- Email: [email protected]
Observation of sharp vibronic bands in the O4− `core ion' by mid infrared predissociation spectroscopy of O4−·Arn clusters
19 August 2002
Chemical Physics Letters 362 (2002) 255–260 www.elsevier.com/locate/cplett
Observation of sharp vibronic bands in the O 4 ‘core ion’ by mid infrared predissociation spectroscopy of O 4 Arn clusters Jude A. Kelley, William H. Robertson, Mark A. Johnson
*
Sterling Chemistry Laboratory, Department of Chemistry, Yale University, P.O. Box 208107, New Haven, CT 06520-8107, USA Received 10 April 2002; in final form 12 June 2002
Abstract We report the first observation of an infrared ðv0;0 4150 cm1 Þ electronic band system arising from excitation of the ground state O 4 ion, which we discuss in the context of the expected transition (J. Chem. Phys. 114 (2001) 3010) between the two low lying isomeric forms of this species. Surprisingly, the band displays sharp vibrational fine structure, opening the way for a detailed spectroscopic characterization of a charge–resonance stabilized dimer ion. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The photochemistry of the negatively charged clusters of molecular oxygen, ðO2 Þn , has proven to be quite complex due to the high density of low lying electronic states arising from the open shell electronic configurations of the asymptotic (O2 and O 2 ) constituents. Early thermochemical and photodetachment work [1–4] established that the first oxygen molecule binds much more strongly to O 2 than subsequent molecules, and the special stability of O 4 was traced to a strong charge–resonance interaction which causes the excess electron to be equally shared between the two
*
Corresponding author. Fax: +1-203-432-6144. E-mail address: [email protected] (M.A. Johnson).
molecular centers [5]. Such delocalization does not proceed beyond the binary complex, however, leading to the conclusion that the O 4 moiety forms a ‘core ion’ in larger ðO2 Þn clusters. This O 4 ðO2 Þn2 structural characterization provides the basis for understanding the wavelength de pendence of the ðO2 Þn photochemical pathways [6], as recently discussed by Zewail and co-workers [7] in the analysis of their time-resolved photodissociation data. Although it is accepted that O 4 is a charge-delocalized species, its structure has been controversial. For example, early matrix isolation studies [8] were interpreted in the context of a trans-planar (C2h ) geometry, while more recent calculations by Aquino et al. [9] indicated that the rectangular (D2h symmetry) configuration lies lowest in energy. The trans-planar state would then exist as a bound
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 0 1 7 - 5
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excited state just above the O 2 þ O2 dissociation asymptote, with another 2 Bg state lying closely above it. This lead Andrews and Chertihin [10] to re-analyze the matrix data, which were found to be more consistent with the rectangular ground state geometry. Continetti and co-workers [11] have considered the electronically excited state structure of O 4 , and pointed out that both the trans-planar 2 Bg and rectangular 2 Au states correlate to ground state (2 Pu and 3 R g ) molecules. Note, however, that in their discussion of correlation, the 2 Bg excited state was constrained to the rectangular (and hence 2 B3g symmetry) configuration. As a result, the state arising from the trans-planar isomer was shown to be repulsive at all distances of approach between the two oxygen centers, masking the fact that this surface is calculated to possess a stationary point in the C2h arrangement. Another curious aspect of the previous work is that although Aquino et al. report that the 2 B3g state is repulsive, the data in their table (IX of [9]) indicate a bound region along a slice corresponding to separation of the two molecules with the intramolecular O–O distances held fixed at the ground state geometry of the cluster. This overall scenario dictates that there should be an optically allowed electronic transition very close to the dissociation energy of the O 4 ion connecting the low lying rectangular and transplanar isomeric forms, in addition to a transition to another repulsive state. Depending on the lifetime of these excited states, one can therefore envision using high resolution electronic spectroscopy to characterize the structure of O 4 . This is significant since the excited states of such charge– resonance stabilized complexes are generally repulsive [12,13] and therefore do not yield sharp spectra. In this Letter, we carry out an argon predissociation study of the O 4 Ar1;2 clusters in order to spectroscopically characterize the electronic transitions of O 4 in the infrared region. 2. Experimental Predissociation spectra of argon-solvated O 4 were obtained the using the Yale tandem time-offlight photofragmentation spectrometer described
previously [14]. Cluster ions were generated by secondary electron attachment arising from electron impact ionization (1 kV) of a supersonic expansion of Ar (backing pressure of approx. 3 atm) into which trace O2 was introduced via entrainment [15]. Mid-IR radiation in the 2400–5300 cm1 region was generated using a combination of optical parametric oscillators (OPOs). For the 2400–4500 region, we used our KTP/KTA-based, oscillator/amplifier from LaserVision, pumped by a Nd:YAG laser (Spectra Physics DCR-3). This arrangement has the unfortunate property, however, that the degeneracy point of the OPO leaves a gap in mid-IR coverage centered at 4698 cm1 , which falls in the middle of the O 4 band system. To overcome this limitation, we constructed another 532-nm pumped oscillator using two (45°) KTP crystals which are usually used in the 1064nm pumped OPA chain. The idler beam from this OPO can be continuously scanned from 3800 to 5300 cm1 with a pulse energy of 2–5 mJ. The reported spectra were obtained using Ar predissociation spectroscopy [16]: O 4 Arn þ hm ! O4 þ n Ar
ð1Þ
with n ¼ 1 and 2. The loosely bound Ar atoms serve the dual purposes of limiting the internal energy content [17] in O 4 as well as providing a ‘messenger’ to enable detection of absorption at photon energies which are insufficient to dissociate the core ion [18,19].
