Photoelectron imaging and photodissociation of ozonide in O3 − ⋅ (O2) n (n = 1-4) clusters Jennifer E. Mann, Mary E. Troyer, and Caroline Chick Jarrold Citation: The Journal of Chemical Physics 142, 124305 (2015); doi: 10.1063/1.4916048 View online: http://dx.doi.org/10.1063/1.4916048 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/142/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High-resolution photoelectron imaging of cold C 60 − anions and accurate determination of the electron affinity of C60 J. Chem. Phys. 140, 224315 (2014); 10.1063/1.4881421 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 platinum carbonyl anion Pt(CO) n − (n = 1-3): Experiment and theory J. Chem. Phys. 137, 204302 (2012); 10.1063/1.4768004 Vibrationally resolved photoelectron imaging of gold hydride cluster anions: AuH − and Au 2 H − J. Chem. Phys. 133, 044303 (2010); 10.1063/1.3456373

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THE JOURNAL OF CHEMICAL PHYSICS 142, 124305 (2015)

Photoelectron imaging and photodissociation of ozonide in O3−·(O2)n (n = 1-4) clusters Jennifer E. Mann, Mary E. Troyer, and Caroline Chick Jarrolda) Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 40405, USA

(Received 13 January 2015; accepted 11 March 2015; published online 25 March 2015) The photoelectron images of O3− and O3−·(O2)n (n = 1–4) have been measured using 3.49 eV photon energy. The spectra exhibit several processes, including direct photodetachment and photodissociation with photodetachment of O− photofragments. Several spectra also exhibit autodetachment of vibrationally excited O2− photofragments. Comparison of the bare O3− photoelectron spectra to that of the complexes shows that the O3− core is preserved upon clustering with several O2 molecules, though subtle changes in the Franck-Condon profile of the ground state photodetachment transition suggest some charge transfer from O3− to the O2 molecules. The electron affinities of the complexes increase by less than 0.1 eV with each additional O2 molecule, which is comparable to the corresponding binding energy [K. Hiraoka, Chem. Phys. 125, 439-444 (1988)]. The relative intensity of the photofragment O− detachment signal to the O3−·(O2)n direct detachment signal increases with cluster size. O2− autodetachment signal is only observed in the O3−, O3−·(O2)3, and O3−·(O2)4 spectra, suggesting that the energy of the dissociative state also varies with the number of O2 molecules present in the cluster. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4916048]

I. INTRODUCTION

The broad importance of oxygen in nature is perspicuous, and both neutral and charged oxygen clusters continue to challenge both experimentalists and theoreticians because of interesting properties that arise from complex electronic structure. Negatively charged O2− and O3− molecules, which play a role in atmospheric processes,1–6 have interesting interactions with neutral O2 molecules. For example, O2− − O2 interactions can result in the formation of O4−, a complicated anion that has been the subject of numerous studies, both experimental 7–12 and theoretical.13,14 Anionic oxygen clusters with an even number of O-atoms are appropriately described as O4−·(O2)n, since the binding energy of additional O2 molecules to the O4− core is relatively small. Interactions between ozonide and molecular oxygen have not been studied as thoroughly as O2− − O2 interactions. It has been shown that O2− − O3 collisions can result in O3− + O2 formation,15,16 which is energetically favored because O3 has a higher electron affinity (the EA of O3 is 2.1028 eV;17,18 the EA of O2 is 0.448 eV19). To our knowledge, the only previous spectroscopic study of O3−·(O2)n clusters was an IR predissociation spectroscopic study by Bopp et al., the results of which suggested that ozonide ion was largely unperturbed by the solvating O2 molecules.20 However, the electronic structure and photochemistry of bare O3−, which has a larger bond dissociation energy than neutral ozone [D0 = 1.705 eV for the O− + O2 X 3Σg− channel, based on D0 (O3) = 1.063 eV,21,22 EA of O = 1.461 11 eV23 and the EA of O3 noted above] have been studied extensively, both experimentally and theoretically. a)Author to whom correspondence should be addressed. Electronic mail:

[email protected] 0021-9606/2015/142(12)/124305/7/$30.00

Figure 1 summarizes the relative energies of O3 and various O3− photodissociation limits relative to the O3− ground electronic state. Photoelectron (PE) spectra of O3− have been reported by several groups,17,18,24 with spectra generally showing transitions to the ground and low-lying excited neutral states along with evidence of photodissociation (O3− + hv → O− + O2).25 Dissociative anion states have been measured26 and calculated to be close in energy to the O− + O2 dissociation limit.27,28 Dissociative attachment of low-energy electrons to O3 also accesses the higher-energy O + O2− dissociation limit.29 In an effort to determine how interactions with molecular oxygen may affect the properties of ozonide, we measured the anion photoelectron spectra of O3−·(O2)n (n = 0–4). Anion PE spectroscopy maps out neutral states relative to the anion ground state, but in the case of these small O3−·(O2)n clusters, the spectra also show how the photodissociation cross section of the O3− core is increased upon complex formation. Evidence of the O2−+ O dissociation channel is also observed, and is sensitive to cluster size.

II. EXPERIMENTAL METHODS

Figure 2 shows a schematic of the newly built photoelectron imaging (PEI) spectrometer used in these studies. The apparatus has three distinct regions for (1) ion production, (2) mass selection/anion detection, and (3) laser interaction and electron detection. O3 is generated by photolysis of O2 with a mercury lamp (UVP, 90-0012-01) in a reservoir pressurized with 60 psi of O2. The resulting gas mixture is supersonically expanded into the vacuum chamber with a pulsed (30 Hz repetition rate) solenoid-type molecular beam valve (General Valve Series 9,

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FIG. 1. Energies of the ground and excited states of O3 relative to O3− [Refs. 17 and 18], O3− Feshbach resonance [Refs. 28 and 29], and various dissociation limits for O3− [based on Refs. 17–19 and 23] and O3 [Refs. 21 and 22]. Autodetaching levels of O2− are indicated with an asterisk (*).

0.5 mm orifice). A plasma is generated when the expansion passes through an electrical discharge consisting of two needles similar to one described by Duncan.30 The needles are situated approximately 2 mm from the orifice and separated by 1 mm. The shot-to-shot stability of the discharge was improved by a continuous, low current 1 keV electron beam. Following production, ions are skimmed (2.5 mm diameter), accelerated to 1 keV, then re-referenced to ground potential with a high voltage switch.31 The ions then enter a 97-cm long time-of-flight Bakker-type mass spectrometer,32,33 through which the beam is electrostatically guided and focused with linear deflectors and an Einzel lens before passing through a 3-mm mass-defining slit located 13-cm upstream of the interaction region. At the end of the flight path, the mass separated ions impinge upon a 25-mm dual microchannel plate. A mass resolution of m/∆m = 144 was achieved in the mass region relevant for the current studies. Prior to colliding with the ion detector, anions are selectively photodetached at the intersection of the ion drift path and a velocity map imaging (VMI) lens assembly using the third harmonic output of a Nd:YAG laser, also operated at 30 Hz. The VMI assembly is based on the design of Eppink and Parker,34 and the image is recorded using a combination CCD camera and phosphor screen, originally demonstrated by Chandler and Houston.35 The projected photoelectrons are

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recorded with a CCD camera, and images are saved using NuACQ 0.9,36 developed and provided by the Suits Group at Wayne State University. The original three dimensional velocity distributions of the photoelectrons are extracted using the BASEX program.37 The resulting velocity distribution is then converted to electron kinetic energy (e−KE) by calibrating the detector to the well-known photoelectron spectrum of O2−.19 The photoelectron spectra are plotted as a function of electron binding energy (e−BE) using e−BE = hν − e−KE, which is independent of photon energy. Photoelectron images are collected over 65k to 230k laser shots, which were largely dependent on ion signal quality. The supplementary material includes a more detailed description of the newly constructed apparatus (S1a), along with PE spectra and raw and reconstructed PEIs of I− and O2− obtained using 3.49 eV photon energy (S1b) to demonstrate the performance of the apparatus on simple systems.38

