Low-energy photoelectron imaging of HS2 anion Zhengbo Qin, Ran Cong, Zhiling Liu, Hua Xie, and Zichao Tang Citation: The Journal of Chemical Physics 141, 204312 (2014); doi: 10.1063/1.4901978 View online: http://dx.doi.org/10.1063/1.4901978 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/141/20?ver=pdfcov Published by the AIP Publishing Articles you may be interested in An investigation into low-lying electronic states of HCS2 via threshold photoelectron imaging J. Chem. Phys. 140, 214318 (2014); 10.1063/1.4879808 Slow photoelectron velocity-map imaging spectroscopy of the C9H7 (indenyl) and C13H9 (fluorenyl) anions J. Chem. Phys. 139, 104301 (2013); 10.1063/1.4820138 Vibrationally resolved photoelectron imaging of gold hydride cluster anions: AuH − and Au 2 H − J. Chem. Phys. 133, 044303 (2010); 10.1063/1.3456373 Low-energy photoelectron imaging spectroscopy of nitromethane anions: Electron affinity, vibrational features, anisotropies, and the dipole-bound state J. Chem. Phys. 130, 074307 (2009); 10.1063/1.3076892 Spectroscopic characterization of the ground and low-lying electronic states of Ga 2 N via anion photoelectron spectroscopy J. Chem. Phys. 124, 064303 (2006); 10.1063/1.2159492
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THE JOURNAL OF CHEMICAL PHYSICS 141, 204312 (2014)
Low-energy photoelectron imaging of HS2 anion Zhengbo Qin,1,2 Ran Cong,2 Zhiling Liu,1 Hua Xie,1 and Zichao Tang1,a) 1
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China 2 Department of Physics, Anhui Normal University, Wuhu, Anhui 241000, China
(Received 2 July 2014; accepted 3 November 2014; published online 26 November 2014) Low-energy photoelectron imaging of HS2 – has been investigated, which provides the vibrational frequencies of the ground state as well as the first excited state of HS2 . It allows us to determine more accurate electron affinity of HS2 , 1.9080 ± 0.0018 eV. Combined with Frank-Condon simulation, the vibrational features have been unveiled related to S-S stretching and S-S-H bending modes for the ground state and S-S stretching, S-S-H bending, and S-H stretching modes for the first excited state. Photoelectron angular distributions are mainly characteristic of electron detachment from two different molecular orbitals (MOs) in HS2 – . With the aid of accurate electron affinity value of HS2 , corresponding thermochemical quantities can be accessed. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4901978] I. INTRODUCTION
There is a growing interest in sulfur-containing compounds on account of their important role in air pollution and acid rain.1–7 Among various sulfur-containing molecules, HS2 radical is one of the most fundamental species responsible for the reaction in combustion, the oxidation of reduced forms of sulfur8–11 and biochemistry.12–14 Extensive studies on the HS2 have been reported for decades. Yamamoto and Saito provided the first experimental information about the ground-state structure of HS2 with a millimeterwave spectroscopy.15 In a recent experiment, the matrixisolation infrared spectra of the HS2 radical following 266 nm photolysis of H2 S2 revealed the S-H stretching and the S-S-H bending modes which were observed to be 2463 and 903 cm−1 , respectively.16 Quite recently, Ashworth and Fink have investigated the chemiluminescence spectrum of the HS2 radical with a high-resolution Fourier-transform spectrometer, and the vibrational parameters have been obtained ranging from 4000 to 9000 cm−1 .17 In addition, photoelectron spectroscopy studies involving transitions from anion to neutral on HS2 have been conducted. Early photoelectron spectrum of HS2 – was reported where S-S stretching mode for the ground state of HS2 has only been extrapolated from low-resolution studies, which was contaminated by S2 − .1 In a recent photoelectron spectroscopic experiment, Entfellner and Boesl have reported an experimental photoelectron study of the HS2 – in which the HS2 was observed in the ground and first excited states with a conventional time of flight photoelectron spectrometry.18 With the aid of an earlier photoelectron spectroscopic investigation and the previous theoretical studies of Peterson et al.,19 the S–S stretching motion for photodetachment into the ground state of HS2 and S–S–H bending motion along with a weak excitation of S–H stretching motion for the photodetachment into the first excited state of HS2 have been a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]. Tel.: +86-411-84379365. Fax: +86-411-84675584. 0021-9606/2014/141(20)/204312/5/$30.00
assigned. On the other hand, considerable theoretical work has focused on HS2 in recent years.19–25 Especially, Owens et al.,21 Denis22 and Peterson et al.19 reported the most accurate results relevant to the geometries and frequencies of the ground and first excited state compared to the previous experimental findings. Moreover, theoretical studies indicated additional weaker and as of yet unresolved vibrational features in the photoelectron spectra of HS2 – .24 In view of importance of its role in combustion and atmosphere, the accurate EA as well as vibrational information would give accurate geometric structure and thermochemistry information. Although comprehensive experimental and theoretical investigations have been conducted on HS2 , there is no thorough investigation into the ground state and the first excited state of HS2 using above-mentioned methods except for conventional photoelectron spectra. Hence, photoelectron imaging with the capability of high sensitivity and high resolution at near-threshold photon energies will help us to resolve such fine vibrational structures with extremely weak excitation in the photodetachment. As far as we know, lowenergy photoelectron imaging of HS2 – is the first such experimental probe into the nature of the neutral and anionic ground states as well as neutral excited state within the vibrational motions manifold. Herein, we present photoelectron spectroscopic and theoretical investigation of HS2 via lowenergy photoelectron imaging. In addition, we compare theoretical MOs (HOMO and HOMO−1) with the photoelectron imaging of HS2 – and probe MO wavefunction nature from the photoelectron anisotropy parameter, β. Furthermore, FranckCondon (FC) simulation of the vibrational structures is performed to interpret the experimental spectra. II. EXPERIMENTAL AND COMPUTATIONAL METHODS
