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Electronic structure of hexafluorobenzene by high-resolution vacuum ultraviolet photo-absorption and He(I) photoelectron spectroscopy
Chemical Physics 328 (2006) 183–189 www.elsevier.com/locate/chemphys
Electronic structure of hexafluorobenzene by high-resolution vacuum ultraviolet photo-absorption and He(I) photoelectron spectroscopy C. Motch a, A. Giuliani a,*, J. Delwiche a, P. Lima˜o-Vieira b, N.J. Mason c, S.V. Hoffmann d, M.-J. Hubin-Franskin a,1 b
a Laboratoire de Spectroscopie d’E´lectrons diffuse´s, Universite´ de Lie`ge, Institut de Chimie B6c, B-4000 Lie`ge 1, Belgium Laborato´rio de Coliso˜es Ato´micas, e Moleculares, Departamento de Fı´sica, CEFITEC, FCT-Universidade Nova de Lisboa, 2829-516 Caparica, Portugal c Centre of Optical and Molecular Sciences, Department of Physics and Astrophysics, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom d Institute for Storage Ring Facilities, University of A˚rhus, Ny Munkegade, DK-8000 A˚rhus C, Denmark
Received 25 March 2006; accepted 25 May 2006 Available online 29 June 2006
Abstract The VUV photo-absorption spectrum of hexafluorobenzene, C6F6, recorded using the synchrotron radiation in the 4–10.75 eV photon range exhibits broad bands between 4 and 8 eV, and fine features above 8 eV. The broad bands are likely to be due to valence states dissociating into neutral fragments. They have been assigned to a p–p* transition, two p–r* transitions and one r–p* transition. A small shoulder observed at 7.36 eV, confirms earlier observations and is evidence of a further valence excited electronic state. Above 8.3 eV numerous fine structures are observed and the features are assigned by comparison with the high-resolution HeI photoelectron spectrum to a nd Rydberg series converging to the ground ionic state and a ns series associated with the first electronic excited state limit. The first two nd terms show vibrational structure. The origin of the vibrational structure is discussed in terms of Jahn–Teller distortion. For the 3s term, excitation of the vibration normal mode m02 is reported for the first time with a value of 0.066 eV, a value consistent with that of the ion. The HeI photoelectron spectrum of C6F6 has also been revisited and a new interpretation of the vibrational structure for the first and the fourth electronic bands is proposed. Finally, using a previously derived solar flux model, the atmospheric lifetime relative to photolysis is estimated and the consequences for atmospheric chemistry briefly discussed. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Photoabsorption; VUV; Synchrotron radiation; Absolute cross-section; Photoelectron spectroscopy; Life time; Photolysis rate
1. Introduction Hexafluorobenzene, C6F6, with cyclo-C4F8 and C2F4, has been proposed as an alternative feed gas in the plasma etching industry. Any replacement feed gas must have a low global warming potential (GWP) i.e. it must not absorb infrared radiation in those regions of the Earth’s atmosphere known as the IR windows. Nor may it have *
Corresponding author. Fax: +32 4 3662941. E-mail address: [email protected] (M.-J. Hubin-Franskin). 1 Present address: CNRS, Laboratoire de Spectrome´trie de masse, Institut de Chimie des Substances Naturelles, avenue de la Terrasse, Gifsur-Yvette CEDEX F-91198, France. 0301-0104/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2006.05.032
a large ozone depletion potential (ODP) i.e. it should not be easily photolysed by sunlight to yield reactive radicals that can catalytically destroy ozone in the terrestrial stratosphere. Accordingly there is much interest in the physical and chemical properties of C6F6. It is particularly important to identify the electronic states of C6F6 since the photolysis rate in the terrestrial atmosphere will be determined by the excitation energy and symmetry of the lowest excited states, while the formation of CFx radicals needed in semiconductor plasma reactors for silicon etching will be dependent upon the dissociation dynamics of these same states. However, to date there is relatively little data, either experimental or theoretical, on either the spectroscopy of the excited states of the neutral hexafluorobenzene
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molecule or those of the positive ion. Several photoelectron spectra [1–13] have been published but at lower resolution than in the present work but there are only four previous photo-absorption studies [14–17] and two spectroscopic studies using electron impact under electric dipolar and non dipolar interaction conditions [18,19], the latter providing the first data on the lowest energy triplet states of hexafluorobenzene. A single study has been reported of the Rydberg states of C6F6 [16]. Part of the fine structure in the 8–11.5 eV region has been assigned to two Rydberg series converging to the two lowest energy ionisation limits and vibrational structure has been observed in the first two terms of the series. The present study therefore aims to provide a more comprehensive study of the electronic spectroscopy of C6F6 and provides some important new high-resolution quantitative data required to model the behaviour of C6F6 in plasma reactors. 2. Experimental methods 2.1. The photoelectron spectrometer The photoelectron spectrometer has been previously described in detail [20,21]. Briefly, HeI photons (58.4 nm) are produced by a dc discharge in helium and the spectra are recorded by sweeping a retarding voltage in steps of 1 meV between the ionisation chamber and the entrance slit of a 180° hemispherical electrostatic analyzer, operating in a constant pass energy mode. The measured spectra are corrected for the transmission of the analyzing system. The ionisation energy scale was calibrated using xenon with the energetic values given in the NIST ChemWebbook [22] (2P3/2 = 12.123 eV and 2P1/2 = 13.436 eV). The accuracy of the energy scale is therefore estimated to be ±0.002 eV. The overall resolution is about 20 meV as measured as the full width at half maximum (FWHM) of Xe+ peaks.