3. Results and discussion Fig. 1 displays the mid-IR spectrum of the 1 O region. The 4 Ar ion in the 3800–5300 cm spectrum is comprised of sharp vibrational features with long progressions involving several modes. Because the bands display rather closely spaced peaks, one might suspect that some of the structure is due to motion of the argon atoms. If this was the case, however, the fine structure pattern would depend on the number of attached argons (as we have observed in several cases involving argon solvation of ion–molecule complexes) [20,21]. To test this point, in Fig. 2, we compare an expanded region of the spectra arising
J.A. Kelley et al. / Chemical Physics Letters 362 (2002) 255–260
Fig. 1. Ar predissociation spectrum of O 4 Ar. The main progression is tentatively assigned to excitation of the symmetric m2 mode [8] of trans-planar (2 Bg ) O 4 . DH indicates the measured [4] dissociation enthalpy of O 4 , vmax is the energy 2 calculated [9] for the 2 B3g Au vertical transition and m0;0 stands for the calculated band origin.
257
from O 4 Ar (lower trace) and O4 Ar2 (upper trace). The main pattern of peaks is quite similar in the two spectra, while several of the peaks display some broadening upon attachment of the second argon. This indicates that the dominant features are indeed due to transitions within the O 4 core ion. Furthermore, although broadened (as is typical for uncomplexed ions in our instrument) the main band origins could be identified in the photodissociation spectra of bare O 4 , again indicating that the bands are unique to O 4. The arrow labeled vmax in Fig. 1 indicates the calculated [9] location (5235 cm1 ) of the 2 2 B3g Au transition preserving the rectangular ground state geometry (i.e. the vertical transition energy), while the arrow labeled DH indicates the measured [4] enthalpy for the dissociation of O 4 into O 2 and O2 . The arrow labeled m0;0 denotes the 2 calculated 2 B3g Au band origin which appears far below the onset of the observed band. Note that the experimental spectrum is much narrower than the theoretical expectation, with a sharp band origin near 4150 cm1 . The structure in the band is curious since it lies above the dissociation limit and both states in this region are calculated to correlate to ground state products [11]. The observation of well-resolved vibronic bands implies relatively long-lived periodic motion on the excited state surface, which in turn requires a barrier to dissociation or the occurrence of isolated resonances in the continuum [22]. The distribution of decay products provides some clues about the decay mechanism of the excited state. For example, one possibility could be rapid vibrational energy redistribution within the excited state leading to argon or O2 evaporation:
O 4 Ar þ hm ! ½O4 þ Ar
! O 2 þ O2 þ Ar O 4 Ar þ hm ! O2 Ar þ O2
ð2aÞ ð2bÞ
while another possibility could be internal conversion of the excited state to the ground state followed by sequential evaporation: Fig. 2. Comparison of the Ar predissociation spectra of O 4 Ar (lower trace) and O 4 Ar2 (upper trace).