The photoelectron velocity-map imaging system has been published in detail elsewhere and is only described here in short.26 The HS2 anion was generated via pulsed UV laser-induced electron attachment method.27 Herein, metal
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target (silver or copper) was ablated in the presence of a supersonic beam of helium (99.999%) carrier gas with 0.2% methanethiol. The formed anions were guided to a McLarenWiley time-of-flight, mass selected, and crossed with a laser beam (Nd:YAG laser and dye laser). The resulting photoelectrons were extracted by a velocity map imaging photoelectron spectrometer and recorded by a charge-coupled device camera. Each image was accumulated with 10 000–50 000 laser shots at 10 Hz repetition rate. The final raw image stood for the projection of the photoelectron density in the 3D laboratory frame onto the 2D imaging detector. The original 3D distribution was reconstructed using the Basis Set Expansion (BASEX) inverse Abel transform method, and the photoelectron spectrum was acquired.28 The photoelectron kinetic energy spectra at near-threshold photon energies were calibrated by the known spectrum of S– . The typical energy resolution was about 3 meV full width at half maximum (FWHM) at low electron kinetic energy (eKE). HS2 –1/0 were optimized at the coupled cluster method including triple excitations CCSD (T) level29 and the double-hybrid density functional theory (mPW2PLYP)30 with long-range dispersion corrections for comparison, and the augmented correlation-consistent polarized valence triple-ζ basis set aug-cc-pVTZ31, 32 was used for all atom. The excited state was obtained by time-dependent DFT (TD-DFT) using a hybrid functional, B3LYP method with the same basis sets.33, 34 Vibrational analysis was employed to check it out whether the optimized structures were the true local minima or not and additionally to get the theoretical frequencies. The excitation energy was obtained by EOM-CCSD35, 36 calculations with the aug-cc-PVTZ basis sets. The adiabatic detachment energy (ADE) was defined as the energy of the origin transition between the ground state of the anion and the ground state of the neutral, which also represents the electron affinity of neutral species. The vertical detachment energy (VDE) was defined as the energy difference between the ground state of the anion and the ground state of the neutral at the anion geometry. All theoretical calculations were performed using the Gaussian09 package.37
III. RESULTS AND DISCUSSION A. Photoelectron spectra
Figs. 1(a)–1(c) display the photoelectron imaging results of HS2 – at 355, 532, and 611.5 nm, respectively. Entfellner and Boesl have already reported the photoelectron spectra of HS2 – at 448 and 305 nm via a conventional negative ion TOF photoelectron spectrometer.18 Their photoelectron spectra exhibit two spectral bands, which represent the ground state and first excited state of HS2 . The current 355 nm photoelectron imaging spectrum (Fig. 1(a)) also allows the ground-state as well as the first excited state transitions to be accessed. One set of peaks with a spacing of 589 ± 12 cm−1 dominates the ground state of HS2 , which corresponds to the S−S stretching mode. Furthermore, an S-S-H bending motion with a frequency of 741 ± 41 cm−1 and a weak S-H stretching motion with a frequency of 2585 ± 81 cm−1 are observed upon photodetachment into the first excited state of HS2 . For the
J. Chem. Phys. 141, 204312 (2014)
(a) 355 nm
k X
β = -0.69
1.0
1.5
2.0
2.5
(b) 532 nm
1.4
1.6
a
1.8
c e gh j i
a' b'
1.6
2.0
2.2
a β = 0.02
(c) 611.5 nm
1.4
3.5
f
-
a' b' c'
Cu-
3.0
d β = -0.11
b
S2
A β = -0.44
m ln op
c'
1.8
b c
2.0
2.2
Electron binding energy (eV)
FIG. 1. Photoelectron velocity-images (left column) and photoelectron spectra (right column) of HS2 – at (a) 355 nm (3.496 eV), (b) 532 nm (2.330 eV), and (c) 611.5 nm (2.028 eV), where X stands for the ground state of HS2 and A stands for the first excited state of HS2 . Each photoelectron velocity-image consists of raw image (left part) and the reconstructed image (right part) after inverse Abel transformation. The double arrow indicates the direction of the laser polarization. The blue dots stand for the photoelectron spectrum for individual S2 – .
ground state and first excited state, both of them exhibit a very weak feature lying in-between main vibrational peaks. The determined values are in good agreement with the previous results.1, 18 At lower photon energy, the higher resolution is achieved for the ground state of HS2 . As shown in Figs. 1(b) and 1(c), the dominated peaks were then wellresolved along with weakly populated vibrational peaks. In addition, the peaks (a, b, and c) with a space of 720 ± 24 cm−1 below the title peaks correspond to the S-S stretching frequency of S2 , which are consistent with the prior experimental observations.1, 18 And the fact is also supported by comparison with the photoelectron spectrum for individual S2 – at 532 nm in Fig. 1(b). Those weakly populated vibrational peaks will be analyzed and identified in Sec. III B, illustrating additional vibrational modes excited in the photodetachment. B. Spectral assignment
To elucidate the structural and electronic properties of neutral and anionic HS2 , we have performed ab initio calculations. The electronic states and geometries are summarized in Table I, along with the literature values of HS2 (X 2 A ). Due to the Cs symmetry, three vibrational modes should be activated upon photodetachment of HS2 – , corresponding to SH stretching, S-S-H bending, and S-S stretching modes. The calculated vibrational frequencies of HS2 – are listed in Table SI at the mPW2PLYP and CCSD(T) level.38 And the vibrational frequencies of the ground state and the first excited state of HS2 are given in Table II along with experimental values obtained here. Franck−Condon (FC) simulations were carried out to assist in the spectral assignment using PESCAL programs,39 which used the Rosenstock-Chen method to calculate FranckCondon factors with harmonic oscillator approximation including Duschinsky rotation. The individual vibrational peak
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TABLE I. The calculated and experimentally observed structures of HS2 in the ground (2 A ) and first excited state (2 A ) and of HS2 – in its ground state (1 A ) (bond lengths are given in Å and angles are given in degrees).