The radiation transmitted through the empty cell (I0) was initially recorded over the limited (11 nm) range. The sample was then introduced into the cell and two scans of transmitted radiation (It) were recorded. The cell was subsequently evacuated and a second (I0) was recorded. The mean values of each of the (I0) and (It) values were then used in the Beer– Lambert law to evaluate the cross-section r: I t ¼ I 0 expðrNxÞ where N is the target gas number density and x is the path length. This averaging procedure compensated effectively for the decay of the radiation intensity as the storage ring current decayed. In the figures presented in this work the spectra are presented on an energy (eV) abscissa. The accuracy of the cross-section is estimated to be better than ±5%. 2.3. Sample Hexafluorobenzene is a liquid at room temperature and under atmospheric pressure. It was provided by Janssens Chemicals with a stated purity of 99%. It was used without further purification although repetitive pump-thaw cycles were performed before introducing the sample into the gas cell to remove any dissolved air. There was no evidence of any impurities in the measured spectra. 3. Properties and electronic structure of hexafluorobenzene The structural constants for hexafluorobenzene, determined experimentally for the gas phase by Almenningen et al. [24] and calculated by Almlo¨f and Faegri [25], are summarized as follows: ˚ Experimental : C–C ¼ 1:391 0:007 A ˚ C–F ¼ 1:327 0:007 A ˚ Calculated : C–C ¼ 1:394 A ˚ C–F ¼ 1:349 A
2.2. The gas phase VUV absorption apparatus Photoabsorption spectrum was recorded at the UV1 ˚ rhus beamline [23] of the storage ring ASTRID at the A University, in Denmark. The normal incidence monochromator was operated using the 2000 l/mm grating with an overall resolution (full width at half maximum, FWHM) of better than 0.1 nm. Synchrotron radiation from the monochromator passed through a LiF window into a gas cell of length 25 cm. A Baratron capacitance manometer monitored the sample pressure (less than 0.5 mbar), and the intensity of the radiation exiting the cell through a second window was detected using an UV enhanced photomultiplier tube (Electron Tube Limited, type 9406B). The spectral range extended from about 5 eV (248 nm) to the LiF cut-off at 10.75 eV (115 nm). This range was covered in sections of 11 nm each, using steps of 0.1 nm; this step size was found to be sufficiently small to resolve fine structure in the measured cross-section curves.
In the gas phase, the hexafluorobenzene molecule is planar and accordingly belongs to the D6h symmetry class in its electronic ground state. This is in contrast with C6Cl6 and C6Br6 that have been found by electron diffraction [26] to be non planar with three alternate halogen atoms above the plane of the ring and three below, thus belonging to the D3d symmetry group. The molecule has 30 normal vibration modes of which there are twenty fundamentals. Out of the twelve different vibrational species available for a D6h molecule, only ten are actually observed. They are divided amongst the symmetry classes as indicated by:2 2a1g ; 1a2g ; 1a2u ; 2b1u ; 2b2g ; 2b2u ; 1e1g ; 3e1u ; 4e2g ; 2e2u The first two values have been inverted to be consistent with Brundle et al. [1].
2
Assuming that the C2 axis goes through a C–F group.
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Table 1 Ionisation energies (eV) and vibrational levels (eV) observed in the HeI photoelectron spectrum of hexafluorobenzene Orbital
Adiabatic
(a)
This work
e1g(p)
Vibration
Adiabatic
Vibration
(b)
9.907 ± 0.002
a2u(p)–Fp
12.580 ± 0.002
b2u(r)–Fr e2g(r)–Fr
13.849 ± 0.002 14.761 ± 0.002
0.192 ± 0.004 0.052 ± 0.004 0.187 ± 0.003 0.067 ± 0.005 0.182 ± 0.002 0.179 ± 0.002 0.171 ± 0.004 –
15.833 ± 0.002
Adiabatic
Vibration
(c)
9.93(6) 12.58 13.84 14.75 15.82
0.180 0.050 0.192 0.066 0.180 0.192 0.060 –
(m1) (m2) (m1) (m2) (m1) (m1) (m2)
9.906 12.577
0.187
13.847 14.760
0.185 0.176 0.053
15.9
m1 = C–F stretch (tot. symm.). m2 = ring breathing mode (tot. symm.). (a) See Ref. [27]. (b) See Ref. [1]. (c) See Ref. [3].
The electronic configuration of the outermost valence shell in the neutral ground state is the following: 2
4
2
4
2
2
. . . ð1b1g Þ ð6e1u Þ ð1a2g Þ ð6e2g Þ ð2b2u Þ ð2a2u Þ ð2e1g Þ
4
e 1 A1g X
This ordering has been determined by photoelectron spectroscopy and calculations [27]. The e1g molecular orbital has been suggested to be a p type (pC–C). The a2u molecular orbital is also a p type, involving the fluorine atoms [27]. The 2b2u molecular orbital has been reported as being a r type. The lowest energy unoccupied molecular orbital is then a p* type. 4. Results 4.1. Ionic states HeI and the HeII photoelectron spectra of C6F6 have been reported by many previous investigators [1–13]. Most of the previous publications, however, were primarily concerned with the energy shifts of the bands with respect to those observed in the photoelectron spectrum of benzene, shifts due to the so-called ‘‘perfluoro’’ effect. Here we shall deal with the interpretation of the fine vibrational features observed in the different bands of the HeI photoelectron spectrum as they provide guidelines for the interpretation of the fine structure of the Rydberg states in photoabsorption. In Table 1, we report the ionisation values measured for the five first bands of the HeI photoelectron spectrum shown in Fig. 1, as well as the energy values for the vibrational features observed, comparisons are made with some data available in the literature. Our data compare very well with those reported by Bieri et al. [3] for the adiabatic ionisation values. Considerable vibrational structure is present in the first four bands. The first and fourth bands deserve special attention as they are subject to Jahn–Teller distortion. It is well known [11] that the Jahn–Teller effect can lead to complicated vibronic structure in photoelectron spectra. Indeed, we observe that, in contrast to the second
Fig. 1. The HeI photoelectron spectrum of hexafluorobenzene. The peaks labelled with a (d) are produced by the Heb line (23.0848 eV). For a complete assignment of the vibrational features see Table 1.
and third bands, the fine structure is poorly resolved in the first band and it does not seem that this apparent lack of resolution can be attributed to rapid dissociation of the ion in its ground state. Indeed, all the fragment ions in the mass spectrum of C6F6 have a rather large appearance energy [22]. We infer from this that the Jahn–Teller distortion plays a role in the shape of the first band of the photoelectron spectrum, inducing the excitation of low wavenumber vibrational modes not resolved in the present experiment. From the theoretical point of view, the vibrational excitation of the ground sate of C6 Fþ 6 has been intensively studied by Sears et al. [28] and Applegate and Miller [29]. There are four e2g modes able to induce Jahn–Teller distortion:3 m015 (1629 cm1/0.202 eV), m016 (1210 cm1/ 0.150 eV), m017 (418 cm1/0.052 eV), and m018 (255 cm1/ 0.032 eV), respectively. Modes 15 and 16 have the largest bond length changes, the first one is predominantly C–C and the second one C–F. Modes 17 and 18 are primarily 3
Herzberg’s notation.