z
O 4 Ar þ hm ! ½O4 Ar ! O4 ðEint Þ þ Ar
ð3aÞ
258 O 4 ðEint Þ ! O2 þ O2
J.A. Kelley et al. / Chemical Physics Letters 362 (2002) 255–260
ð3bÞ
The evaporation pathways on the electronically excited surface become available when the vibrational excitation exceeds the bonding energies of either the O2 or Ar. While we expect Ar to be similarly bound in the ground and excited states, the O 4 dissociation energy should be much smaller in the excited state, and therefore O2 loss can compete with argon as a statistical evaporation pathway [23]. If O2 is ejected, then the O 2 Ar complex (2b) can be stable since most of the energy is dissipated in dissociating the relatively strongly bound O 4 species. On the other hand, if the argon evaporates (2a) from the excited state, the resulting O 4 (i.e. left on the excited state) can only decay to O 2 since the electronic origin lies above the O4 dissociation energy (neglecting fluorescence due to the low frequency of the emitted light). On the other hand, when there is insufficient vibrational excitation in the excited state to eject either O2 or Ar, the only relaxation pathway available is internal conversion which leads to hot O 4 Ar (3a). This can then result in production of stable O 4 via argon evaporation, or sequential loss of both Ar and O2 depending on the available internal energy, Eint . All three expected anionic photoproducts are formed upon excitation of O 4 Ar, and Fig. 3 compares the relative action spectra for produc tion of O 2 (top trace), O2 Ar (middle trace), and O4 (lower trace). Argon loss dominates near the origin but is displaced first by O2 loss and then by formation of O 2 resulting from ejection of both O2 and Ar. Preferential formation of O 4 near the origin suggests decay by internal conversion, while significant O2 loss starts at the second strong doublet located about 242 cm1 above the band origin. This indicates that the effective binding energy of O 4 in the excited state is on this order, which is less than the binding energy of argon to 1 O in the case of 4 in the ground state ( 700 cm O2 H2 O). Finally, the O2 yield gradually increases throughout the band, which can arise from either prompt internal conversion or internal conversion following argon evaporation from the excited state (Eqs. (2a) and (3b), respectively). Thus, the decay pathways indicate that the upper
Fig. 3. Action spectra of O 4 Ar, monitoring the relative yield in each photofragmentation channel. The lowest trace results from the O 4 channel, the middle trace from O2 Ar, and the top trace from the production of O2 .
state in the observed electronic transition is very weakly bound with respect to O2 ejection. Note that the action spectra for O2 ejection (middle trace in Fig. 3) is quite structured, indicating that the system must be predissociated. The important open question is, therefore, whether this state is undergoing vibrational or electronic predissociation. The expected character of the states at play in the observed transition raises some interesting issues regarding the assignments of the bands. Vertical excitation from the rectangular ground state to the trans-planar excited state, for example, would place the nuclei near the transition state region for interconversion between the two degenerate configurations of the 2 Bg state, as depicted in the sketch of the potential energy curves shown in Fig. 4. Andrews and Chertihin [10] have reported the six calculated vibrational fundamentals for the 2 Bg state, of which three (136, 234, and
J.A. Kelley et al. / Chemical Physics Letters 362 (2002) 255–260
Fig. 4. Schematic view of potential curves along with calculated [9] structures for the rectangular and trans-planar isomers of O 4.
280 cm1 ) lie in the energy range below 300 cm1 , where activity is observed in the spectrum. Of these, only the Ag O2 –O2 stretch at 280 cm1 is totally symmetric and would therefore give rise to a simple Franck–Condon progression. The spectrum can be decomposed into a primary anharmonic progression (we ¼ 240 cm1 , we xe ¼ 8 cm1 ) as indicated in Fig. 1, with substructure on each member of this series. A Birge–Sponer extrapolation based on the vibrational constants yields an effective dissociation asymptote about 1676 cm1 above the origin, well below the next
259
available limit (7904 cm1 for the formation of 2 O2 a1 Dg and O 2 X Pg relative to ground state products) but interestingly close to the O2 vibrational spacing (1556 cm1 ). Moreover, while close to the calculated energy for the O2 –O2 stretching motion between the two molecular centers in a rectangular-to-trans-planar transition, it is not obvious why this progression would dominate the spectrum of such a weakly bound excited state when this coordinate leads to dissociation of the trans-planar state. Another problem with the rectangular ! transplanar assignment for the observed band system is that the lower frequency fine structure on each O2 – O2 stretching band does not correspond to any of the calculated harmonic frequencies for trans-planar O 4 . Moreover, it is observed to be quite anharmonic. In fact, it displays negative anharmonicity in the progression near the band origin with peak spacings of 41, 49, and 72 cm1 between members in the first series. If the band near 4200 cm1 arises from a transition between the rectangular and trans-planar isomers, excitation from the rectangular ground state should lead to maximum vibrational overlap near the barrier to interconversion between the two equivalent minima on the 2 Bg surface as illustrated in Fig. 4. The fact that the intensity maximum in the spectrum lies close to the origin suggests that the barrier does not extend far above these minima, which in turn indicates that tunneling should be important even for the low vibrational levels of the frustrated geared rotation. The complexity of the band pattern and unusual nature of the vibronic transition would greatly benefit from a theoretical perspective on the dynamics at play on a realistic surface describing the trans-planar form of this system.