III. RESULTS AND ANALYSIS A. O3− photodetachment and photodissociation energies

The reconstructed PEIs and PE spectra obtained using hν = 3.49 eV of the O3−·(O2)n , (n = 0–4) cluster ions are shown in Figure 3. The PE spectrum of O3− has been reported by several other groups,18,17,24 and is presented here to facilitate direct comparison with spectra of the larger clusters obtained on a single apparatus. Assignments discussed here are based on these previous studies. There are three distinct features in the PE spectrum, in the order of increasing electron binding energy, labeled, O (3P), X, and A. The single peak centered at 1.44 eV has been observed previously18,25 and is the result of a two-photon process, photodissociation of the parent ion, O3− + hv → O− + O2, followed by photodetachment of the atomic oxygen anion, O− + hv → O + e−. The PE spectrum was collected at several different laser fluences and the results confirmed that this peak is due to a two-photon process. Band X is broad with a vibrational progression spaced by 1100 ± 50 cm−1, consistent with the 1110 cm−1 symmetric stretch of O3.39 This band is present in the other reported spectra of O3− and is assigned to the O3( X˜ 1A1) + e− ← O3− ( X˜ 2B1) transition. Higher resolution PE spectra have shown that the bend mode is also active.17 The unpaired electron in O3− nominally occupies the lowest-energy antibonding π∗-like b1 orbital, resulting in longer O–O internuclear distances and a

FIG. 2. Diagram of the newly constructed massspectrometer/photoelectron imaging apparatus used in this study. Details on ion production, mass selection, and photoelectron imaging are given in the text and the supplementary material.38

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exhibits only the lowest energy transitions to these open-shell states. Referring to Fig. 1, only two excited states of O3 (plus their bend vibrational fundamentals) which are within 3.49 eV of the anion ground state will be present in our spectrum. However, our spectrum has several additional peaks that were not observed previously, with positions that are consistent with electron autodetachment from vibrationally excited O2− photofragments. Figure 4 shows this region of the spectrum on an expanded scale, with the position of autodetachment peaks indicated with vertical dashed lines. A plot showing slices through the PEI at different angles relative to the laser polarization is included in the supplementary material (S3).38 The plot shows that features in this energy range have isotropic angular distributions [ β (E) = 0], which is a characteristic of autodetachment signal. Fig. 1 includes the energies of the υ = 0–6 vibrational levels of the O2−2Πu state at the O2−+ O dissociation limit relative to O2 + O; the υ = 4, 5, and 6 levels of the O2−2Πu state are energetically accessible with 3.49 eV photon energy, and are unstable with respect to O2 + e−. Photodissociation of O3− into O + O2− was observed by Hiller and Vestal26 at photon energies greater than 2.41 eV, and Allan et al. observed production of both bound and vibrationally autodetaching O2− in low-energy electron impact experiments on O3.29 However, to our knowledge, autodetachment has not been observed previously in the photoelectron FIG. 3. Reconstructed PEIs and PE spectra obtained for O3− ·(O2)n (n = 0–4). Laser polarization is vertical with respect to the images. The spectrum of O− is the red trace superimposed on the spectrum of O3− ·(O2)2, and the spectrum of O2− is the blue trace superimposed on the spectrum of O3− ·(O2)4. The energies of the excited triplet neutral states of O3 are indicated with royal blue vertical lines on the O3− spectrum; the lighter blue lines associated with the O3− ·O2 spectrum show the position of the excited triplet states shifted by the same energy as band X is shifted relative to the bare O3− spectrum.

smaller bond angle relative to the 1A1 neutral. From the image shown in Fig. 3, band X exhibits an anisotropic distribution. The photoelectron angular distribution is given by40  ( ) ∂σ σt ot al 3 1 = 1 + β (E) cos2θ − , ∂Ω 4π 2 2 where σtotal is the total photodetachment cross section, and β (E) is an energy-dependent asymmetry parameter, which can be related to the symmetry of the orbital associated with the detachment. In molecular systems, the relationship is more complex than in atomic systems, but it can still be a useful analytical tool (vide infra). The value of β (E) determined for band X in the O3− spectrum, using β (E) =

I0 − I90 1 2 I0 + I90

is −0.8 ± 0.1. A plot of the integrated intensity of band X in the reconstructed image is included in the supplementary material (S2).38 Band A features a series of peaks in the range of 3.180 eV –3.49 eV. In the spectrum obtained with 4.66 eV photon energy reported by Arnold et al.,17 peaks at energies indicated by blue lines in this range and to higher binding energy were assigned to a number of low lying triplet and open-shell singlet excited states of O3. With 3.49 eV photon energy, our spectrum

FIG. 4. High e −BE region of the photoelectron spectra of O3− ·(O2)n (n = 0 − 4) indicating the position of transitions to excited triplet states of bare O3 (heavy royal blue lines), the 1 D 2 ← 2 P transition of the O− photofragment, and O2− vibrational autodetachment peaks (dashed gray lines, e −KE = 3.493 eV − e −BE).