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bending modes, the first one more C–C–C and the second one more C–C–F. The fourth band, of e2g symmetry, presents an overall shape that is somewhat similar to the first one, although the vibrational fine structure is more apparent than in the first band. We distinguish two vibrational progressions. The first one with a spacing of 0.179 eV and a rapid decrease of intensity and a second one with a spacing of 0.171 eV, shifted with respect to the first one with a progressive increase of intensity followed by a decrease. In the absence of any calculations for this state and because of the existence of a Jahn–Teller distortion we cannot suggest an assignment for these two vibrational progressions. As shown in Table 1, the second and the third bands appearing in the photoelectron spectrum also show vibrational features. They can be attributed, in absence of Jahn–Teller distortion to excitation of the two totally symmetrical normal vibration modes m01 and m02 for band two and to m01 only for band three. 4.2. Valence and Rydberg states The absolute photo-absorption cross-section (Fig. 2) is reported from 4.3 to 10.8 eV. The present data extends previously recorded spectra to slightly higher energies [14,15]. The cross-section is dominated by a quite intense absorption centered at 7.11 eV with a local maximum cross-section of 180 Mb although it exhibits a series of broad bands located between 4.3 and 8 eV with considerable fine structure being observed in the region between 8.0 and 10.1 eV. 4.2.1. Valence excited states The valence state region extends from 4.3 to 8.0 eV. The spectrum (Fig. 2) is composed of several broad bands already reported previously using photo-absorption and electron impact under dipolar interaction conditions. The energy values of the band maxima are given in Table 2 where they are also compared with earlier data. Excellent agreement is found for all the features.
Despite the excellent energy resolution in the present work, there is no evidence for any vibrational feature in these absorption bands. The lowest energy band centered at 4.86 eV, only partly resolved from the second band centred at 5.38 eV, corresponds to the excitation of the symmetry forbidden singlet–singlet transition, e 1 A1g 11 B2u . The second broad band located at X 5.38 eV has been labelled in the literature as ‘‘band C’’. Its origin has been largely discussed by Philis et al. [14], Hitchcock et al. [15] and also by Frueholz et al. [19]. One possible assignment is that it is related to the e 1 A1g 11 E2g observed in benzene. An altertransition X native assignment is that it involves excitation of the p electron of a fluorine atom to the unoccupied r* molecular orbital as suggested by Philis et al. [14] and by Hitchcock et al. [15] (Table 2). The third band with a maximum at 5.66 eV is only partly resolved from the transition at 5.38 eV despite the high resolution of our measurements. It has been assigned to a p ! r* type excitation by some authors [30,31], involving r*(e2g). At higher energy, the band centered at 6.39 eV (Fig. 2) has been suggested to be the second p ! p* transition of hexafluorobenzene corresponding to the excitation of the 11B1u state, a symmetry forbidden transition. The most intense absorption (Fig. 2, Table 2) is located at 7.11 eV with a maximum absorption value of 180 Mb. This corresponds to the optically allowed transition to the 1E1u excited state. On the high energy side of the band, three features are only partly resolved at 7.25, 7.36, and 7.7 eV, respectively. The 7.7 eV feature has been observed in previous studies where it has been assigned to a valence state involving a r ! p* transition and the 1A2u state [14]. The weak shoulders around 7.25 and 7.36 eV are also present in the spectrum of Philis et al. [14] and Smith et al. [16] but have not been assigned. 4.2.2. Rydberg state region from 8 to 10.75 eV As shown in Fig. 3, the spectrum is characterized by numerous fine structures, albeit of rather low cross-section. In the present spectra these are slightly better resolved than in the only previously study, reported by Smith et al. [16]. The features have been labelled by numbers to assist in the discussion of their assignments. The corresponding energy values are given in Table 3. The agreement with the values reported by Smith et al. [16] is excellent except for the features located between 8.3 and 8.7 eV. These were only very weakly resolved by Smith et al. [16] study due to their lower resolution. These features may be assigned using the Rydberg formula: En ¼ IE
Fig. 2. The absolute absorption cross-section of hexafluorobenzene in the 4.2–10.8 eV energy range.
R ðn dÞ2
where En is the energy of the experimental band, IE is the ionisation limit of the series, R is the Rydberg constant (13.606 eV), n is the principal quantum number, and d is the quantum defect.
C. Motch et al. / Chemical Physics 328 (2006) 183–189
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Table 2 Energy position (eV) and attribution of the electronic bands observed in the 4–8 eV energy range of the absorption spectrum of hexafluorobenzene Excitation energy (eV)
Attribution
Electron impact (25 eV, h = 80°)
UV photo-absorption
(a)
(b)
(c)
This work
4.80 5.32
4.70 5.39 5.72 6.25 7.00
(4.28) 5.37 5.75 6.40 7.11
4.86 5.38 5.66 6.39 7.11 7.25 7.36 7.70
6.36 7.10
7.70
p ! p*: X1A1g ! 11B2u C band/p ! r* p ! r* p ! p*: X1A1g! 11B1u p ! p*: X1A1g! 1E1u Valence electronic state (?) Valence electronic state (?) r ! p*: 1A1g ! 1A2u
(a) See Ref. [18,19]. (b) See Ref. [15]. (c) See Ref. [16].
Table 3 Energy position (eV) and attribution of the features observed in the 8– 10 eV energy region in the absorption spectrum of hexafluorobenzene
Fig. 3. The absolute absorption cross-section of hexafluorobenzene in the 8.2–10.1 eV energy range. We show our assignments for the vibrational features of the first members of the nd and ns Rydberg series. For a complete assignment, see Table 3.
The first series starting at 8.315 eV (Table 3) has a quantum defect of 0.05 which is consistent with a nd series. The series with the first term at 8.771 eV corresponds to a ns series as suggested by the quantum defect value of 1.05. We expect that the excited electronic states members of the nd Rydberg series converging to the electronic ground state present some Jahn–Teller distortion as it is the case for the ion in its ground electronic state. We have therefore attempted to interpret the vibrational features that are associated with the lowest terms of the series taking into account our interpretation of the photoelectron spectrum for the first band and the calculations of Applegate et al. [29]. As shown in Table 3 and Fig. 3, the data are readily assigned in terms of excitation of m015 , m016 , m017 , and m018 . For the first term of the ns series converging to the first excited electronic state of the cation, we have based our interpretation of the vibrational spacings observed in the corresponding band in the photoelectron spectrum. We have identified two vibrational progressions for m01 and m02 , the two totally symmetrical vibrations.