4. Summary We report a predissociated electronic band system of the O 4 ion which occurs in the vicinity predicted for the transition between the two isomeric forms of this species. The band occurs above the O 4 dissociation energy and is observed to eject an O2 molecule in preference to a weakly bound argon atom upon excitation near the peak of the
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vibrational envelope. The fine structure is not straightforward to reconcile with the calculated properties of the trans-planar state. Nonetheless, this discovery of a discrete electronic spectrum for the O 4 core ion introduces a new capability in the study of anionic clusters such as O 4 ðH2 OÞn , where we can finally exploit the highly developed arsenal of electronic spectroscopy tools (RIDIRS, etc.) [24] to sort out isomers and facilitate band assignments by double resonance. Furthermore, with access to a spectroscopic signature of the core ion structure, one can now envision following the solvent-induced changes in this core ion using the electronic spectrum in conjunction with the usual vibrational probes of the solvent structure. Such studies are presently underway in our laboratory.
Acknowledgements We thank the National Science Foundation (experimental physical chemistry division) for support of this work.
References [1] P.C. Cosby, R.A. Bennet, J.R. Peterson, J.T. Moseley, J. Chem. Phys. 63 (1975) 1612. [2] D.C. Conway, L.E. Nesbitt, J. Chem. Phys. 48 (1968) 509. [3] L.A. Posey, M.J. Deluca, M.A. Johnson, Chem. Phys. Lett. 131 (1986) 170. [4] K. Hiraoka, J. Chem. Phys. 89 (1988) 3190.
[5] D.C. Conway, J. Chem. Phys. 50 (1969) 3864. [6] M.J. Deluca, C.C. Han, M.A. Johnson, J. Chem. Phys. 93 (1990) 268. [7] D.H. Paik, T.M. Bernhardt, N.J. Kim, A.H. Zewail, J. Chem. Phys. 115 (2001) 612. [8] W.E. Thompson, M.E. Jacox, J. Chem. Phys. 91 (1989) 3826. [9] A.J.A. Aquino, P.R. Taylor, S.P. Walch, J. Chem. Phys. 114 (2001) 3010. [10] G.V. Chertihin, L. Andrews, J. Chem. Phys. 108 (1998) 6404. [11] R. Li, K.A. Hanold, M.C. Garner, A.K. Luong, R.E. Continetti, Faraday Discuss. 108 (1997) 115. [12] T. Tsukuda, M.A. Johnson, T. Nagata, Chem. Phys. Lett. 268 (1997) 429. [13] T. Dresch, H. Kramer, Y. Thurner, R. Weber, Chem. Phys. Lett. 177 (1991) 383. [14] M.A. Johnson, W.C. Lineberger, in: J.M. Farrar, W. Saunders (Eds.), Techniques for the Study of Gas-Phase Ion Molecule Reactions, Wiley, New York, 1988, p. 591. [15] W.H. Robertson, J.A. Kelley, M.A. Johnson, Rev. Sci. Instrum. 71 (2000) 4431. [16] P. Ayotte, G.H. Weddle, J. Kim, M.A. Johnson, Chem. Phys. 239 (1998) 485. [17] C.E. Klots, J. Chem. Phys. 83 (1985) 5854. [18] M. Okumura, L.I. Yeh, J.D. Myers, Y.T. Lee, J. Phys. Chem. 94 (1990) 3416. [19] M. Okumura, L.I. Yeh, J.D. Myers, Y.T. Lee, J. Chem. Phys. 85 (1986) 2328. [20] S.B. Nielsen, P. Ayotte, J.A. Kelley, M.A. Johnson, J. Chem. Phys. 111 (1999) 9593. [21] S.A. Corcelli, J.A. Kelley, J.C. Tully, M.A. Johnson, J. Phys. Chem. A 106 (2002) 4872. [22] E. Deeber, E.H. Kim, D.M. Neumark, R.F. Gunion, W.C. Lineberger, J. Phys. Chem. 99 (1995) 13627. [23] S. Nandi, A. Sanov, N. Delaney, J. Faeder, R. Parson, W.C. Lineberger, J. Phys. Chem. A 102 (1998) 8827. [24] R.N. Pribble, F. Hagemeister, T.S. Zwier, J. Chem. Phys. 106 (1997) 2145.