(a)
Parameter State HS2 – (X 1 A ) HS2 (X 2 A )
HS2 (A 2 A ) a
Method
rS-S
rS-H
θ H-S-S
mPW2PLYP/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ mPW2PLYP/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ Expt.a TD-B3LYP/aug-cc-pVTZ
2.107 2.120 1.981 1.993 1.9603 2.102
1.347 1.350 1.350 1.352 1.3523 1.349
101.61 101.03 101.45 100.91 101.74 94.09
1.6
1.8
2.0
2.2
2.4
(b)
From Ref. 45.
2.6
contours were approximated as a Gaussian function with 20 and 30 meV FWHM (full width at half-maximum) for the ground state in the 532 nm and the first excited state in the 355 nm spectra, respectively (Fig. 2). The simulated vibrational temperature of 180 K was obtained for the anion. As shown in Figs. 2(a) and 2(b), the simulations fit the spectral bands very well except for the intensity close to the detachment threshold (at 532 nm) due to Wigner threshold effect.40 Further FC simulation of the ground state in the 355 nm photoelectron spectrum of HS2 was also carried out and indeed excellently agrees with the spectral profile as shown in Figure S1 in the supplementary material.38 Upon photodetachment into the ground state of HS2 at 532 and 611.5 nm, the peak assignments are unambiguous and yield frequencies for the S-S stretching (589 ± 12 cm−1 ) and SS-H bending modes (895 ± 24 cm−1 ). For the first excited state of HS2 , all three vibrational frequencies are resolved and determined as 490 ± 41 cm−1 (S-S stretching motion), 741 ± 41 cm−1 (S-S-H bending motion), and 2585 ± 81 cm−1 (S-H stretching motion). The values measured in the ground state of HS2 are in good agreement with the values obtained by microwave spectroscopy,15 matrix-isolation infrared spectroscopy,16 Fourier-transform chemiluminescence spectroscopy,17 and photoelectron spectroscopy,18 respec-
2.8 3.0 3.2 Electron binding energy (eV)
3.4
FIG. 2. A comparison between the photoelectron spectra and FranckCondon simulations for (a) the ground-state at 532 nm and (b) first excited state detachment transition at 355 nm for HS2 – using PESCAL. The black dots represent the experimental data and the blue line is the Franck-Condon simulated spectrum. The vertical sticks represent the calculated FranckCondon factors.
tively (Table II). For the first excited state of HS2 , the vibrational frequencies in current work accord with the previous reported results17, 18 except for the S-S-H bending frequency (808 ± 10 cm−1 ) determined by Fourier-transform chemiluminescence spectroscopy.17 As shown in Fig. 1(c), significant high resolution is achieved near the threshold at 611.5 nm and the peak (a) indicated by the arrow defines the 0-0 transition. Thus, the more accurate electron affinity (EA) of HS2 is determined as 1.9080 ± 0.0018 eV from the 0-0 transition as listed in Table III. In addition, the experimental VDE of current work and term energy T1 (Table III) are also nicely in line with theoretical calculations. Generally, vibrational modes showing up in the photoelectron spectra involving transitions from anion to neutral are rooted in relatively large change in the equilibrium structure between the initial anion and corresponding neutral species
TABLE II. Calculated vibrational frequencies (unscaled) of HS2 compared to experimental observations. (cm−1 ). X (2 A ) Method mPW2PLYP/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ TD-B3LYP/aug-cc-pVTZ Expt.
A (2 A )
S-S stretch. S-S-H bend. S-H stretch. S-S stretch. S-S-H bend. S-H stretch. 597 584
918 904
589(12)a 600(120)b 596.3c 595d
895(24)a 892(10)c 904d 903e
2623 2605 495 490(41)a 504.5c
763 741(41)a 750(200)b 808(10)c
2647 2585(81)a 2550(200)b
2463e
610(80)f a
Current work. From Ref. 18. c From Ref. 17. d From Refs. 15. e From Ref. 16. f From Ref. 1. b
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TABLE III. Comparison of calculated ADEs, VDEs and term energies of HS2 with experimental observations. HS2 EA (eV)
VDE (eV)
T1 d (eV) T0 f (eV)
Method
X 2 A
mPW2PLYP/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ Exp.
1.836 1.880 1.9080(18)a 1.916(15)b 1.915(22)c 1.907(23)c 1.980 2.019 2.0572(41)a
mPW2PLYP/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ Expt. EOM-CCSD/ aug-cc-pVTZe Expt. Expt.