Feature number
Energy
Assignments
1 2 3 4 5 6 7 8 10 11 13 14 15 17 18 19 20 21 22 23 25 26 27 28 29 30 31 32 9 10 12 15 16 20 24
8.316 8.349 8.371 8.406 8.463 8.510 8.670 8.713 8.837 8.894 9.024 9.063 9.076 9.211 9.253 9.273 9.343 9.407 9.460 9.515 9.619 9.686 9.709 9.732 9.770 9.809 9.871 9.950 8.771 8.894 8.965 9.076 9.164 9.343 9.537
2e1g ! 3d 1m018 ð3dÞ 1m017 ð3dÞ 2m017 ð3dÞ 1m016 ð3dÞ 1m015 ð3dÞ 1m015 þ 1m016 ð3dÞ 2m015 ð3dÞ 3m015 ð3dÞ 2e1g ! 4d 3m015 þ 1m016 ð3dÞ 4m015 ð3dÞ 1m015 ð4dÞ 1m015 þ 1m018 ð4dÞ 1m015 þ 1m017 ð4dÞ 2e1g ! 5d 2m015 ð4dÞ 2m015 þ m017 ð4dÞ 2e1g ! 6d 2e1g ! 7d 2e1g ! 8d 1m018 ð8dÞ 2e1g ! 9d 2e1g ! 10d 2e1g ! 12d
2a2u ! 3s 1m02 ð3sÞ 1m01 ð3sÞ 1m01 þ m02 ð3sÞ 2m01 ð3sÞ 3m01 ð3sÞ 4m01 ð3sÞ
5. Absolute cross-sections and UV photolysis rates Absolute photo-absorption cross-sections have been reported by only two groups. The present data are in excellent agreement with the previous results [14,16]. Such
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assigned to the excitation of non symmetrical normal modes for the electronic states members of the nd series. For the first term of the ns series, we have identified vibrational features that are assigned to the excitation of the two symmetrical normal modes. The measured photo-absorption cross-sections have been used to estimate photolysis rates in the terrestrial atmosphere and the lifetime to solar photolysis shown to be long in the lower atmosphere. Acknowledgements
Fig. 4. Total photolysis rate and local lifetime of hexafluorobenzene between 0 and 50 km.
absolute cross-sections may be used to derive the atmospheric lifetime for a given molecular species to photolysis. The photolysis rates of C6F6 may be evaluated as the product of the solar actinic flux [32] and molecular photo-absorption cross-section at different altitudes and wavelengths. The total rates shown in Fig. 4 are the summation over the wavelength range of these partial rates assuming that the quantum yield for photo-dissociation is unity. The local lifetime to photolysis at a given altitude, also shown in Fig. 4, is then simply the reciprocal of the total photolysis rate. The lifetimes calculated are for a molecule with fixed altitude in a sunlit, clear sky atmosphere. Our results show that the local photolysis lifetime of C6F6 varies from about 280 years around 20 km to a few days at 40–50 km. This indicates that C6F6 will be stable to solar photolysis in the troposphere but can be broken up fairly easily at high altitude leading probably to Cm Fn radicals and F atoms in the terrestrial stratosphere with potential for ozone depletion. Similarly if C6F6 has a strong infrared absorption cross-section, such a long tropospheric lifetime may lead to a significant GWP. However C6F6 may be reactive with OH radicals and/or soluble in water leading to ‘washout’ significantly reducing the atmospheric lifetime of C6F6. 6. Conclusions In the present study we have been able to provide a new interpretation of the vibrational fine structure for the first and the fourth electronic bands of the HeI photoelectron spectrum of hexafluorobenzene. Our interpretation takes into account the most recent ab initio calculations and the existence of Jahn–Teller distortion. From this we propose excitation of non symmetrical normal modes to be responsible for the observed structure. We also suggest a new interpretation, based on the photoelectron spectrum, of the absorption in the 8.0–10.1 eV Rydberg states region, where the vibrational structure is
The ‘‘Patrimoine de l’Universite´ de Lie`ge’’, the ‘‘Fonds National de la Recherche Scientifique’’, and the ‘‘Fonds de la Recherche Fondamentale Collective’’ (Contract No. 2.4503.04 F) have supported this research. M.J.H.F. acknowledge the ‘‘Fonds National de la Recherche Scientifique’’ for her research position. P.L.V. acknowledges the honorary research fellow position at University College London, a visiting fellowship position at CEMOS, The Open University, UK and, together with M.-J.H.-F, financial support from the Portuguese–Belgian joint collaboration. The authors wish to acknowledge the beam time at ˚ rhus, Denthe ISA synchrotron facility, University of A mark, and the support from the European Commission for access to research infrastructure action of the improving human potential programme. Some of this work forms part of the EU network programme EPIC, HPRN-CT2002-00179. References [1] C.R. Brundle, M.B. Robin, N.A. Kuebler, J. Am. Chem. Soc. 94 (1972) 1466. [2] I.D. Clark, D.C. Frost, J. Am. Chem. Soc. 89 (1967) 244. ˚ sbrink, W. von Niessen, J. Electron. Spectrosc. Relat. [3] G. Bieri, L. A Phenom. 23 (1981) 281. [4] D.G. Streets, D.G. Ceasar, Mol. Phys. 26 (1973) 1037. [5] T. Kobayashi, S. Nagakura, J. Electron. Spectrosc. Relat. Phenom. 7 (1975) 187. [6] T. Kobayashi, Phys. Lett. 69 (1978) 105. [7] B.C. Trudel, S.J.W. Price, Can. J. Chem. 57 (1979) 2256. [8] J.A. Sell, D.M. Mintz, A. Kupperman, Chem. Phys. Lett. 58 (1978) 601. [9] J. Bastide, D. Hall, E. Heibronner, J.P. Maier, J. Electron. Spectrosc. Relat. Phenom. 16 (1979) 205. [10] J.P. Maier, F. Thommen, Chem. Phys. 57 (1981) 319. [11] J.W. Rabalais, T. Bergmark, L.O. Werme, L. Karlsson, K. Siegbahn, Phys. Scripta 3 (1971) 13. [12] A.W. Potts, W.C. Price, D.G. Streets, T.A. Williams, Faraday Discuss. Chem. Soc. 54 (1972) 168. [13] W.C. Price, D.A.W. Potts, T.A. Williams, Chem. Phys. Lett. 37 (1976) 12. [14] J. Philis, A. Bolovinos, G. Andritsopoulos, E. Pantos, P. Tsekeris, J. Phys B: At. Mol. Phys. 14 (1981) 3621. [15] A.P. Hitchcock, P. Fisher, A. Gedanken, M.B. Robin, J. Phys. Chem. 91 (1987) 531. [16] D.R. Smith, J.W. Raymonda, Chem. Phys. Lett. 12 (1971) 269. [17] R. Gilbert, P. Sauvageau, C. Sandorfy, Can. J. Chem. 50 (1972) 543. [18] R.P. Frueholz, W.M. Flicker, O.A. Mosher, A. Kuppermann, Phys. Lett. 52 (1977) 86.