Chemical Physics Letters 362 (2002) 255–260 www.elsevier.com/locate/cplett
Observation of sharp vibronic bands in the O 4 ‘core ion’ by mid infrared predissociation spectroscopy of O 4 Arn clusters Jude A. Kelley, William H. Robertson, Mark A. Johnson
*
Sterling Chemistry Laboratory, Department of Chemistry, Yale University, P.O. Box 208107, New Haven, CT 06520-8107, USA Received 10 April 2002; in final form 12 June 2002
Abstract We report the first observation of an infrared ðv0;0 4150 cm1 Þ electronic band system arising from excitation of the ground state O 4 ion, which we discuss in the context of the expected transition (J. Chem. Phys. 114 (2001) 3010) between the two low lying isomeric forms of this species. Surprisingly, the band displays sharp vibrational fine structure, opening the way for a detailed spectroscopic characterization of a charge–resonance stabilized dimer ion. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The photochemistry of the negatively charged clusters of molecular oxygen, ðO2 Þn , has proven to be quite complex due to the high density of low lying electronic states arising from the open shell electronic configurations of the asymptotic (O2 and O 2 ) constituents. Early thermochemical and photodetachment work [1–4] established that the first oxygen molecule binds much more strongly to O 2 than subsequent molecules, and the special stability of O 4 was traced to a strong charge–resonance interaction which causes the excess electron to be equally shared between the two
*
Corresponding author. Fax: +1-203-432-6144. E-mail address: [email protected] (M.A. Johnson).
molecular centers [5]. Such delocalization does not proceed beyond the binary complex, however, leading to the conclusion that the O 4 moiety forms a ‘core ion’ in larger ðO2 Þn clusters. This O 4 ðO2 Þn2 structural characterization provides the basis for understanding the wavelength de pendence of the ðO2 Þn photochemical pathways [6], as recently discussed by Zewail and co-workers [7] in the analysis of their time-resolved photodissociation data. Although it is accepted that O 4 is a charge-delocalized species, its structure has been controversial. For example, early matrix isolation studies [8] were interpreted in the context of a trans-planar (C2h ) geometry, while more recent calculations by Aquino et al. [9] indicated that the rectangular (D2h symmetry) configuration lies lowest in energy. The trans-planar state would then exist as a bound
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 0 1 7 - 5
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J.A. Kelley et al. / Chemical Physics Letters 362 (2002) 255–260
excited state just above the O 2 þ O2 dissociation asymptote, with another 2 Bg state lying closely above it. This lead Andrews and Chertihin [10] to re-analyze the matrix data, which were found to be more consistent with the rectangular ground state geometry. Continetti and co-workers [11] have considered the electronically excited state structure of O 4 , and pointed out that both the trans-planar 2 Bg and rectangular 2 Au states correlate to ground state (2 Pu and 3 R g ) molecules. Note, however, that in their discussion of correlation, the 2 Bg excited state was constrained to the rectangular (and hence 2 B3g symmetry) configuration. As a result, the state arising from the trans-planar isomer was shown to be repulsive at all distances of approach between the two oxygen centers, masking the fact that this surface is calculated to possess a stationary point in the C2h arrangement. Another curious aspect of the previous work is that although Aquino et al. report that the 2 B3g state is repulsive, the data in their table (IX of [9]) indicate a bound region along a slice corresponding to separation of the two molecules with the intramolecular O–O distances held fixed at the ground state geometry of the cluster. This overall scenario dictates that there should be an optically allowed electronic transition very close to the dissociation energy of the O 4 ion connecting the low lying rectangular and transplanar isomeric forms, in addition to a transition to another repulsive state. Depending on the lifetime of these excited states, one can therefore envision using high resolution electronic spectroscopy to characterize the structure of O 4 . This is significant since the excited states of such charge– resonance stabilized complexes are generally repulsive [12,13] and therefore do not yield sharp spectra. In this Letter, we carry out an argon predissociation study of the O 4 Ar1;2 clusters in order to spectroscopically characterize the electronic transitions of O 4 in the infrared region. 2. Experimental Predissociation spectra of argon-solvated O 4 were obtained the using the Yale tandem time-offlight photofragmentation spectrometer described
previously [14]. Cluster ions were generated by secondary electron attachment arising from electron impact ionization (1 kV) of a supersonic expansion of Ar (backing pressure of approx. 3 atm) into which trace O2 was introduced via entrainment [15]. Mid-IR radiation in the 2400–5300 cm1 region was generated using a combination of optical parametric oscillators (OPOs). For the 2400–4500 region, we used our KTP/KTA-based, oscillator/amplifier from LaserVision, pumped by a Nd:YAG laser (Spectra Physics DCR-3). This arrangement has the unfortunate property, however, that the degeneracy point of the OPO leaves a gap in mid-IR coverage centered at 4698 cm1 , which falls in the middle of the O 4 band system. To overcome this limitation, we constructed another 532-nm pumped oscillator using two (45°) KTP crystals which are usually used in the 1064nm pumped OPA chain. The idler beam from this OPO can be continuously scanned from 3800 to 5300 cm1 with a pulse energy of 2–5 mJ. The reported spectra were obtained using Ar predissociation spectroscopy [16]: O 4 Arn þ hm ! O4 þ n Ar
ð1Þ
with n ¼ 1 and 2. The loosely bound Ar atoms serve the dual purposes of limiting the internal energy content [17] in O 4 as well as providing a ‘messenger’ to enable detection of absorption at photon energies which are insufficient to dissociate the core ion [18,19].