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spectra of O3−, though past experiments either did not have sufficient photon energy to access the O + O2− photodissociation channel,25 or used electron extraction/detection methods that were less sensitive to low energy electrons.17 Peak positions are consistent with large vibrational energy relaxation of the anion, with the final state being O2 (υ = 0) + e− (hν − e−KE = 3.182, 3.303, and 3.427 eV, corresponding to autodetachment from the υ = 6, 5, and 4 levels of O2−, as indicated in Fig. 1). An additional peak at the extreme e−BE edge of the spectrum has an energy consistent with autodetachment from the υ = 5 level of the anion to the υ = 1 level of the neutral. Note that the e−KE of electrons resulting from autodetachment are independent of the photon energy, so the position of the peaks on the e−BE scale would change if the photon energy was varied. Allan et al. proposed the existence of a core excited Feshbach resonance with one of the low-lying valence states of ozone in the 1-2 eV range as a source for both bound and vibrationally autodetaching O2−,29 which Nestman et al. attributed to the 2 A2 excited state of O3−;28 the energy range of the Feshbach resonance has also been included in Fig. 1. The photon energy of 3.49 eV is within the range of the Feshbach resonance and could be the source of O2− autodetachment signal observed in the spectrum. B. O3−·(O2)n (n = 1–4) PE spectra 1. Shift in energy and Franck-Condon profile of band X

The O3−·(O2)n cluster spectra (Fig. 3) each have a single peak at lower binding energy, consistent with detachment of the O− photofragment. Broad, vibrationally resolved transitions centered between 2.3 and 2.5 eV similar in appearance to band X in the bare O3− spectrum are also observed, consistent with direct detachment of a core O3− ground state to the neutral O3 ground state. With each sequential addition of neutral O2 to the core O3−, the origin of band X shifts towards higher binding energy by less than 0.1 eV, as summarized in Table I, with the overall increase in binding energy reaching 0.26 eV with the addition of four O2 molecules. In general, anion-neutral interactions are considerably more stabilizing than neutral-neutral interaction energies, resulting in a net increase in e−BE. Typical e−BE increases range from 0.25–1 eV relative to that of the bare anion.41–44 For examples related to anionic oxygen complexes, O2−·H2O45,46 and O2−·benzene 47 have e−BE’s that are 1.12 eV and 0.61 eV higher, respectively, than the bare O2−e−BE. Again, the increase in e−BE is primarily due to enhanced stability of the anion, the implication of which is that O2 molecules are bound by ca. ≤0.1 eV to O3−. Hiraoka’s gas-phase ion equilibria measurements for O3−·(O2)48 n gave binding energies of 0.09 eV for the first several O2 molecules, which are very similar to O4−·(O2)n binding energies.49,50 By comparing Hiraoka’s O3−·(O2)n−1 − O2 energies to the shifts observed in our spectra (Table I), we infer that the neutral O3·(O2)n−1 − O2 binding energy is on the order of 0.02 eV for the first several O2 molecules. Bands X in the spectra of both O3− · O2 and O3−·(O2)2 exhibit well-resolved vibrational progressions, with vibrational spacings that are identical to bare O3−, within the

J. Chem. Phys. 142, 124305 (2015) TABLE I. Electron affinities of O3 ·(O2)n clusters, determined from the origin of band X, the “core” O3− ground state direct detachment band. The uncertainty in the absolute energy position is 0.015 eV; relative spacings are good to 0.005 eV. The origin of band X in the O3− ·(O2)4 spectrum could not be unambiguously identified, and the uncertainty in the EA should be taken to be ca.0.07 eV.

