 1851.3 eV, (e) 1860.2 eV, and (f) 1951.8 eV. See the text for labels added to some structures like V, V , and so forth. Bars at the left end denote background fluctuations.
TABLE I. Spectral structures at Si 1s photoabsorption of Si-containing molecules. Uncertainty in excitation energy for the MCDS is ±0.3 eV. I.E.: ionization energy. Excitation energy (eV) Cl3 SiSi(CH3 )3
1842.7 1844.4 1846.5 1846.9 1849.2 1851.3
1860.2
Assignment
Term value (eV)
Si[Me]: 1s−1 u1 4.2 4.8 Si[Cl]: 1s−1 u2 Si[Cl]: high Rydberg 2.7 (or Si[Me]: high Rydberg) 0.4 Si[Me]: I.E. Si[Cl]: I.E. Doubly excited [or shape Si[Cl]: −2.1 resonance] Si[Me]: −4.4 Shape resonance Si[Cl]: −11 Si[Me]: −13.3
Si(CH3 )4 a 1843.3 1846.3
6t2 (and 4p) I.E.
3.0
1844.8 1846.2 1850.7 1851.3 1857.7
8a1 9t2 I.E. High Rydberg Doubly excited
5.9 4.5
SiCl4 b FIG. 1. Total ion yield (TIY) spectrum of Cl3 SiSi(CH3 )3 (MCDS) in the Si 1s excitation region, as well as calculated photoabsorption (XA) spectra. (a) Experimental TIY spectrum. (b) Calculated XA spectrum of MCDS. (c) Calculated XA spectrum for Si[Cl] excitation in the MCDS. (d) Calculated XA spectrum for Si[Me] excitation. Bars with hatching denote the ionization energies, 1846.9 eV for Si[Me] and 1849.2 eV for Si[Cl]. Arrows denote peak structures or intensity increases.
a b
Data cited from Ref. 24. Data cited from Ref. 35.
−0.6 −7.0
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The shoulder at 1842.7 eV comes from the excitation of the Si[Me] 1s electron into the vacant valence orbital (u1 ). The highest peak at 1844.4 eV mainly originates from the excitation of the Si[Cl] 1s electron into the vacant valence orbital (u2 ) and small contribution to this excitation probably comes from the Si[Me] 1s transition to a Rydberg orbital. These orbitals, u1 and u2 , are assigned to the valence orbitals largely composed of p-type orbitals of Si[Me] atom and Si[Cl] atom, respectively. The structure at 1846.5 eV probably corresponds to the excitation of the Si[Cl] 1s electron into a Rydberg orbital of a principal quantum number (n) of 4 or so (see below). This structure partly includes a contribution from the excitation of the Si[Me] 1s electron into a higher-lying Rydberg orbital, 7p or so. The structure around 1851.3 eV is presumed to come from a shape resonance effect or from excitation of two electrons into vacant orbitals; that is, excitation to doubly excited states, 1s−1 v−1 w1 w2 , where v indicates a valence orbital, and w1 and w2 denote vacant valence orbitals. It seems more probable that the doubly excited states exist there, because the present calculation holding one-electron character does not show clear increase around 1851 eV. A small intensity increase around 1860 eV probably corresponds to a shape resonance. The term values for the excited states are listed in Table I. A comparison of the term values of the vacant valence orbitals among the three molecules suggests an interesting point. The term value for the valence orbital for the Si[Me] of the MCDS, 4.2 eV, is higher than that for Si(CH3 )4 , 3.0 eV, and that for Si[Cl] of the MCDS, 4.8 eV, is lower than that for SiCl4 , 5.9 eV. This finding is the same as the characteristics of the relation among these molecules for the 1s ionization energy. These results reflect that the electronic structure near the Si atoms in the present molecule consists of a convolution from the electronic structures of the Si(CH3 )4 and SiCl4 . Figure 2 shows electron kinetic energy spectra of Si KL23 L23 resonant Auger decay, together with the normal Auger electron spectrum. Figs. 2(a)–2(e) exhibit the spectra measured at photon energies of 1842.7 eV, 1844.4 eV, 1846.5 eV, 1851.3 eV, and 1860.2 eV, respectively. Fig. 2(f) depicts the normal Auger spectrum measured at 1951.8 eV. The clear peaks for the normal Auger electrons in Fig. 2(f) indicate the energy values of 1602.6 eV and 1604.2 eV, which correspond to the formation of the doubly charged state of 1 D2 of Si[Cl] and that of Si[Me], respectively. Here the atomic representation on the Si atom state is utilized because the Auger decay usually takes place as pseudo-atomic process. These processes are represented with the following: M + hv → M+ (Si 1s−1 ) + e → M+2 (Si 2p−2 :1 D2 ) + 2e. (1) Here M denotes the MCDS molecule. The states of 1 S0 of Si atoms are presumed to correspond to small structures appearing around 1597 eV and 1600 eV marginally. In order to examine electron kinetic energy spectra closely, we have measured these spectra as a function of the incident photon energy near the Si 1s ionization thresholds. The resultant two dimensional (2D) map is exhibited in Fig. 3, which includes the TIY spectrum at the right end. In this figure, a long vertical line around the electron kinetic en-
J. Chem. Phys. 139, 174314 (2013)
FIG. 3. Two-dimensional map of electron emission spectrum in Si KL23 L23 Auger decays as a parameter of photon energy. The total ion yield spectrum is displayed at the right end. See the text for island structures named as V, V , and R.