3. Results and discussion Fig. 1 displays the mid-IR spectrum of the 1 O region. The 4 Ar ion in the 3800–5300 cm spectrum is comprised of sharp vibrational features with long progressions involving several modes. Because the bands display rather closely spaced peaks, one might suspect that some of the structure is due to motion of the argon atoms. If this was the case, however, the fine structure pattern would depend on the number of attached argons (as we have observed in several cases involving argon solvation of ion–molecule complexes) [20,21]. To test this point, in Fig. 2, we compare an expanded region of the spectra arising
J.A. Kelley et al. / Chemical Physics Letters 362 (2002) 255–260
Fig. 1. Ar predissociation spectrum of O 4 Ar. The main progression is tentatively assigned to excitation of the symmetric m2 mode [8] of trans-planar (2 Bg ) O 4 . DH indicates the measured [4] dissociation enthalpy of O 4 , vmax is the energy 2 calculated [9] for the 2 B3g Au vertical transition and m0;0 stands for the calculated band origin.
257
from O 4 Ar (lower trace) and O4 Ar2 (upper trace). The main pattern of peaks is quite similar in the two spectra, while several of the peaks display some broadening upon attachment of the second argon. This indicates that the dominant features are indeed due to transitions within the O 4 core ion. Furthermore, although broadened (as is typical for uncomplexed ions in our instrument) the main band origins could be identified in the photodissociation spectra of bare O 4 , again indicating that the bands are unique to O 4. The arrow labeled vmax in Fig. 1 indicates the calculated [9] location (5235 cm1 ) of the 2 2 B3g Au transition preserving the rectangular ground state geometry (i.e. the vertical transition energy), while the arrow labeled DH indicates the measured [4] enthalpy for the dissociation of O 4 into O 2 and O2 . The arrow labeled m0;0 denotes the 2 calculated 2 B3g Au band origin which appears far below the onset of the observed band. Note that the experimental spectrum is much narrower than the theoretical expectation, with a sharp band origin near 4150 cm1 . The structure in the band is curious since it lies above the dissociation limit and both states in this region are calculated to correlate to ground state products [11]. The observation of well-resolved vibronic bands implies relatively long-lived periodic motion on the excited state surface, which in turn requires a barrier to dissociation or the occurrence of isolated resonances in the continuum [22]. The distribution of decay products provides some clues about the decay mechanism of the excited state. For example, one possibility could be rapid vibrational energy redistribution within the excited state leading to argon or O2 evaporation:
O 4 Ar þ hm ! ½O4 þ Ar
! O 2 þ O2 þ Ar O 4 Ar þ hm ! O2 Ar þ O2
ð2aÞ ð2bÞ
while another possibility could be internal conversion of the excited state to the ground state followed by sequential evaporation: Fig. 2. Comparison of the Ar predissociation spectra of O 4 Ar (lower trace) and O 4 Ar2 (upper trace).