O3− (O2)n

Electron affinity (eV)

Band X shift relative to O3− (eV)

Hiraoka’s binding energy48 (eV)

O3− O3− ·(O2) O3− ·(O2)2 O3− ·(O2)3 O3− ·(O2)4

2.092 2.16 2.24 2.28 2.35

... 0.07 0.15 0.20 0.26

... 0.09 0.18 0.27 0.33

resolution of the experiment. The photoelectron angular distribution in band X is largely preserved in the complexes, becoming slightly less negative in the larger clusters. For example, β (E) = −0.30 ± 0.15 for band X in the O3−·(O2)3 spectrum. Plots of slices through the O3−·(O2)3 at different angles relative to the electric field of the detachment laser are included in the supplementary material (S3).38 In addition, there is a subtle change in the Franck-Condon profile within band X for each cluster. This observation is most clearly observed in bands X of O3−, O3− · O2, and O3−·(O2)2: The intensity of transitions to lower vibrational levels increases slightly relative to higher vibrational levels with cluster size. Small energy shifts and changes in the Franck-Condon profile of band X notwithstanding, the PE spectra are consistent with the findings of Bopp et al., whose vibrational predissociation spectra of O3−·(O2)n clusters showed very minimal change in the O3− vibrational frequency in the O3−·(O2)n complexes.20 Vibrational spectra of O3−·(O2)n also showed some O2 vibrational excitation, which becomes “allowed” in the asymmetric cluster environment. The O2− local frequency was slightly lower than the free O2 frequencies, suggesting some charge delocalization into the πg orbitals of O2. Given this spectroscopic feature, the subtle change in the FranckCondon profile with additional O2 molecules can also be explained as charge delocalization from O3− to the surrounding (O2)n molecules, resulting in an O3− core that is incrementally more similar in structure to neutral O3. 2. Relative intensity of O − detachment signal

The photoelectron spectrum of each cluster was collected at several different laser fluences (not shown) and the changes in the relative intensities of the peak labeled O(3P) and band X confirmed that in all spectra, peak O(3P) corresponds to a two-photon transition (photodissociation of O3− followed by photodetachment of O−). Given constant laser fluence, the intensity of the O(3P) peak increases relative to band X with cluster size, suggesting that the ozonide photodissociation cross section increases relative to the direct detachment cross section at 355 nm. The approximate ratios of direct detachment to two-photon detachment for the spectra shown, obtained with the same fluence, are given in Table II. The trend is unambiguous for O3− · O2 and O3−·(O2)2, but it becomes non-monoatonic at the point when additional direct

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detachment in the O3− · (O2)3 and O3−·(O2)4 (vide infra). Overall, the implication of this finding is that O3− in a collision complex with one or two O2 molecules will more readily photodissociate to form O− + O2 than bare O3−. 3. High electron binding energy region

Features in the 3.0–3.5 eV binding energy range of the O3−·(O2)n (n = 0–4) PE spectra vary substantially, as shown in Fig. 4. As with band X, the addition of each O2 molecule should cause the open-shell neutral states to shift to higher binding energy relative to the anion ground state. The lowest energy peak in band A of the bare O3− spectrum that is not assigned to O2− autodetachment is at 3.280(5) eV. However, in both the O3− · O2 and O2−·(O2)2 PE spectra, the lowest energy transition above 3.0 eV is a distinct peak at 3.424(10) eV, 0.14 eV higher in energy than the direct detachment transition in the O3− spectrum. We assign the peak at 3.424 eV in the O2− · O2 and − O2 ·(O2)2 PE spectra to the 1 D ← 2 P(3/2 and 1/2) transition arising from the O− photofragment, which must be present in all of the spectra based on the pronounced signal associated with the 3 P ← 2 P(3/2 and 1/2) transition. The bare O− PE spectrum is superimposed on the O3−·(O2)2 spectrum in both Figs. 3 and 4 to illustrate where the 1 D ← 2 P(3/2 and 1/2) transition lies. With higher resolution at lower e−KE values, a partially resolved doublet is observed, with the two peaks spaced by 0.024(3) eV, consistent with reported spin-orbit splitting of 22 meV between the 2P3/2 and 2P1/2 states of O−.51 As is evident in Fig. 4, the O(1D) transition observed in the O3−·(O2)2 spectrum is not solvent shifted or broadened compared to the bare O− photoelectron spectrum, suggesting that the O− photofragment is photodetached outside the O2 cluster environment. Since the 3.49 eV photon energy is significantly higher than the O− + O2 dissociation limit of O3−, the O− is feasibly “desolvated.” The O(1D) peak in the O3−·O2 spectrum, however, is broader than the peak in the O3−·(O2)2 spectrum, and there is an additional peak at the edge of the binding energy limit. It is possible that some O2− vibrational autodetachment is occurring, but the signal pattern is very different from the bare O3− spectrum.52 On the other hand, if we expect shifts in the positions of the O3 triplet states to be in line with shifts in band X, as indicated with the pale blue vertical lines, direct detachment signal should overlap with the O(1D) peak. The spectrum of O3−·(O2)2 is uniquely simple: No O2− autodetachment signal or transitions to excited states of the ozone core are observed. It, therefore, appears that the O + O2− dissociation channel is closed, and the excited O3·(O2)2 neutral TABLE II. Ratio of the integrated direct detachment signal to the integrated O− photofragment detachment signal in the PE spectra of O3− ·(O2)n clusters. O3−(O2) n