ergy of 1604 eV is found, together with a thinner line around 1603 eV. This long thicker line comes from the normal Auger decay generating the Si[Me] 1 D2 core, and the thinner line corresponds to the normal Auger decay to the Si[Cl] 1 D2 core. It is found that the thicker line becomes slightly bent near the ionization threshold because of the post-collision interaction, while the thinner one does not provide a clear bent feature owing to low signal yields. The island (V) with the highest intensity is seen around the electron kinetic energy of 1608 eV and the photon energy of 1844 eV. This island comes from the spectator resonant Auger decay of the Si[Cl] 1s−1 u2 state. This decay is expressed as follows: M∗ (Si[Cl]1s −1 u2 ) → M+ (Si[Cl]2p−2 :1 D2 u2 ) + e.
(2)
The island (V ) on the higher electron energy side and on the lower photon energy side originates from the similar Auger decay. It is found that there are an island (R) around the electron energy of 1608 eV and some structures around 1605 eV at the photon energy of 1846.5 eV. These structures correspond to the resonant Auger decays accompanied by shake effects (see below). Let us explain assignments of electron signals using the spectra in Fig. 2. The spectrum in Fig. 2(a), which has been measured at the photon energy of 1842.7 eV, shows the highest peak at 1610.2 eV (V ). This peak originates from a resonant Auger decay of spectator type. In this decay the valence electron initially excited from the Si[Me] 1s orbital behaves as a spectator and then remains in this excited orbital during the Auger decay. This decay is expressed in the following manner: M + hv → M∗ (Si[Me]1s −1 u1 ) → M+ (Si[Me]2p−2 :1 D2 u1 ) + e.
(3)
The small peak at 1607.7 eV (V) comes from the spectator resonant Auger decay, Si[Cl] 1s−1 u2 to Si[Cl]2p−2 : 1 D2 u2 . The transition from the Si[Cl] 1s orbital becomes possible through overlapping of the wide energy width of the deep core holes. These spectating phenomena of the valence electrons have been found to occur in the instances of the resonant
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TABLE II. Characteristic energies related to Si KL23 L23 Auger decays in Cl3 Si-Si(Me)3 (in units of eV). Note that the term of 2p−2 is of 1 D2 except for indication of 1 S0 . See text for several labels such as T, n∗ , and so forth. Values in the parentheses denote those estimated on the assumption that the relevant orbitals are of Rydberg-type. Photon energy
Tneu
n∗
n
δ
Electron energy
Tion
n∗
n
δ
1842.7
4.2
(1.8
3
1.2)
Si[Me]1s−1 u1
1610.2 1607.7 1603.0
10.2 11.6 3.0 (or 7.5)
(2.3 (2.2 4.3 (2.7
4 4 6 4
1.7) 1.8) 1.7 1.3)
Si[Me], u1 Si[Cl], u2 Si[Me], R1 (or Si[Me]:1 S0 u1 )
1844.4
4.8
(1.7
3
1.3)
Si[Cl]1s−1 u2
1609.4 1602.9
11.6 5.1 (or 10.3)
(2.2 3.3 (2.3
4 5 4
1.8) 1.7 1.7)
Si[Cl], u2 Si[Cl], R2 (or Si[Cl]:1 S0 u2 )
1846.5
2.7 0.4
2.2 5.8
4 7
1.8 1.2
Si[Cl]1s−1 R (or Si[Me]1s−1 R)
1608.4
8.5 (or 4.6 2.2 (or 6.1
2.5 3.4 5.0 3.0
4 5 6 4
1.5 1.6) 1.0 1.0)
Si[Cl], R3 (or Si[Me], R3 ) Si[Me], R4 (or Si[Cl], R4 )
Assignment
1606 1851.3
Doubly excited (or shape res.)