z
O 4 Ar þ hm ! ½O4 Ar ! O4 ðEint Þ þ Ar
ð3aÞ
258 O 4 ðEint Þ ! O2 þ O2
J.A. Kelley et al. / Chemical Physics Letters 362 (2002) 255–260
ð3bÞ
The evaporation pathways on the electronically excited surface become available when the vibrational excitation exceeds the bonding energies of either the O2 or Ar. While we expect Ar to be similarly bound in the ground and excited states, the O 4 dissociation energy should be much smaller in the excited state, and therefore O2 loss can compete with argon as a statistical evaporation pathway [23]. If O2 is ejected, then the O 2 Ar complex (2b) can be stable since most of the energy is dissipated in dissociating the relatively strongly bound O 4 species. On the other hand, if the argon evaporates (2a) from the excited state, the resulting O 4 (i.e. left on the excited state) can only decay to O 2 since the electronic origin lies above the O4 dissociation energy (neglecting fluorescence due to the low frequency of the emitted light). On the other hand, when there is insufficient vibrational excitation in the excited state to eject either O2 or Ar, the only relaxation pathway available is internal conversion which leads to hot O 4 Ar (3a). This can then result in production of stable O 4 via argon evaporation, or sequential loss of both Ar and O2 depending on the available internal energy, Eint . All three expected anionic photoproducts are formed upon excitation of O 4 Ar, and Fig. 3 compares the relative action spectra for produc tion of O 2 (top trace), O2 Ar (middle trace), and O4 (lower trace). Argon loss dominates near the origin but is displaced first by O2 loss and then by formation of O 2 resulting from ejection of both O2 and Ar. Preferential formation of O 4 near the origin suggests decay by internal conversion, while significant O2 loss starts at the second strong doublet located about 242 cm1 above the band origin. This indicates that the effective binding energy of O 4 in the excited state is on this order, which is less than the binding energy of argon to 1 O in the case of 4 in the ground state ( 700 cm O2 H2 O). Finally, the O2 yield gradually increases throughout the band, which can arise from either prompt internal conversion or internal conversion following argon evaporation from the excited state (Eqs. (2a) and (3b), respectively). Thus, the decay pathways indicate that the upper
Fig. 3. Action spectra of O 4 Ar, monitoring the relative yield in each photofragmentation channel. The lowest trace results from the O 4 channel, the middle trace from O2 Ar, and the top trace from the production of O2 .
state in the observed electronic transition is very weakly bound with respect to O2 ejection. Note that the action spectra for O2 ejection (middle trace in Fig. 3) is quite structured, indicating that the system must be predissociated. The important open question is, therefore, whether this state is undergoing vibrational or electronic predissociation. The expected character of the states at play in the observed transition raises some interesting issues regarding the assignments of the bands. Vertical excitation from the rectangular ground state to the trans-planar excited state, for example, would place the nuclei near the transition state region for interconversion between the two degenerate configurations of the 2 Bg state, as depicted in the sketch of the potential energy curves shown in Fig. 4. Andrews and Chertihin [10] have reported the six calculated vibrational fundamentals for the 2 Bg state, of which three (136, 234, and
J.A. Kelley et al. / Chemical Physics Letters 362 (2002) 255–260
Fig. 4. Schematic view of potential curves along with calculated [9] structures for the rectangular and trans-planar isomers of O 4.
280 cm1 ) lie in the energy range below 300 cm1 , where activity is observed in the spectrum. Of these, only the Ag O2 –O2 stretch at 280 cm1 is totally symmetric and would therefore give rise to a simple Franck–Condon progression. The spectrum can be decomposed into a primary anharmonic progression (we ¼ 240 cm1 , we xe ¼ 8 cm1 ) as indicated in Fig. 1, with substructure on each member of this series. A Birge–Sponer extrapolation based on the vibrational constants yields an effective dissociation asymptote about 1676 cm1 above the origin, well below the next
259
available limit (7904 cm1 for the formation of 2 O2 a1 Dg and O 2 X Pg relative to ground state products) but interestingly close to the O2 vibrational spacing (1556 cm1 ). Moreover, while close to the calculated energy for the O2 –O2 stretching motion between the two molecular centers in a rectangular-to-trans-planar transition, it is not obvious why this progression would dominate the spectrum of such a weakly bound excited state when this coordinate leads to dissociation of the trans-planar state. Another problem with the rectangular ! transplanar assignment for the observed band system is that the lower frequency fine structure on each O2 – O2 stretching band does not correspond to any of the calculated harmonic frequencies for trans-planar O 4 . Moreover, it is observed to be quite anharmonic. In fact, it displays negative anharmonicity in the progression near the band origin with peak spacings of 41, 49, and 72 cm1 between members in the first series. If the band near 4200 cm1 arises from a transition between the rectangular and trans-planar isomers, excitation from the rectangular ground state should lead to maximum vibrational overlap near the barrier to interconversion between the two equivalent minima on the 2 Bg surface as illustrated in Fig. 4. The fact that the intensity maximum in the spectrum lies close to the origin suggests that the barrier does not extend far above these minima, which in turn indicates that tunneling should be important even for the low vibrational levels of the frustrated geared rotation. The complexity of the band pattern and unusual nature of the vibronic transition would greatly benefit from a theoretical perspective on the dynamics at play on a realistic surface describing the trans-planar form of this system.