Direct detachment to two-photon detachment ratio

O3− O3− ·(O2) O3− ·(O2)2 O3− ·(O2)3 O3− ·(O2)4

12 3 1.3 1.8 1.0

states are shifted beyond the range of the photon energy. The only signal observed can be attributed to the O(3P) ← O− (2P), (1D) ← O−(2P), and O3( X˜ 1A1) + e− ← O3−( X˜ 2B1) transitions. Autodetachment signal is pronounced in the O3−·(O2)3 spectrum, as indicated in Fig. 4. While the signal to noise ratio in the O3−·(O2)4 spectrum is lower, peaks in the same energy range align with O2− autodetachment peak positions, as well. O2− autodetachment signal is observed in at least three of the spectra shown in Fig. 4, but there is no pronounced O2− direct detachment signal. However, all spectra exhibit low-intensity signal in the binding energy region of the O2− direct detachment spectrum.19 The PE spectrum of O2− is superimposed on the O3−·(O2)4 spectrum in Fig. 3 to illustrate where direct O2− detachment signal is expected. There are clearly low-intensity peaks in this spectrum and the O3−·(O2)3 spectrum that coincide with O2− direct detachment transitions. 4. Low-lying excited electronic states observed in the O3 −·(O2 )3 and O3 −·(O2 )4 spectra

The PE spectrum of O3−·(O2)3 exhibits a broad transition with an approximate onset of 2.8 eV that partially overlaps with band X, labeled with an asterisk (*). This band is not present in the spectra of smaller clusters, which exhibit baseline over part of the same energy range. It is difficult to assign a band origin or maximum to this feature because of overlap with band X as well as the O2− autodetachment peaks. The energy of the transition is approximately 0.6 eV higher than band X. The signal in this energy region exhibits an isotropic angular distribution (see the supplementary materials, S338), which raises the possibility that this band is an extension of O2− autodetachment signal. It is also possible that detachment of O3−·(O3)2 species are contributing to the spectrum, since neutral O3 will also be generated in the UV-discharge source. The IR predissociation spectrum of O3−·(O2)3 reported by Bopp et al. did not exhibit any spectroscopic features unique among the On − (n = odd) clusters, though their ion production method was significantly different than the method used in this study.20 The PE spectrum of O3−·(O2)4 exhibits continuous, partially structured signal from band X up to the detection limit. There is not a pronounced band in this range as is observed in the O3−·(O2)3 spectrum. However, the spectrum is clearly dominated by O− signal. IV. DISCUSSION

Based on the results presented here, weakly bound ozonideoxygen encounter complexes have higher photodissociation cross sections than bare O3−. The implication of this finding is that collision complexes formed between O3− and O2 will more readily photodissociate to produce O− than bare O3−, which may be relevant to atmospheric ion chemistry modeling. Photodissociation of larger complexes provides intriguing hints into the nature of the higher-lying dissociative channel. Based on high-level elastic electron-O3 scattering calculations reported by Nestman et al.,28 a Feshbach resonance calculated to lie 1.3 eV above the neutral ground state is consistent with experimental scattering results in which vibrationally