1604.6 1603.2 1610.0 1608 1597
1860.2
Shape res.
1951.8
KLL Auger decay of other Si-containing molecules like SiCl4 and Si(CH3 )4 .22–24, 35 In particular, the electron signals for this type of the decay have provided very high yield. The small structure around 1603 eV probably corresponds to a shake-up transition, as represented with the following: ∗
M + hv → M (Si[Me]1s +
−1
u1 )
−2 1
→ M (Si[Me]2p : D2 R1 ) + e,
(4)
where the valence electron, initially excited from the Si[Me] 1s orbital, is shaken up to a Rydberg orbital, R1 . Branching ratio for these processes, V : V: R1 , is approximately estimated to be 7:2:1. At this point, it is convenient to introduce a term value for a resonant Auger final state. The term value is calculated using the following equation: Tion = (EK − EA ) − (EPR − ERA ).
(5)
Here EK , EA , EPR , and ERA denote the ionization energy of the Si 1s electron, the normal Auger electron energy of KL23 L23 forming the 1 D2 core holes, the photon energy yielding the resonance excitation, and the electron energy of the resonant Auger decay, respectively. Table II lists the characteristics of the peaks and structures exhibited in Fig. 2 as well as characteristic energies for resonant and normal Auger electron spectra measured at several photon energies. Tneu denotes a term value for the Si 1s photoexcited state. Effective quantum numbers, n∗ (= n – δ), and quantum defects (δ) are estimated for Rydberg type orbitals in Si 1s photoexcited states and reso-
5.3 (or 1.4 3.3
3.2 6.2 4.1
5 8 6
1.8 1.8) 1.9
Assignment (Si 2p−2 :1 D2 )
NA, Si[Me] NA, Si[Cl] Re-capture, Si[Cl], R5 (or re-capture, Si[Me], R5 ) Re-capture, Si[Cl], R6 NA, Si[Cl]: 1 S0
1604.6 1603.2 1597
NA, Si[Me] NA, Si[Cl]) NA, Si[Cl]: 1 S0
1604.2 1602.6 1600 1597
NA, Si[Me] NA, Si[Cl] NA, Si[Me]:1 S0 NA, Si[Cl]: 1 S0
nant Auger final states. The effective quantum numbers are calculated using a simple Rydberg equation, Tion or Tneu =Zc2 Ry/(n∗ )2 .