A 2 A
0.760 0.754(10)a 0.903(10)a 0.899(25)g 0.899h
the S-S bond to nearly unchanged. Accordingly, in the photoelectron spectra of ground state HS2 , the S–S stretching motion has been investigated due to the significant shortening of the S–S bond going from anion to neutral in line with the theoretical prediction. A slightly more pronounced S-S-H bending motion has also been revealed with weak features in the experiment owning to the slight decrease of S-S-H angle (θ ∼ 0.12◦ at the CCSD(T) level). In the case of the first excited state, we observed the dominant S–S–H bending motion instead of S–S stretching motion because major structural changes between anionic and excited neutral geometry are induced by S-S-H angle decreasing. The excitations in the S–H and S–S stretching motions are significantly less pronounced and bear out relatively featureless features in the photoelectron spectrum owning to the less significant geometric changes.
a
Current work. From Ref. 18. From Ref. 1. d T1 = VDE(HS2 A 2 A ) – VDE(HS2 X 2 A ). e The geometry of HS2 – is used at the CCSD(T)/aug-cc-pVTZ level. f T0 = EA(HS2 A 2 A ) – EA(HS2 X 2 A ) g From Ref. 18. h From Ref. 17. b
C. PADs
c
along the normal coordinate. Geometry discrepancy between the ground state of HS2 – and the ground state and the first excited state of HS2 can be interpreted using the MO scheme in Fig. 3. As listed in Table I, the S-S bond distance, S-H bond distance, and HSS angle for the ground state of HS2 at the mPW2PLYP/aug-cc-pVTZ level are better consistent with the prior results calculated at the CCSD(T) level (CBS limit).22 For the first excited state of HS2 , the geometric parameters at the TD-B3LYP/aug-cc-pVTZ level get closer to the reported results using B3LYP method in contrast to that using the CCSD(T) method.22 For an electron detached from HOMO with respect to the X 2 A state of HS2 , the antibonding interaction of S-S bond in the HOMO will lead to the decrease of S-S bond distance. The prediction is in line with the calculated reduction by 0.13 Å (in both mPW2PLYP and CCSD(T) method) detached from the ground state of HS2 – to form the ground state of HS2 . On the other hand, the HOMO −1 of HS2 – is mainly composed of atomic S p orbital and will diminish sharing of electron density between the two sulfur atoms. Removal of an electron from the HOMO −1, which results in the A 2 A excited state of HS2 , would cause
The PADs (yielding anisotropy parameter, β, ranging from −1 to 2) from photoelectron imaging can also reveal qualitative insight into the photodetachment process about the parent MO.41 The valence MOs of HS2 – are presented in Fig. 3, suggesting that the main component of HOMO and HOMO−1 orbitals possesses electron density relevant to the out-of-plane and in-plane S atom p-orbitals, respectively. The HOMO of HS2 – has two nodal planes, which resembles an atomic d orbital (initial orbital angular momentum quantum number of the electron: l = 2). Hence, as shown in Fig. 1, the trend of the anisotropy parameter (β = 0.02 in Fig. 1(c) to β = −0.69 in Fig. 1(a) for the ground state) formed by the detachment of HOMO appears to be consistent with the CooperZare center-potential model for the d orbital transition.42 In the case of HOMO-1 of HS2 – , it is mainly composed of S atomic p orbital. For electron detached from the p-type parent orbital, by reason of conservation of angular momentum, the outgoing electron waves must have either s or d character. Thus, the s-wave (l = 0) dominates near threshold as a result of the threshold law,40, 43 giving rise to more isotropic angular distributions. With increasing kinetic energies, the d-wave contributes more and interference between the s- and d-wave results in the perpendicular angular distributions (β = −0.44 in Fig. 1(a) for the first excited state of HS2 ). D. Thermochemical quantities
The electron affinity of HS2 can be utilized to access its application to the relevant thermochemical quantities. The dissociation energy (D0 ) of the anion can be obtained by utilizing thermodynamic cycle with EA(S2 ) and EA(HS2 ), D0 (S− 2 -H) = D0 (S2 -H) − EA(S2 ) + EA(HS2 ).
FIG. 3. The contour plots of the occupied valence orbitals of the ground state of HS2 – calculated at the mPW2PLYP /aug-cc-pVTZ level (iso = 0.04 a.u.).
(1)
Utilizing D0 (S2 -H) = 60.5 ± 1.5 kcal mol−1 ,44 EA (S2 ) = 38.61 ± 0.08 kcal mol−1 ,27 and EA (HS2 ) = 44.00 ± 0.08 kcal mol−1 measured in this work, D0 (S2 – -H) is determined to be 65.89 ± 1.5 kcal mol−1 . It is noted that bond energy varies little in accordance with the nonbonding character of the added electron.
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The gas-phase acidity scale of H2 S2 can be evaluated through the following thermochemical cycle: acid H0 (HS2 -H) = BDE(HS2 -H) + IE(H) − EA(HS2 ). (2) Here, acid H0 (HS2 -H) is the S-H deprotonation enthalpy of HS2 -H at 0 K, which is equivalent to the gas-phase acidity at 0 K, EA(HS2 ) is the electron affinity of the HS2 radical, and IE(H) is the ionization energy of the H atom (313.592 kcal mol−1 ). The BDE of HS2 -H is evaluated from previous study to be about 70 ± 1.5 kcal mol−1 .44 The resulting BDE of HS2 H yields a deprotonation enthalpy for the HSSH acid between 338.1 and 341.1 kcal mol−1 (i.e., acid H0 (HSS-H) = 339.6 ± 1.5 kcal mol−1 ). IV. CONCLUSIONS
The electronic structure of HS2 has been reinvestigated using low-energy photoelectron imaging and ab initio calculations. In agreement with previous photoelectron spectra, the dominated S-S stretching mode for the ground state and S-S-H bending and S-H stretching modes for the first excited state of HS2 were obtained. With the help of theoretical calculations and the Frank-Condon simulations, vibrational frequencies of 895 ± 24 cm−1 for the ground state, 490 ± 41 cm−1 for the excited state of HS2 are also firstly observed and assigned to the S-S-H bending and S-S stretching modes, respectively. In this work, more accurate electron affinity of HS2 was deduced from low-energy photoelectron spectrum as 1.9080 ± 0.0018 eV. Obtained electron affinity allows for the corresponding gas acidity determination of H2 S2 , acid H0 (HS2 -H) = 339.6 ± 1.5 kcal mol−1 . We expect this initial application to provide further insight into the nature of fine vibrational structures with extremely weak vibrational excitation in the photodetachment for many small molecules. ACKNOWLEDGMENTS
This work is supported by the National Natural Science Foundation of China (Grant Nos. 21073186, 21273233, and 21103186), the Ministry of Science and Technology of China (Grant No. 2011YQ09000505), and the Chinese Academy of Sciences. 1 S.