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excited and autodetaching O2− fragments were observed.29 The Feshbach resonance involves the 2 A2 state of O3−, which is nominally accessed from the anion ground state by exciting an electron from the lowest-energy non-bonding (a2) orbital to the singly occupied b1 orbital, resulting in an overall loss in bond order. For convenience, depictions of the bonding, nonbonding, and lowest-energy anti-bonding (singly occupied HOMO of the 2 B1 anion state) are included in the supplementary material (S4).38 The calculated energy of the Feshbach resonance (1.3 eV above the neutral ground state; 3.4 eV higher in energy than the anion ground state, based on the neutral EA) coincides with the 3.49 eV photon energy used in this study. Based on this, we tentatively assign the 2 A2 state to the source of autodetaching O2− photofragments evident in this study. The O + O2− dissociation channel accessed by 3.49 eV absorption by bare O3− appears closed in the O3− · O2 and O3−·(O2)2 complexes, but becomes accessible in the O3−·(O2)3 and O3−·(O2)4 complexes, suggesting that the energy of the tentatively assigned 2 A2 dissociative state relative to the ground state is very sensitive to the cluster environment. The intensification of autodetachment signal [and what may be a particularly large increase in autodetachment signal in the case of O3− · (O2)3] with the larger clusters suggests that the 2 A2 state is stabilized more by O2 clusters than the 2 B1 ground state upon and addition of the third O2 molecule to the cluster, proffering additional vibrational excitation in the O2− photofragments. A remaining question is why the O2− photofragments are generated in high vibrational levels. The O3− O–O internuclear distance is 1.36 ± 0.02 Å,17 which is similar to the O2− bondlength, 1.348 ± 0.008 Å.19 The dissociation dynamics initiated by excitation to the 2 A2 state must include a large change in the O2− photofragment bondlength. Sanov and coworkers have observed pronounced O2− vibrational autodetachment signal in PEI spectra of O4−·(O2)n and O4−·(O2)n · H2O clusters,50 and put forward a chargehopping mechanism. As shown in Fig. 1, this mechanism would be energetically viable in the case of O3− if the O2 (b1Σg+υ = 0, 1) + O−(2P) dissociation limit was accessed, with subsequent charge hopping to form O2−(X 2Π3/2, υ = high) + O (3P). The charge-hopping mechanism is, therefore, a possible alternative to the observation of O2− autodetachment. A more thorough analysis of the spectra reported here would be advanced with high level ab initio calculations. Calculations on the anion and neutral clusters will be complicated by (1) electron correlation in both anionic and neutral clusters and (2) weak binding between the ozonide and O2 molecules, which complicates the search for any definitive ground state structure (a problem that vexed calculations on O3−·(O2)n reported by Bopp et al.).20 Understanding the electronic structure of neutral O3 has been in itself historically challenging from a theoretical standpoint.53–55 However, while all experiments to date have demonstrated that the O3−·(O2)n−1 − O2 binding energy is small, the photophysics and electronic structure of the clusters appear to be significantly affected by the O2 solvent molecules, and further study is warranted.

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V. CONCLUSIONS

The evolution of features observed in the photoelectron spectra of the O3−·(O2)n (n = 0–4) supports previous findings that O2 molecules are fairly weakly bound by ≤ 0.1 eV to the ozonide core, and that the electronic structure of ozonide is largely unaffected by clustering O2 molecules. However, subtle changes in the Franck-Condon profile of the lowest energy O3− direct detachment transition suggest some charge transfer from ozonide to the clustering O2 molecules. In addition, the photodissociation cross section of ozonide appears to increase in the O2 clusters, based on the increase in detachment signal observed from O− photofragments. Photodissociation to O + O2− followed by autodetachment from υ = 4, 5, and 6 vibrational levels of O2− is evident in the photoelectron spectra of O3−·(O2)n n = 0, 3, and 4 complexes, but not n = 1 and 2, so the manner of photodissociation is cluster size-dependent. ACKNOWLEDGMENTS

The authors gratefully acknowledge the National Science Foundation NSF CHE 1265991 for support of this research. C.C.J. wishes to thank Professor John Stanton for illuminating conversations about ozone. C.C.J. and J.E.M. also thank Professor Arthur Suits for invaluable assistance with implementation of image recording. 1S. Matejcik, A. Kiendler, P. Cicman, J. Skalny, P. Stampfli, E. Illenberger, Y.

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Photoelectron imaging and photodissociation of ozonide in O3(-)⋅(O2)n (n = 1-4) clusters.

The photoelectron images of O3 (-) and O3 (-) ⋅ (O2)n (n = 1-4) have been measured using 3.49 eV photon energy. The spectra exhibit several processes,...
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