(6)
In this equation, Ry denotes the Rydberg energy, 13.6 eV, and Zc indicates the core charge. Since the quantum number is an integer (n > 3), a couple of integer values are assumed for n after the calculation of n∗ and the corresponding δ values are calculated at several photon energies. On account of reasonability for excited core states of Si molecules, the dataset for n and δ values has been decided and listed in Table II. The resonant Auger final state corresponding to the small peak at 1603 eV has a quantum number of 6 if it is yielded through the shake-up process. In Fig. 2(b), the sharp peak at 1609.4 eV comes from a spectator resonant Auger decay, whose final state is represented with Si[Cl]2p−2 :1 D2 u2 . The orbital u2 in this state is different from the u1 in process (3) and thus the term values are different from each other: 11.6 eV for this u2 orbital and 10.2 eV for the u1 in process (3), as indicated in Table II. The small peak around 1602.9 eV probably corresponds to a shake-up transition of the initially excited electron into a Rydberg orbital, Si[Cl]2p−2 2 D1 R2 (branching ratio of V:R2 is 9: 1). Another possibility is suggested that this structure partly comes from a spectator resonant Auger decay in which the core hole state is 1 S0 and the spectator electron is the excited valence electron (u2 ). This suggestion is based on the finding that the energy difference between 1609.4 eV and 1602.9 eV, 6.5 eV, is not so different from that between the normal
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Auger electron energies, 5.2 eV calculated from 1602.6 eV for Si[Cl]2p−2 :1 D2 and 1597.4 eV for Si[Cl]2p−2 :1 S0 . The spectrum in Fig. 2(c) exhibits a profile different from the spectra in Figs. 2(a) and 2(b) discussed above. The highest peak at 1608.4 eV probably comes from the resonant Auger decay into the state of Si[Cl]2p−2 :1 D2 R3 (R3 :4p or so), as expressed with the following: M + hv → M∗ (Si[Cl]1s −1 R3 ) → M+ (Si[Cl]2p−2 :1 D2 R3 ) + e,
(7)
where the excited Rydberg electron remains in the same orbital. Further, it is possible to expect that the 1s excited state is of Si[Me]1s−1 R3 and that the final state is of Si[Me]2p−2 :1 D2 R3 . Here R3 and R3 are presumed to be 7p and 5p or so, respectively. The broad peak structure on the lower energy side, about 1606 eV, is presumed to correspond to the resonant decay into the state of Si[Me]2p−2 :1 D2 R4 (R4 :6p or so). In this decay the Rydberg electron initially excited is shaken down from 7p into 6p or so, as denoted in Table II. Here the approximate branching ratio of R3 :R4 is 6:4. Faint structures around 1601 eV on the lower energy side seems to correspond to the resonant Auger decay of the shake-up or no change in the principal quantum number. These findings seem peculiar in consideration of shake phenomena of the excited Rydberg electron in other Si-containing molecules.22–24 Many shake-up Auger decays of the Rydberg electrons had been observed in the resonant KLL Auger transitions and the quantum number of the Rydberg electron was promoted by 1, i.e., from n into n + 1. This promotion was in agreement with the expectation based on the hydrogenic model, where the n value in the singly charged core turns into n in the doubly charged core, n = n[(Zc + 1)/Zc ]1/2 .13 The peculiar shake effect of the present molecule seems to result from the spatial overlapping of higher-lying Rydberg orbitals with large orbital radii in the two Si atoms, which have direct bonding with each other. The bond distance between the two Si atoms is estimated to be 2.37 Å.25 Thus, it is natural that high Rydberg orbitals of the Si atoms are closely distributed and effectively coupled. Another possibility seems to be connected with mixing between Rydberg orbitals and valence orbitals. It cannot be concluded at present which effect plays a major role on the shake phenomena of this molecule. In the spectrum in Fig. 2(d), measured at 1851.3 eV, a twin peak with tailing into higher energy is found to have a considerable yield at 1603.2 eV and 1604.6 eV. This twin peak originates from the normal Auger decay into states of Si[Cl]2p−2 :1 D2 and Si[Me]2p−2 :1 D2 and the tailing results from the post-collision interaction with a photoelectron having low kinetic energy.22, 35 Some structures around 1608 eV and 1610 eV seem to correspond to resonant Auger decay and then these come from higher-lying Rydberg states which are generated through a re-capture of the slow photoelectron (branching ratio of NA:R5 + R6 is 7:3). In other words, the photoelectron is re-trapped by the 2p−2 :1 D2 core of the Si atom during the Auger transition. It is not possible, at present, to clarify which Si atom on the two sites plays a significant role in this trapping phenomenon. As mentioned above, the slight intensity-increase at this photon energy in the TIY prob-
FIG. 4. Schematic drawing of KL23 L23 Auger decay pathways from Si 1s holes states. Bars with labels denote Si 2p−2 : 1 D2 u (or R) states. Bars with labels and hatchings indicate doubly charged states of 1 D2 core holes and 1s−1 energy levels. Arrows denote decay pathways and arrows with small circles indicate those of electron emission following photoelectron re-capture.