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THE JOURNAL OF CHEMICAL PHYSICS 141, 204312 (2014)
Low-energy photoelectron imaging of HS2 anion Zhengbo Qin,1,2 Ran Cong,2 Zhiling Liu,1 Hua Xie,1 and Zichao Tang1,a) 1
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China 2 Department of Physics, Anhui Normal University, Wuhu, Anhui 241000, China
(Received 2 July 2014; accepted 3 November 2014; published online 26 November 2014) Low-energy photoelectron imaging of HS2 – has been investigated, which provides the vibrational frequencies of the ground state as well as the first excited state of HS2 . It allows us to determine more accurate electron affinity of HS2 , 1.9080 ± 0.0018 eV. Combined with Frank-Condon simulation, the vibrational features have been unveiled related to S-S stretching and S-S-H bending modes for the ground state and S-S stretching, S-S-H bending, and S-H stretching modes for the first excited state. Photoelectron angular distributions are mainly characteristic of electron detachment from two different molecular orbitals (MOs) in HS2 – . With the aid of accurate electron affinity value of HS2 , corresponding thermochemical quantities can be accessed. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4901978] I. INTRODUCTION
There is a growing interest in sulfur-containing compounds on account of their important role in air pollution and acid rain.1–7 Among various sulfur-containing molecules, HS2 radical is one of the most fundamental species responsible for the reaction in combustion, the oxidation of reduced forms of sulfur8–11 and biochemistry.12–14 Extensive studies on the HS2 have been reported for decades. Yamamoto and Saito provided the first experimental information about the ground-state structure of HS2 with a millimeterwave spectroscopy.15 In a recent experiment, the matrixisolation infrared spectra of the HS2 radical following 266 nm photolysis of H2 S2 revealed the S-H stretching and the S-S-H bending modes which were observed to be 2463 and 903 cm−1 , respectively.16 Quite recently, Ashworth and Fink have investigated the chemiluminescence spectrum of the HS2 radical with a high-resolution Fourier-transform spectrometer, and the vibrational parameters have been obtained ranging from 4000 to 9000 cm−1 .17 In addition, photoelectron spectroscopy studies involving transitions from anion to neutral on HS2 have been conducted. Early photoelectron spectrum of HS2 – was reported where S-S stretching mode for the ground state of HS2 has only been extrapolated from low-resolution studies, which was contaminated by S2 − .1 In a recent photoelectron spectroscopic experiment, Entfellner and Boesl have reported an experimental photoelectron study of the HS2 – in which the HS2 was observed in the ground and first excited states with a conventional time of flight photoelectron spectrometry.18 With the aid of an earlier photoelectron spectroscopic investigation and the previous theoretical studies of Peterson et al.,19 the S–S stretching motion for photodetachment into the ground state of HS2 and S–S–H bending motion along with a weak excitation of S–H stretching motion for the photodetachment into the first excited state of HS2 have been a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]. Tel.: +86-411-84379365. Fax: +86-411-84675584. 0021-9606/2014/141(20)/204312/5/$30.00
assigned. On the other hand, considerable theoretical work has focused on HS2 in recent years.19–25 Especially, Owens et al.,21 Denis22 and Peterson et al.19 reported the most accurate results relevant to the geometries and frequencies of the ground and first excited state compared to the previous experimental findings. Moreover, theoretical studies indicated additional weaker and as of yet unresolved vibrational features in the photoelectron spectra of HS2 – .24 In view of importance of its role in combustion and atmosphere, the accurate EA as well as vibrational information would give accurate geometric structure and thermochemistry information. Although comprehensive experimental and theoretical investigations have been conducted on HS2 , there is no thorough investigation into the ground state and the first excited state of HS2 using above-mentioned methods except for conventional photoelectron spectra. Hence, photoelectron imaging with the capability of high sensitivity and high resolution at near-threshold photon energies will help us to resolve such fine vibrational structures with extremely weak excitation in the photodetachment. As far as we know, lowenergy photoelectron imaging of HS2 – is the first such experimental probe into the nature of the neutral and anionic ground states as well as neutral excited state within the vibrational motions manifold. Herein, we present photoelectron spectroscopic and theoretical investigation of HS2 via lowenergy photoelectron imaging. In addition, we compare theoretical MOs (HOMO and HOMO−1) with the photoelectron imaging of HS2 – and probe MO wavefunction nature from the photoelectron anisotropy parameter, β. Furthermore, FranckCondon (FC) simulation of the vibrational structures is performed to interpret the experimental spectra. II. EXPERIMENTAL AND COMPUTATIONAL METHODS
The photoelectron velocity-map imaging system has been published in detail elsewhere and is only described here in short.26 The HS2 anion was generated via pulsed UV laser-induced electron attachment method.27 Herein, metal