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Electronic structure of hexafluorobenzene by high-resolution vacuum ultraviolet photo-absorption and He(I) photoelectron spectroscopy C. Motch a, A. Giuliani a,*, J. Delwiche a, P. Lima˜o-Vieira b, N.J. Mason c, S.V. Hoffmann d, M.-J. Hubin-Franskin a,1 b
a Laboratoire de Spectroscopie d’E´lectrons diffuse´s, Universite´ de Lie`ge, Institut de Chimie B6c, B-4000 Lie`ge 1, Belgium Laborato´rio de Coliso˜es Ato´micas, e Moleculares, Departamento de Fı´sica, CEFITEC, FCT-Universidade Nova de Lisboa, 2829-516 Caparica, Portugal c Centre of Optical and Molecular Sciences, Department of Physics and Astrophysics, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom d Institute for Storage Ring Facilities, University of A˚rhus, Ny Munkegade, DK-8000 A˚rhus C, Denmark
Received 25 March 2006; accepted 25 May 2006 Available online 29 June 2006
Abstract The VUV photo-absorption spectrum of hexafluorobenzene, C6F6, recorded using the synchrotron radiation in the 4–10.75 eV photon range exhibits broad bands between 4 and 8 eV, and fine features above 8 eV. The broad bands are likely to be due to valence states dissociating into neutral fragments. They have been assigned to a p–p* transition, two p–r* transitions and one r–p* transition. A small shoulder observed at 7.36 eV, confirms earlier observations and is evidence of a further valence excited electronic state. Above 8.3 eV numerous fine structures are observed and the features are assigned by comparison with the high-resolution HeI photoelectron spectrum to a nd Rydberg series converging to the ground ionic state and a ns series associated with the first electronic excited state limit. The first two nd terms show vibrational structure. The origin of the vibrational structure is discussed in terms of Jahn–Teller distortion. For the 3s term, excitation of the vibration normal mode m02 is reported for the first time with a value of 0.066 eV, a value consistent with that of the ion. The HeI photoelectron spectrum of C6F6 has also been revisited and a new interpretation of the vibrational structure for the first and the fourth electronic bands is proposed. Finally, using a previously derived solar flux model, the atmospheric lifetime relative to photolysis is estimated and the consequences for atmospheric chemistry briefly discussed. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Photoabsorption; VUV; Synchrotron radiation; Absolute cross-section; Photoelectron spectroscopy; Life time; Photolysis rate
1. Introduction Hexafluorobenzene, C6F6, with cyclo-C4F8 and C2F4, has been proposed as an alternative feed gas in the plasma etching industry. Any replacement feed gas must have a low global warming potential (GWP) i.e. it must not absorb infrared radiation in those regions of the Earth’s atmosphere known as the IR windows. Nor may it have *
Corresponding author. Fax: +32 4 3662941. E-mail address: [email protected] (M.-J. Hubin-Franskin). 1 Present address: CNRS, Laboratoire de Spectrome´trie de masse, Institut de Chimie des Substances Naturelles, avenue de la Terrasse, Gifsur-Yvette CEDEX F-91198, France. 0301-0104/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2006.05.032
a large ozone depletion potential (ODP) i.e. it should not be easily photolysed by sunlight to yield reactive radicals that can catalytically destroy ozone in the terrestrial stratosphere. Accordingly there is much interest in the physical and chemical properties of C6F6. It is particularly important to identify the electronic states of C6F6 since the photolysis rate in the terrestrial atmosphere will be determined by the excitation energy and symmetry of the lowest excited states, while the formation of CFx radicals needed in semiconductor plasma reactors for silicon etching will be dependent upon the dissociation dynamics of these same states. However, to date there is relatively little data, either experimental or theoretical, on either the spectroscopy of the excited states of the neutral hexafluorobenzene
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molecule or those of the positive ion. Several photoelectron spectra [1–13] have been published but at lower resolution than in the present work but there are only four previous photo-absorption studies [14–17] and two spectroscopic studies using electron impact under electric dipolar and non dipolar interaction conditions [18,19], the latter providing the first data on the lowest energy triplet states of hexafluorobenzene. A single study has been reported of the Rydberg states of C6F6 [16]. Part of the fine structure in the 8–11.5 eV region has been assigned to two Rydberg series converging to the two lowest energy ionisation limits and vibrational structure has been observed in the first two terms of the series. The present study therefore aims to provide a more comprehensive study of the electronic spectroscopy of C6F6 and provides some important new high-resolution quantitative data required to model the behaviour of C6F6 in plasma reactors. 2. Experimental methods 2.1. The photoelectron spectrometer The photoelectron spectrometer has been previously described in detail [20,21]. Briefly, HeI photons (58.4 nm) are produced by a dc discharge in helium and the spectra are recorded by sweeping a retarding voltage in steps of 1 meV between the ionisation chamber and the entrance slit of a 180° hemispherical electrostatic analyzer, operating in a constant pass energy mode. The measured spectra are corrected for the transmission of the analyzing system. The ionisation energy scale was calibrated using xenon with the energetic values given in the NIST ChemWebbook [22] (2P3/2 = 12.123 eV and 2P1/2 = 13.436 eV). The accuracy of the energy scale is therefore estimated to be ±0.002 eV. The overall resolution is about 20 meV as measured as the full width at half maximum (FWHM) of Xe+ peaks.