ably comes from the transition into the doubly excited states. If the two excited electrons remain in the excited orbitals, the spectator Auger decay induces emission of electrons with higher kinetic energy than the NA electrons. These electrons seem to correspond to the structures around 1610 eV in Fig. 2(d). The quantum number of the electrons in the final state is calculated to be 5 or so, on the assumption using the Rydberg formula. Thus, the possibility cannot be excluded that the structures around 1610 eV slightly come from the spectator process of the two electrons. The faint structures around 1597 eV are presumed to correspond to the normal Auger decay of another core state, 2p−2 :1 S0 . At the photon energy of 1860.2 eV (Fig. 2(e)), two peaks with considerable tailing are found at 1603 eV and 1604.6 eV in the spectrum. These peaks come from the normal Auger decay into 1 D2 core holes on both Si-sites. Faint structure is seen around 1598 eV, probably corresponding to the 1 S0 core holes. Schematic diagram of the resonant KL23 L23 Auger decays from Si 1s holes states is illustrated in Fig. 4. Bars with some labels denote the Si 2p−2 :1 D2 u (or R) states with a single charge, together with the Si 1s holes states. Bars with labels and hatchings indicate the doubly charged states of 1 D2 core holes as well as Si 1s−1 energy levels. The arrows depict the decay pathways with substantial yields from the Si 1s holes states into the Si 2p−2 :1 D2 core holes states. Arrows with small circles denote the decay pathways of electron emission following photoelectron re-capture. The photoexcitation into vacant valence orbitals mainly induces the spectator Auger decay to the 1 D2 u states and partly generates the Auger shake-up transitions. The excitation into the Rydberg states yields the Auger shake-down transition. The photoionization slightly above the ionization thresholds induces the photoelectron re-capture accompanied by the resonant Auger transitions with appreciable yields, together with the normal Auger decay. It is important to compare the present results of the MCDS to those of F3 SiCH2 CH2 Si(CH3 )3 (FSMSE) in the resonant KL23 L23 Auger decay.22 In the previous instance, the
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FSMSE indicated a clear site-selectivity in the Auger decay following the excitation into the Rydberg orbital, where the shake-down transition into the valence orbital takes place as well as the shake-up one into the higher-lying Rydberg orbital on the F-site while only shake-up transitions occur on the CH3 -site. In the present instance of the MCDS, the Rydberg excitation induces apparently peculiar effects in Auger processes, in which a high Rydberg electron seems to shakedown to a lower-lying Rydberg orbital on the CH3 -site while a Rydberg electron remains in the same orbital on the Cl-site. The present result shows a clear contrast to the previous findings. This contrast probably originates from the significant spatial-overlapping between wave functions of the Rydberg electrons of the two Si atoms directly bonding each other in the MCDS.
V. SUMMARY
Resonant Auger electron spectra of Si KL23 L23 type in Cl3 SiSi(CH3 )3 have been measured using the electron spectrometer combined with monochromatized undulator radiation. A spectator electron excited through initial photoabsorption remains dominantly at the excited valence orbital, initially populated, during the Si KL23 L23 Auger decay at the photon energy showing the highest photoabsorption yield, 1844.4 eV. Photoexcitation of Si 1s electron into some Rydberg orbitals induces Auger shake-down transitions, which probably originate from the mixing effect between Rydberg orbitals of the two Si atoms on the CH3 -side and the Cl-side. A resonant Auger decay from the state formed via the re-capture of a slow photoelectron has been observed at the photon energy slightly above the Si 1s ionization thresholds. The present results provide that the Si-site dependent decay behavior takes place in the resonant Auger decay to some extent and that this dependence is lost partly. These findings clarified dynamics of excited electrons in resonant KL23 L23 Auger decays, giving important information on electronelectron interaction in molecules and on energy levels of core-excited molecular ions. The present results are expected to stimulate theoretical studies on photoexcitation and decay processes associated with core excited states of molecules.
ACKNOWLEDGMENTS
The authors wish to express sincere thanks to the members of the research team for soft X-ray photochemistry for their fruitful comments, and to the SPring-8 facility staff for their assistance during the course of the experiments. The authors are indebted to Professor Joji Ohshita of Hiroshima University providing us with MCDS. I.H.S. expresses an appreciation for support by Professor Kenji Ito of Photon Factory during this study. O.T. was supported by a Grant-in-Aid for Scientific Research (C) (Contract No. 23540476) of the Ministry of Education, Culture, Sports, Science, and Technology, Japan. This research was carried out with the approval of the SPring-8 Program Advisory Committee (Proposal No. 2012A1067).
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