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target (silver or copper) was ablated in the presence of a supersonic beam of helium (99.999%) carrier gas with 0.2% methanethiol. The formed anions were guided to a McLarenWiley time-of-flight, mass selected, and crossed with a laser beam (Nd:YAG laser and dye laser). The resulting photoelectrons were extracted by a velocity map imaging photoelectron spectrometer and recorded by a charge-coupled device camera. Each image was accumulated with 10 000–50 000 laser shots at 10 Hz repetition rate. The final raw image stood for the projection of the photoelectron density in the 3D laboratory frame onto the 2D imaging detector. The original 3D distribution was reconstructed using the Basis Set Expansion (BASEX) inverse Abel transform method, and the photoelectron spectrum was acquired.28 The photoelectron kinetic energy spectra at near-threshold photon energies were calibrated by the known spectrum of S– . The typical energy resolution was about 3 meV full width at half maximum (FWHM) at low electron kinetic energy (eKE). HS2 –1/0 were optimized at the coupled cluster method including triple excitations CCSD (T) level29 and the double-hybrid density functional theory (mPW2PLYP)30 with long-range dispersion corrections for comparison, and the augmented correlation-consistent polarized valence triple-ζ basis set aug-cc-pVTZ31, 32 was used for all atom. The excited state was obtained by time-dependent DFT (TD-DFT) using a hybrid functional, B3LYP method with the same basis sets.33, 34 Vibrational analysis was employed to check it out whether the optimized structures were the true local minima or not and additionally to get the theoretical frequencies. The excitation energy was obtained by EOM-CCSD35, 36 calculations with the aug-cc-PVTZ basis sets. The adiabatic detachment energy (ADE) was defined as the energy of the origin transition between the ground state of the anion and the ground state of the neutral, which also represents the electron affinity of neutral species. The vertical detachment energy (VDE) was defined as the energy difference between the ground state of the anion and the ground state of the neutral at the anion geometry. All theoretical calculations were performed using the Gaussian09 package.37
III. RESULTS AND DISCUSSION A. Photoelectron spectra
Figs. 1(a)–1(c) display the photoelectron imaging results of HS2 – at 355, 532, and 611.5 nm, respectively. Entfellner and Boesl have already reported the photoelectron spectra of HS2 – at 448 and 305 nm via a conventional negative ion TOF photoelectron spectrometer.18 Their photoelectron spectra exhibit two spectral bands, which represent the ground state and first excited state of HS2 . The current 355 nm photoelectron imaging spectrum (Fig. 1(a)) also allows the ground-state as well as the first excited state transitions to be accessed. One set of peaks with a spacing of 589 ± 12 cm−1 dominates the ground state of HS2 , which corresponds to the S−S stretching mode. Furthermore, an S-S-H bending motion with a frequency of 741 ± 41 cm−1 and a weak S-H stretching motion with a frequency of 2585 ± 81 cm−1 are observed upon photodetachment into the first excited state of HS2 . For the
J. Chem. Phys. 141, 204312 (2014)
(a) 355 nm
k X
β = -0.69
1.0
1.5
2.0
2.5
(b) 532 nm
1.4
1.6
a
1.8
c e gh j i
a' b'
1.6
2.0
2.2
a β = 0.02
(c) 611.5 nm
1.4
3.5
f
-
a' b' c'
Cu-
3.0
d β = -0.11
b
S2
A β = -0.44
m ln op
c'
1.8
b c
2.0
2.2
Electron binding energy (eV)
FIG. 1. Photoelectron velocity-images (left column) and photoelectron spectra (right column) of HS2 – at (a) 355 nm (3.496 eV), (b) 532 nm (2.330 eV), and (c) 611.5 nm (2.028 eV), where X stands for the ground state of HS2 and A stands for the first excited state of HS2 . Each photoelectron velocity-image consists of raw image (left part) and the reconstructed image (right part) after inverse Abel transformation. The double arrow indicates the direction of the laser polarization. The blue dots stand for the photoelectron spectrum for individual S2 – .
ground state and first excited state, both of them exhibit a very weak feature lying in-between main vibrational peaks. The determined values are in good agreement with the previous results.1, 18 At lower photon energy, the higher resolution is achieved for the ground state of HS2 . As shown in Figs. 1(b) and 1(c), the dominated peaks were then wellresolved along with weakly populated vibrational peaks. In addition, the peaks (a, b, and c) with a space of 720 ± 24 cm−1 below the title peaks correspond to the S-S stretching frequency of S2 , which are consistent with the prior experimental observations.1, 18 And the fact is also supported by comparison with the photoelectron spectrum for individual S2 – at 532 nm in Fig. 1(b). Those weakly populated vibrational peaks will be analyzed and identified in Sec. III B, illustrating additional vibrational modes excited in the photodetachment. B. Spectral assignment
To elucidate the structural and electronic properties of neutral and anionic HS2 , we have performed ab initio calculations. The electronic states and geometries are summarized in Table I, along with the literature values of HS2 (X 2 A ). Due to the Cs symmetry, three vibrational modes should be activated upon photodetachment of HS2 – , corresponding to SH stretching, S-S-H bending, and S-S stretching modes. The calculated vibrational frequencies of HS2 – are listed in Table SI at the mPW2PLYP and CCSD(T) level.38 And the vibrational frequencies of the ground state and the first excited state of HS2 are given in Table II along with experimental values obtained here. Franck−Condon (FC) simulations were carried out to assist in the spectral assignment using PESCAL programs,39 which used the Rosenstock-Chen method to calculate FranckCondon factors with harmonic oscillator approximation including Duschinsky rotation. The individual vibrational peak
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TABLE I. The calculated and experimentally observed structures of HS2 in the ground (2 A ) and first excited state (2 A ) and of HS2 – in its ground state (1 A ) (bond lengths are given in Å and angles are given in degrees).