4. Summary We report a predissociated electronic band system of the O 4 ion which occurs in the vicinity predicted for the transition between the two isomeric forms of this species. The band occurs above the O 4 dissociation energy and is observed to eject an O2 molecule in preference to a weakly bound argon atom upon excitation near the peak of the
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vibrational envelope. The fine structure is not straightforward to reconcile with the calculated properties of the trans-planar state. Nonetheless, this discovery of a discrete electronic spectrum for the O 4 core ion introduces a new capability in the study of anionic clusters such as O 4 ðH2 OÞn , where we can finally exploit the highly developed arsenal of electronic spectroscopy tools (RIDIRS, etc.) [24] to sort out isomers and facilitate band assignments by double resonance. Furthermore, with access to a spectroscopic signature of the core ion structure, one can now envision following the solvent-induced changes in this core ion using the electronic spectrum in conjunction with the usual vibrational probes of the solvent structure. Such studies are presently underway in our laboratory.
Acknowledgements We thank the National Science Foundation (experimental physical chemistry division) for support of this work.
References [1] P.C. Cosby, R.A. Bennet, J.R. Peterson, J.T. Moseley, J. Chem. Phys. 63 (1975) 1612. [2] D.C. Conway, L.E. Nesbitt, J. Chem. Phys. 48 (1968) 509. [3] L.A. Posey, M.J. Deluca, M.A. Johnson, Chem. Phys. Lett. 131 (1986) 170. [4] K. Hiraoka, J. Chem. Phys. 89 (1988) 3190.
[5] D.C. Conway, J. Chem. Phys. 50 (1969) 3864. [6] M.J. Deluca, C.C. Han, M.A. Johnson, J. Chem. Phys. 93 (1990) 268. [7] D.H. Paik, T.M. Bernhardt, N.J. Kim, A.H. Zewail, J. Chem. Phys. 115 (2001) 612. [8] W.E. Thompson, M.E. Jacox, J. Chem. Phys. 91 (1989) 3826. [9] A.J.A. Aquino, P.R. Taylor, S.P. Walch, J. Chem. Phys. 114 (2001) 3010. [10] G.V. Chertihin, L. Andrews, J. Chem. Phys. 108 (1998) 6404. [11] R. Li, K.A. Hanold, M.C. Garner, A.K. Luong, R.E. Continetti, Faraday Discuss. 108 (1997) 115. [12] T. Tsukuda, M.A. Johnson, T. Nagata, Chem. Phys. Lett. 268 (1997) 429. [13] T. Dresch, H. Kramer, Y. Thurner, R. Weber, Chem. Phys. Lett. 177 (1991) 383. [14] M.A. Johnson, W.C. Lineberger, in: J.M. Farrar, W. Saunders (Eds.), Techniques for the Study of Gas-Phase Ion Molecule Reactions, Wiley, New York, 1988, p. 591. [15] W.H. Robertson, J.A. Kelley, M.A. Johnson, Rev. Sci. Instrum. 71 (2000) 4431. [16] P. Ayotte, G.H. Weddle, J. Kim, M.A. Johnson, Chem. Phys. 239 (1998) 485. [17] C.E. Klots, J. Chem. Phys. 83 (1985) 5854. [18] M. Okumura, L.I. Yeh, J.D. Myers, Y.T. Lee, J. Phys. Chem. 94 (1990) 3416. [19] M. Okumura, L.I. Yeh, J.D. Myers, Y.T. Lee, J. Chem. Phys. 85 (1986) 2328. [20] S.B. Nielsen, P. Ayotte, J.A. Kelley, M.A. Johnson, J. Chem. Phys. 111 (1999) 9593. [21] S.A. Corcelli, J.A. Kelley, J.C. Tully, M.A. Johnson, J. Phys. Chem. A 106 (2002) 4872. [22] E. Deeber, E.H. Kim, D.M. Neumark, R.F. Gunion, W.C. Lineberger, J. Phys. Chem. 99 (1995) 13627. [23] S. Nandi, A. Sanov, N. Delaney, J. Faeder, R. Parson, W.C. Lineberger, J. Phys. Chem. A 102 (1998) 8827. [24] R.N. Pribble, F. Hagemeister, T.S. Zwier, J. Chem. Phys. 106 (1997) 2145.