The radiation transmitted through the empty cell (I0) was initially recorded over the limited (11 nm) range. The sample was then introduced into the cell and two scans of transmitted radiation (It) were recorded. The cell was subsequently evacuated and a second (I0) was recorded. The mean values of each of the (I0) and (It) values were then used in the Beer– Lambert law to evaluate the cross-section r: I t ¼ I 0 expðrNxÞ where N is the target gas number density and x is the path length. This averaging procedure compensated effectively for the decay of the radiation intensity as the storage ring current decayed. In the figures presented in this work the spectra are presented on an energy (eV) abscissa. The accuracy of the cross-section is estimated to be better than ±5%. 2.3. Sample Hexafluorobenzene is a liquid at room temperature and under atmospheric pressure. It was provided by Janssens Chemicals with a stated purity of 99%. It was used without further purification although repetitive pump-thaw cycles were performed before introducing the sample into the gas cell to remove any dissolved air. There was no evidence of any impurities in the measured spectra. 3. Properties and electronic structure of hexafluorobenzene The structural constants for hexafluorobenzene, determined experimentally for the gas phase by Almenningen et al. [24] and calculated by Almlo¨f and Faegri [25], are summarized as follows: ˚ Experimental : C–C ¼ 1:391 0:007 A ˚ C–F ¼ 1:327 0:007 A ˚ Calculated : C–C ¼ 1:394 A ˚ C–F ¼ 1:349 A
2.2. The gas phase VUV absorption apparatus Photoabsorption spectrum was recorded at the UV1 ˚ rhus beamline [23] of the storage ring ASTRID at the A University, in Denmark. The normal incidence monochromator was operated using the 2000 l/mm grating with an overall resolution (full width at half maximum, FWHM) of better than 0.1 nm. Synchrotron radiation from the monochromator passed through a LiF window into a gas cell of length 25 cm. A Baratron capacitance manometer monitored the sample pressure (less than 0.5 mbar), and the intensity of the radiation exiting the cell through a second window was detected using an UV enhanced photomultiplier tube (Electron Tube Limited, type 9406B). The spectral range extended from about 5 eV (248 nm) to the LiF cut-off at 10.75 eV (115 nm). This range was covered in sections of 11 nm each, using steps of 0.1 nm; this step size was found to be sufficiently small to resolve fine structure in the measured cross-section curves.
In the gas phase, the hexafluorobenzene molecule is planar and accordingly belongs to the D6h symmetry class in its electronic ground state. This is in contrast with C6Cl6 and C6Br6 that have been found by electron diffraction [26] to be non planar with three alternate halogen atoms above the plane of the ring and three below, thus belonging to the D3d symmetry group. The molecule has 30 normal vibration modes of which there are twenty fundamentals. Out of the twelve different vibrational species available for a D6h molecule, only ten are actually observed. They are divided amongst the symmetry classes as indicated by:2 2a1g ; 1a2g ; 1a2u ; 2b1u ; 2b2g ; 2b2u ; 1e1g ; 3e1u ; 4e2g ; 2e2u The first two values have been inverted to be consistent with Brundle et al. [1].
2
Assuming that the C2 axis goes through a C–F group.
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Table 1 Ionisation energies (eV) and vibrational levels (eV) observed in the HeI photoelectron spectrum of hexafluorobenzene Orbital
Adiabatic
(a)
This work
e1g(p)
Vibration
Adiabatic
Vibration
(b)
9.907 ± 0.002
a2u(p)–Fp
12.580 ± 0.002
b2u(r)–Fr e2g(r)–Fr
13.849 ± 0.002 14.761 ± 0.002
0.192 ± 0.004 0.052 ± 0.004 0.187 ± 0.003 0.067 ± 0.005 0.182 ± 0.002 0.179 ± 0.002 0.171 ± 0.004 –
15.833 ± 0.002
Adiabatic
Vibration
(c)
9.93(6) 12.58 13.84 14.75 15.82
0.180 0.050 0.192 0.066 0.180 0.192 0.060 –
(m1) (m2) (m1) (m2) (m1) (m1) (m2)
9.906 12.577
0.187
13.847 14.760
0.185 0.176 0.053
15.9
m1 = C–F stretch (tot. symm.). m2 = ring breathing mode (tot. symm.). (a) See Ref. [27]. (b) See Ref. [1]. (c) See Ref. [3].
The electronic configuration of the outermost valence shell in the neutral ground state is the following: 2
4
2
4
2
2
. . . ð1b1g Þ ð6e1u Þ ð1a2g Þ ð6e2g Þ ð2b2u Þ ð2a2u Þ ð2e1g Þ
4
e 1 A1g X
This ordering has been determined by photoelectron spectroscopy and calculations [27]. The e1g molecular orbital has been suggested to be a p type (pC–C). The a2u molecular orbital is also a p type, involving the fluorine atoms [27]. The 2b2u molecular orbital has been reported as being a r type. The lowest energy unoccupied molecular orbital is then a p* type. 4. Results 4.1. Ionic states HeI and the HeII photoelectron spectra of C6F6 have been reported by many previous investigators [1–13]. Most of the previous publications, however, were primarily concerned with the energy shifts of the bands with respect to those observed in the photoelectron spectrum of benzene, shifts due to the so-called ‘‘perfluoro’’ effect. Here we shall deal with the interpretation of the fine vibrational features observed in the different bands of the HeI photoelectron spectrum as they provide guidelines for the interpretation of the fine structure of the Rydberg states in photoabsorption. In Table 1, we report the ionisation values measured for the five first bands of the HeI photoelectron spectrum shown in Fig. 1, as well as the energy values for the vibrational features observed, comparisons are made with some data available in the literature. Our data compare very well with those reported by Bieri et al. [3] for the adiabatic ionisation values. Considerable vibrational structure is present in the first four bands. The first and fourth bands deserve special attention as they are subject to Jahn–Teller distortion. It is well known [11] that the Jahn–Teller effect can lead to complicated vibronic structure in photoelectron spectra. Indeed, we observe that, in contrast to the second
Fig. 1. The HeI photoelectron spectrum of hexafluorobenzene. The peaks labelled with a (d) are produced by the Heb line (23.0848 eV). For a complete assignment of the vibrational features see Table 1.
and third bands, the fine structure is poorly resolved in the first band and it does not seem that this apparent lack of resolution can be attributed to rapid dissociation of the ion in its ground state. Indeed, all the fragment ions in the mass spectrum of C6F6 have a rather large appearance energy [22]. We infer from this that the Jahn–Teller distortion plays a role in the shape of the first band of the photoelectron spectrum, inducing the excitation of low wavenumber vibrational modes not resolved in the present experiment. From the theoretical point of view, the vibrational excitation of the ground sate of C6 Fþ 6 has been intensively studied by Sears et al. [28] and Applegate and Miller [29]. There are four e2g modes able to induce Jahn–Teller distortion:3 m015 (1629 cm1/0.202 eV), m016 (1210 cm1/ 0.150 eV), m017 (418 cm1/0.052 eV), and m018 (255 cm1/ 0.032 eV), respectively. Modes 15 and 16 have the largest bond length changes, the first one is predominantly C–C and the second one C–F. Modes 17 and 18 are primarily 3
Herzberg’s notation.