(a)
Parameter State HS2 – (X 1 A ) HS2 (X 2 A )
HS2 (A 2 A ) a
Method
rS-S
rS-H
θ H-S-S
mPW2PLYP/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ mPW2PLYP/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ Expt.a TD-B3LYP/aug-cc-pVTZ
2.107 2.120 1.981 1.993 1.9603 2.102
1.347 1.350 1.350 1.352 1.3523 1.349
101.61 101.03 101.45 100.91 101.74 94.09
1.6
1.8
2.0
2.2
2.4
(b)
From Ref. 45.
2.6
contours were approximated as a Gaussian function with 20 and 30 meV FWHM (full width at half-maximum) for the ground state in the 532 nm and the first excited state in the 355 nm spectra, respectively (Fig. 2). The simulated vibrational temperature of 180 K was obtained for the anion. As shown in Figs. 2(a) and 2(b), the simulations fit the spectral bands very well except for the intensity close to the detachment threshold (at 532 nm) due to Wigner threshold effect.40 Further FC simulation of the ground state in the 355 nm photoelectron spectrum of HS2 was also carried out and indeed excellently agrees with the spectral profile as shown in Figure S1 in the supplementary material.38 Upon photodetachment into the ground state of HS2 at 532 and 611.5 nm, the peak assignments are unambiguous and yield frequencies for the S-S stretching (589 ± 12 cm−1 ) and SS-H bending modes (895 ± 24 cm−1 ). For the first excited state of HS2 , all three vibrational frequencies are resolved and determined as 490 ± 41 cm−1 (S-S stretching motion), 741 ± 41 cm−1 (S-S-H bending motion), and 2585 ± 81 cm−1 (S-H stretching motion). The values measured in the ground state of HS2 are in good agreement with the values obtained by microwave spectroscopy,15 matrix-isolation infrared spectroscopy,16 Fourier-transform chemiluminescence spectroscopy,17 and photoelectron spectroscopy,18 respec-
2.8 3.0 3.2 Electron binding energy (eV)
3.4
FIG. 2. A comparison between the photoelectron spectra and FranckCondon simulations for (a) the ground-state at 532 nm and (b) first excited state detachment transition at 355 nm for HS2 – using PESCAL. The black dots represent the experimental data and the blue line is the Franck-Condon simulated spectrum. The vertical sticks represent the calculated FranckCondon factors.
tively (Table II). For the first excited state of HS2 , the vibrational frequencies in current work accord with the previous reported results17, 18 except for the S-S-H bending frequency (808 ± 10 cm−1 ) determined by Fourier-transform chemiluminescence spectroscopy.17 As shown in Fig. 1(c), significant high resolution is achieved near the threshold at 611.5 nm and the peak (a) indicated by the arrow defines the 0-0 transition. Thus, the more accurate electron affinity (EA) of HS2 is determined as 1.9080 ± 0.0018 eV from the 0-0 transition as listed in Table III. In addition, the experimental VDE of current work and term energy T1 (Table III) are also nicely in line with theoretical calculations. Generally, vibrational modes showing up in the photoelectron spectra involving transitions from anion to neutral are rooted in relatively large change in the equilibrium structure between the initial anion and corresponding neutral species
TABLE II. Calculated vibrational frequencies (unscaled) of HS2 compared to experimental observations. (cm−1 ). X (2 A ) Method mPW2PLYP/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ TD-B3LYP/aug-cc-pVTZ Expt.
A (2 A )
S-S stretch. S-S-H bend. S-H stretch. S-S stretch. S-S-H bend. S-H stretch. 597 584
918 904
589(12)a 600(120)b 596.3c 595d
895(24)a 892(10)c 904d 903e
2623 2605 495 490(41)a 504.5c
763 741(41)a 750(200)b 808(10)c
2647 2585(81)a 2550(200)b
2463e
610(80)f a
Current work. From Ref. 18. c From Ref. 17. d From Refs. 15. e From Ref. 16. f From Ref. 1. b
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TABLE III. Comparison of calculated ADEs, VDEs and term energies of HS2 with experimental observations. HS2 EA (eV)
VDE (eV)
T1 d (eV) T0 f (eV)
Method
X 2 A
mPW2PLYP/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ Exp.
1.836 1.880 1.9080(18)a 1.916(15)b 1.915(22)c 1.907(23)c 1.980 2.019 2.0572(41)a
mPW2PLYP/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ Expt. EOM-CCSD/ aug-cc-pVTZe Expt. Expt.
A 2 A
0.760 0.754(10)a 0.903(10)a 0.899(25)g 0.899h
the S-S bond to nearly unchanged. Accordingly, in the photoelectron spectra of ground state HS2 , the S–S stretching motion has been investigated due to the significant shortening of the S–S bond going from anion to neutral in line with the theoretical prediction. A slightly more pronounced S-S-H bending motion has also been revealed with weak features in the experiment owning to the slight decrease of S-S-H angle (θ ∼ 0.12◦ at the CCSD(T) level). In the case of the first excited state, we observed the dominant S–S–H bending motion instead of S–S stretching motion because major structural changes between anionic and excited neutral geometry are induced by S-S-H angle decreasing. The excitations in the S–H and S–S stretching motions are significantly less pronounced and bear out relatively featureless features in the photoelectron spectrum owning to the less significant geometric changes.