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bending modes, the first one more C–C–C and the second one more C–C–F. The fourth band, of e2g symmetry, presents an overall shape that is somewhat similar to the first one, although the vibrational fine structure is more apparent than in the first band. We distinguish two vibrational progressions. The first one with a spacing of 0.179 eV and a rapid decrease of intensity and a second one with a spacing of 0.171 eV, shifted with respect to the first one with a progressive increase of intensity followed by a decrease. In the absence of any calculations for this state and because of the existence of a Jahn–Teller distortion we cannot suggest an assignment for these two vibrational progressions. As shown in Table 1, the second and the third bands appearing in the photoelectron spectrum also show vibrational features. They can be attributed, in absence of Jahn–Teller distortion to excitation of the two totally symmetrical normal vibration modes m01 and m02 for band two and to m01 only for band three. 4.2. Valence and Rydberg states The absolute photo-absorption cross-section (Fig. 2) is reported from 4.3 to 10.8 eV. The present data extends previously recorded spectra to slightly higher energies [14,15]. The cross-section is dominated by a quite intense absorption centered at 7.11 eV with a local maximum cross-section of 180 Mb although it exhibits a series of broad bands located between 4.3 and 8 eV with considerable fine structure being observed in the region between 8.0 and 10.1 eV. 4.2.1. Valence excited states The valence state region extends from 4.3 to 8.0 eV. The spectrum (Fig. 2) is composed of several broad bands already reported previously using photo-absorption and electron impact under dipolar interaction conditions. The energy values of the band maxima are given in Table 2 where they are also compared with earlier data. Excellent agreement is found for all the features.
Despite the excellent energy resolution in the present work, there is no evidence for any vibrational feature in these absorption bands. The lowest energy band centered at 4.86 eV, only partly resolved from the second band centred at 5.38 eV, corresponds to the excitation of the symmetry forbidden singlet–singlet transition, e 1 A1g 11 B2u . The second broad band located at X 5.38 eV has been labelled in the literature as ‘‘band C’’. Its origin has been largely discussed by Philis et al. [14], Hitchcock et al. [15] and also by Frueholz et al. [19]. One possible assignment is that it is related to the e 1 A1g 11 E2g observed in benzene. An altertransition X native assignment is that it involves excitation of the p electron of a fluorine atom to the unoccupied r* molecular orbital as suggested by Philis et al. [14] and by Hitchcock et al. [15] (Table 2). The third band with a maximum at 5.66 eV is only partly resolved from the transition at 5.38 eV despite the high resolution of our measurements. It has been assigned to a p ! r* type excitation by some authors [30,31], involving r*(e2g). At higher energy, the band centered at 6.39 eV (Fig. 2) has been suggested to be the second p ! p* transition of hexafluorobenzene corresponding to the excitation of the 11B1u state, a symmetry forbidden transition. The most intense absorption (Fig. 2, Table 2) is located at 7.11 eV with a maximum absorption value of 180 Mb. This corresponds to the optically allowed transition to the 1E1u excited state. On the high energy side of the band, three features are only partly resolved at 7.25, 7.36, and 7.7 eV, respectively. The 7.7 eV feature has been observed in previous studies where it has been assigned to a valence state involving a r ! p* transition and the 1A2u state [14]. The weak shoulders around 7.25 and 7.36 eV are also present in the spectrum of Philis et al. [14] and Smith et al. [16] but have not been assigned. 4.2.2. Rydberg state region from 8 to 10.75 eV As shown in Fig. 3, the spectrum is characterized by numerous fine structures, albeit of rather low cross-section. In the present spectra these are slightly better resolved than in the only previously study, reported by Smith et al. [16]. The features have been labelled by numbers to assist in the discussion of their assignments. The corresponding energy values are given in Table 3. The agreement with the values reported by Smith et al. [16] is excellent except for the features located between 8.3 and 8.7 eV. These were only very weakly resolved by Smith et al. [16] study due to their lower resolution. These features may be assigned using the Rydberg formula: En ¼ IE
Fig. 2. The absolute absorption cross-section of hexafluorobenzene in the 4.2–10.8 eV energy range.
R ðn dÞ2
where En is the energy of the experimental band, IE is the ionisation limit of the series, R is the Rydberg constant (13.606 eV), n is the principal quantum number, and d is the quantum defect.
C. Motch et al. / Chemical Physics 328 (2006) 183–189
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Table 2 Energy position (eV) and attribution of the electronic bands observed in the 4–8 eV energy range of the absorption spectrum of hexafluorobenzene Excitation energy (eV)
Attribution
Electron impact (25 eV, h = 80°)
UV photo-absorption
(a)
(b)
(c)
This work
4.80 5.32
4.70 5.39 5.72 6.25 7.00
(4.28) 5.37 5.75 6.40 7.11
4.86 5.38 5.66 6.39 7.11 7.25 7.36 7.70
6.36 7.10
7.70
p ! p*: X1A1g ! 11B2u C band/p ! r* p ! r* p ! p*: X1A1g! 11B1u p ! p*: X1A1g! 1E1u Valence electronic state (?) Valence electronic state (?) r ! p*: 1A1g ! 1A2u
(a) See Ref. [18,19]. (b) See Ref. [15]. (c) See Ref. [16].
Table 3 Energy position (eV) and attribution of the features observed in the 8– 10 eV energy region in the absorption spectrum of hexafluorobenzene
Fig. 3. The absolute absorption cross-section of hexafluorobenzene in the 8.2–10.1 eV energy range. We show our assignments for the vibrational features of the first members of the nd and ns Rydberg series. For a complete assignment, see Table 3.
The first series starting at 8.315 eV (Table 3) has a quantum defect of 0.05 which is consistent with a nd series. The series with the first term at 8.771 eV corresponds to a ns series as suggested by the quantum defect value of 1.05. We expect that the excited electronic states members of the nd Rydberg series converging to the electronic ground state present some Jahn–Teller distortion as it is the case for the ion in its ground electronic state. We have therefore attempted to interpret the vibrational features that are associated with the lowest terms of the series taking into account our interpretation of the photoelectron spectrum for the first band and the calculations of Applegate et al. [29]. As shown in Table 3 and Fig. 3, the data are readily assigned in terms of excitation of m015 , m016 , m017 , and m018 . For the first term of the ns series converging to the first excited electronic state of the cation, we have based our interpretation of the vibrational spacings observed in the corresponding band in the photoelectron spectrum. We have identified two vibrational progressions for m01 and m02 , the two totally symmetrical vibrations.
Feature number
Energy
Assignments
1 2 3 4 5 6 7 8 10 11 13 14 15 17 18 19 20 21 22 23 25 26 27 28 29 30 31 32 9 10 12 15 16 20 24
8.316 8.349 8.371 8.406 8.463 8.510 8.670 8.713 8.837 8.894 9.024 9.063 9.076 9.211 9.253 9.273 9.343 9.407 9.460 9.515 9.619 9.686 9.709 9.732 9.770 9.809 9.871 9.950 8.771 8.894 8.965 9.076 9.164 9.343 9.537
2e1g ! 3d 1m018 ð3dÞ 1m017 ð3dÞ 2m017 ð3dÞ 1m016 ð3dÞ 1m015 ð3dÞ 1m015 þ 1m016 ð3dÞ 2m015 ð3dÞ 3m015 ð3dÞ 2e1g ! 4d 3m015 þ 1m016 ð3dÞ 4m015 ð3dÞ 1m015 ð4dÞ 1m015 þ 1m018 ð4dÞ 1m015 þ 1m017 ð4dÞ 2e1g ! 5d 2m015 ð4dÞ 2m015 þ m017 ð4dÞ 2e1g ! 6d 2e1g ! 7d 2e1g ! 8d 1m018 ð8dÞ 2e1g ! 9d 2e1g ! 10d 2e1g ! 12d
2a2u ! 3s 1m02 ð3sÞ 1m01 ð3sÞ 1m01 þ m02 ð3sÞ 2m01 ð3sÞ 3m01 ð3sÞ 4m01 ð3sÞ
5. Absolute cross-sections and UV photolysis rates Absolute photo-absorption cross-sections have been reported by only two groups. The present data are in excellent agreement with the previous results [14,16]. Such
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assigned to the excitation of non symmetrical normal modes for the electronic states members of the nd series. For the first term of the ns series, we have identified vibrational features that are assigned to the excitation of the two symmetrical normal modes. The measured photo-absorption cross-sections have been used to estimate photolysis rates in the terrestrial atmosphere and the lifetime to solar photolysis shown to be long in the lower atmosphere. Acknowledgements
Fig. 4. Total photolysis rate and local lifetime of hexafluorobenzene between 0 and 50 km.
absolute cross-sections may be used to derive the atmospheric lifetime for a given molecular species to photolysis. The photolysis rates of C6F6 may be evaluated as the product of the solar actinic flux [32] and molecular photo-absorption cross-section at different altitudes and wavelengths. The total rates shown in Fig. 4 are the summation over the wavelength range of these partial rates assuming that the quantum yield for photo-dissociation is unity. The local lifetime to photolysis at a given altitude, also shown in Fig. 4, is then simply the reciprocal of the total photolysis rate. The lifetimes calculated are for a molecule with fixed altitude in a sunlit, clear sky atmosphere. Our results show that the local photolysis lifetime of C6F6 varies from about 280 years around 20 km to a few days at 40–50 km. This indicates that C6F6 will be stable to solar photolysis in the troposphere but can be broken up fairly easily at high altitude leading probably to Cm Fn radicals and F atoms in the terrestrial stratosphere with potential for ozone depletion. Similarly if C6F6 has a strong infrared absorption cross-section, such a long tropospheric lifetime may lead to a significant GWP. However C6F6 may be reactive with OH radicals and/or soluble in water leading to ‘washout’ significantly reducing the atmospheric lifetime of C6F6. 6. Conclusions In the present study we have been able to provide a new interpretation of the vibrational fine structure for the first and the fourth electronic bands of the HeI photoelectron spectrum of hexafluorobenzene. Our interpretation takes into account the most recent ab initio calculations and the existence of Jahn–Teller distortion. From this we propose excitation of non symmetrical normal modes to be responsible for the observed structure. We also suggest a new interpretation, based on the photoelectron spectrum, of the absorption in the 8.0–10.1 eV Rydberg states region, where the vibrational structure is
The ‘‘Patrimoine de l’Universite´ de Lie`ge’’, the ‘‘Fonds National de la Recherche Scientifique’’, and the ‘‘Fonds de la Recherche Fondamentale Collective’’ (Contract No. 2.4503.04 F) have supported this research. M.J.H.F. acknowledge the ‘‘Fonds National de la Recherche Scientifique’’ for her research position. P.L.V. acknowledges the honorary research fellow position at University College London, a visiting fellowship position at CEMOS, The Open University, UK and, together with M.-J.H.-F, financial support from the Portuguese–Belgian joint collaboration. The authors wish to acknowledge the beam time at ˚ rhus, Denthe ISA synchrotron facility, University of A mark, and the support from the European Commission for access to research infrastructure action of the improving human potential programme. Some of this work forms part of the EU network programme EPIC, HPRN-CT2002-00179. References [1] C.R. Brundle, M.B. Robin, N.A. Kuebler, J. Am. Chem. Soc. 94 (1972) 1466. [2] I.D. Clark, D.C. Frost, J. Am. Chem. Soc. 89 (1967) 244. ˚ sbrink, W. von Niessen, J. Electron. Spectrosc. Relat. [3] G. Bieri, L. A Phenom. 23 (1981) 281. [4] D.G. Streets, D.G. Ceasar, Mol. Phys. 26 (1973) 1037. [5] T. Kobayashi, S. Nagakura, J. Electron. Spectrosc. Relat. Phenom. 7 (1975) 187. [6] T. Kobayashi, Phys. Lett. 69 (1978) 105. [7] B.C. Trudel, S.J.W. Price, Can. J. Chem. 57 (1979) 2256. [8] J.A. Sell, D.M. Mintz, A. Kupperman, Chem. Phys. Lett. 58 (1978) 601. [9] J. Bastide, D. Hall, E. Heibronner, J.P. Maier, J. Electron. Spectrosc. Relat. Phenom. 16 (1979) 205. [10] J.P. Maier, F. Thommen, Chem. Phys. 57 (1981) 319. [11] J.W. Rabalais, T. Bergmark, L.O. Werme, L. Karlsson, K. Siegbahn, Phys. Scripta 3 (1971) 13. [12] A.W. Potts, W.C. Price, D.G. Streets, T.A. Williams, Faraday Discuss. Chem. Soc. 54 (1972) 168. [13] W.C. Price, D.A.W. Potts, T.A. Williams, Chem. Phys. Lett. 37 (1976) 12. [14] J. Philis, A. Bolovinos, G. Andritsopoulos, E. Pantos, P. Tsekeris, J. Phys B: At. Mol. Phys. 14 (1981) 3621. [15] A.P. Hitchcock, P. Fisher, A. Gedanken, M.B. Robin, J. Phys. Chem. 91 (1987) 531. [16] D.R. Smith, J.W. Raymonda, Chem. Phys. Lett. 12 (1971) 269. [17] R. Gilbert, P. Sauvageau, C. Sandorfy, Can. J. Chem. 50 (1972) 543. [18] R.P. Frueholz, W.M. Flicker, O.A. Mosher, A. Kuppermann, Phys. Lett. 52 (1977) 86.
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