a
Current work. From Ref. 18. From Ref. 1. d T1 = VDE(HS2 A 2 A ) – VDE(HS2 X 2 A ). e The geometry of HS2 – is used at the CCSD(T)/aug-cc-pVTZ level. f T0 = EA(HS2 A 2 A ) – EA(HS2 X 2 A ) g From Ref. 18. h From Ref. 17. b
C. PADs
c
along the normal coordinate. Geometry discrepancy between the ground state of HS2 – and the ground state and the first excited state of HS2 can be interpreted using the MO scheme in Fig. 3. As listed in Table I, the S-S bond distance, S-H bond distance, and HSS angle for the ground state of HS2 at the mPW2PLYP/aug-cc-pVTZ level are better consistent with the prior results calculated at the CCSD(T) level (CBS limit).22 For the first excited state of HS2 , the geometric parameters at the TD-B3LYP/aug-cc-pVTZ level get closer to the reported results using B3LYP method in contrast to that using the CCSD(T) method.22 For an electron detached from HOMO with respect to the X 2 A state of HS2 , the antibonding interaction of S-S bond in the HOMO will lead to the decrease of S-S bond distance. The prediction is in line with the calculated reduction by 0.13 Å (in both mPW2PLYP and CCSD(T) method) detached from the ground state of HS2 – to form the ground state of HS2 . On the other hand, the HOMO −1 of HS2 – is mainly composed of atomic S p orbital and will diminish sharing of electron density between the two sulfur atoms. Removal of an electron from the HOMO −1, which results in the A 2 A excited state of HS2 , would cause
The PADs (yielding anisotropy parameter, β, ranging from −1 to 2) from photoelectron imaging can also reveal qualitative insight into the photodetachment process about the parent MO.41 The valence MOs of HS2 – are presented in Fig. 3, suggesting that the main component of HOMO and HOMO−1 orbitals possesses electron density relevant to the out-of-plane and in-plane S atom p-orbitals, respectively. The HOMO of HS2 – has two nodal planes, which resembles an atomic d orbital (initial orbital angular momentum quantum number of the electron: l = 2). Hence, as shown in Fig. 1, the trend of the anisotropy parameter (β = 0.02 in Fig. 1(c) to β = −0.69 in Fig. 1(a) for the ground state) formed by the detachment of HOMO appears to be consistent with the CooperZare center-potential model for the d orbital transition.42 In the case of HOMO-1 of HS2 – , it is mainly composed of S atomic p orbital. For electron detached from the p-type parent orbital, by reason of conservation of angular momentum, the outgoing electron waves must have either s or d character. Thus, the s-wave (l = 0) dominates near threshold as a result of the threshold law,40, 43 giving rise to more isotropic angular distributions. With increasing kinetic energies, the d-wave contributes more and interference between the s- and d-wave results in the perpendicular angular distributions (β = −0.44 in Fig. 1(a) for the first excited state of HS2 ). D. Thermochemical quantities
The electron affinity of HS2 can be utilized to access its application to the relevant thermochemical quantities. The dissociation energy (D0 ) of the anion can be obtained by utilizing thermodynamic cycle with EA(S2 ) and EA(HS2 ), D0 (S− 2 -H) = D0 (S2 -H) − EA(S2 ) + EA(HS2 ).
FIG. 3. The contour plots of the occupied valence orbitals of the ground state of HS2 – calculated at the mPW2PLYP /aug-cc-pVTZ level (iso = 0.04 a.u.).
(1)
Utilizing D0 (S2 -H) = 60.5 ± 1.5 kcal mol−1 ,44 EA (S2 ) = 38.61 ± 0.08 kcal mol−1 ,27 and EA (HS2 ) = 44.00 ± 0.08 kcal mol−1 measured in this work, D0 (S2 – -H) is determined to be 65.89 ± 1.5 kcal mol−1 . It is noted that bond energy varies little in accordance with the nonbonding character of the added electron.
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The gas-phase acidity scale of H2 S2 can be evaluated through the following thermochemical cycle: acid H0 (HS2 -H) = BDE(HS2 -H) + IE(H) − EA(HS2 ). (2) Here, acid H0 (HS2 -H) is the S-H deprotonation enthalpy of HS2 -H at 0 K, which is equivalent to the gas-phase acidity at 0 K, EA(HS2 ) is the electron affinity of the HS2 radical, and IE(H) is the ionization energy of the H atom (313.592 kcal mol−1 ). The BDE of HS2 -H is evaluated from previous study to be about 70 ± 1.5 kcal mol−1 .44 The resulting BDE of HS2 H yields a deprotonation enthalpy for the HSSH acid between 338.1 and 341.1 kcal mol−1 (i.e., acid H0 (HSS-H) = 339.6 ± 1.5 kcal mol−1 ). IV. CONCLUSIONS
The electronic structure of HS2 has been reinvestigated using low-energy photoelectron imaging and ab initio calculations. In agreement with previous photoelectron spectra, the dominated S-S stretching mode for the ground state and S-S-H bending and S-H stretching modes for the first excited state of HS2 were obtained. With the help of theoretical calculations and the Frank-Condon simulations, vibrational frequencies of 895 ± 24 cm−1 for the ground state, 490 ± 41 cm−1 for the excited state of HS2 are also firstly observed and assigned to the S-S-H bending and S-S stretching modes, respectively. In this work, more accurate electron affinity of HS2 was deduced from low-energy photoelectron spectrum as 1.9080 ± 0.0018 eV. Obtained electron affinity allows for the corresponding gas acidity determination of H2 S2 , acid H0 (HS2 -H) = 339.6 ± 1.5 kcal mol−1 . We expect this initial application to provide further insight into the nature of fine vibrational structures with extremely weak vibrational excitation in the photodetachment for many small molecules. ACKNOWLEDGMENTS
This work is supported by the National Natural Science Foundation of China (Grant Nos. 21073186, 21273233, and 21103186), the Ministry of Science and Technology of China (Grant No. 2011YQ09000505), and the Chinese Academy of Sciences. 1 S.
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