Ionization energies for valence electrons of CF3SF5 were measured in the 12-35 eV region by means of angle-resolved photoelectron spectroscopy. The observed ionization energies were basically consistent with previous ones measured by threshold photoelectron and HeI photoelectron spectroscopy, and the ionization levels at 22.60, 26.92 and 27.86 eV were newly observed. Asymmetry parameter derived at hν = 684.7 eV was a constant of β = 1.02 ± 0.04 in the ionization energy lower than 20 eV, suggesting that lone pair electron of F atom is probably released through 2p → εd like electron emission. Characters of the 22 valence bands were elucidated with the help of ab initio calculations and of the nature of valence orbitals for CF4 and SF

Ibuki, T.

Morishita, Y.

Nagaoka, S.

Okada, Kazumasa

Suzuki, I.H.

Takemoto, M.

Tamenori, Y.

Journal of Electron Spectroscopy and Related Phenomena

Hiroshima University Institutional Repository

 - 1 - On the ionization energy of CF3SF5 in the valence region measured by angle-resolved photoelectron spectroscopy  T. Ibuki a*, Y. Tamenori b, K. Okada c, M. Takemoto d, S. Nagaoka d, Y. Morishita e, I.H. Suzuki e  a Kyoto University of Education, Fushimi-ku, Kyoto 612-8522, Japan b SPring-8/Japan Synchrotron Radiation Research Institute, Mikazuki, Hyogo 679-5198, Japan c Department of Chemistry, Hiroshima University, Higashi-Hiroshima 739-8526, Japan d Department of Chemistry, Ehime University, Matsuyama 790-8577, Japan  e National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8568, Japan  Received  Keywords: Angle-resolved photoelectron spectroscopy, asymmetry parameter, ionization energy, valence orbital  Abstract  Ionization energies for valence electrons of CF3SF5 were measured in the 12−35 eV region by means of angle-resolved photoelectron spectroscopy. The observed ionization energies were basically consistent with previous ones measured by threshold photoelectron and HeI photoelectron spectroscopy, and the ionization levels at 22.60, 26.92 and 27.86 eV were newly observed. Asymmetry parameter derived at hν = 684.7 eV was a constant of β = 1.02 ± 0.04 in the ionization energy lower than 20 eV, suggesting that lone pair electron of F atom is probably released through 2p → εd like electron emission. Characters of the 22 valence bands were elucidated with the help of ab initio calculations and of the nature of valence orbitals for CF4 and SF6.                                                    * Corresponding author: Tel and Fax: +81-774-27-2924, e-mail: ibuki@kyokyo-u.ac.jp.  - 2 - Introduction  Trifluoromethyl sulfur pentafluoride, CF3SF5, is a polyatomic molecule detected in the terrestrial atmosphere in 1999. The molecule has the highest global warming potential of ≅22000 and is estimated to live in the atmosphere on the order of 1000 years by measuring infrared absorption [1]. Photoabsorption data are the most essential for understanding photochemistry of CF3SF5 in the atmosphere. Total photoabsorption cross sections were measured in the 4−26 eV region by means of photoabsorption [2-6] and electron energy loss spectroscopy [6, 7]. The onset of photoabsorption is around 6 eV and then the cross section increases up to ≅120 Mb at 23 eV with several strong peaks [2, 5-7]. Fragment ion with the lowest appearance energy has been reported to be CF3+ at 12.92 ± 0.18 eV [8]. Assignment for the photoabsorption bands still remains open to question since data for ionization energies (IEs) of the valence electrons of CF3SF5 are limited. To the best of our knowledge two papers have been published for the valence IEs: Chim et al [8] employed threshold photoelectron spectroscopy (TPES) and reported eight IE levels up to 25 eV. Limão-Vieira et al [6] recorded a HeI (21.22 eV) photoelectron spectrum with ten IE values and then assigned the photoabsorption peaks to Rydberg transitions. The IEs of the two papers are, however, not wholly in agreement. In this paper we present IEs of the valence electrons for CF3SF5 measured by angle-resolved photoelectron spectroscopy at photon energy of 684.7 eV, being just before the F K-edge absorption. Details of photoabsorption cross sections and decay pathways for the C, F and S core-excited CF3SF5 were already reported in previous publications［9, 10]. Photoelectron spectroscopy measured at high photon energy has advantage for characterization of valence molecular orbitals (MOs) though the photoabsorption cross section is small: Kinetic energies of photoelectrons released from valence MOs are high and lie close, and thus detection efficiency of the electrons is hardly affected by the kinetic energies. Photoexcitation by high photon energy is free from resonance excitation of a valence electron to give electron emission. Consequently peak intensity of photoelectron with high kinetic energy is proportional to the number of electrons in a valence MO. We think that the IE values for CF3SF5 obtained by the present method different from the previous ones [6, 8] are useful to evaluate them more reliably.     - 3 - 2. Experimental and MO calculations  The light source was synchrotron radiation (SR) from a high resolution monochromator with varied-line-space plane gratings at the c-branch of the soft X-ray photochemistry beamline BL27SU of the SPring-8 facility [11]. Three gratings were installed in the monochromator to cover photon energies of 180−2800 eV. The dispersed photon flux was monitored by measuring the drain current of the post-focusing mirror coated by gold. The radiation from a Figure-8 undulator was linearly polarized either in the plane of the electron storage ring (the 1st order harmonic) or in the plane perpendicular to the ring (the 0.5-th order harmonic) [12]. Experimental setup for measuring photoelectron spectra was described in a previous paper [13]. In brief, an electron energy analyzer of Gammadata Scienta SES-2002 was used for the angle-resolved electron emission measurements by changing the undulator gap from the 1st order to the 0.5-th order harmonic without rotating the electron energy analyzer. Energy scale of the electron spectrometer was calibrated by measuring the M4N4,5N4,5 and M5N4,5N4,5 Auger electron emissions from xenon [14]. The resolution of the energy analyzer was set to 300 meV, and that for the exciting photon was 150 meV. Commercial sample of CF3SF5 with a stated purity of >99% (SynQuest Lab.) was used without further purification. The base pressure of the electron energy analyzer was lower than 1 × 10-7 Pa, and the sample pressure was kept at ≅7 × 10-4 Pa during experiments. Ab initio self-consistent field calculations were carried out for valence molecular orbitals with the Gaussian 03 program [15]. The basis set used in the present calculation was 6-311G*.    3. Results and discussion  Photoelectron spectrum (PES) at photon energy of 684.7 eV is shown in Fig. 1 where the scales of x-axis are given by kinetic energy at the top and by ionization energy in the bottom. Panel a shows the PES measured by detecting photoelectrons emitted into the direction parallel to the electric vector of SR (the 1st order harmonic and denoted by 0°-spectrum hereafter) and panel b does that at the 0.5-th order harmonic (90°-spectrum). The 0°- and 90°-spectra were normalized by the photon flux monitored. The experimental data are given  - 4 - by thick dots. Thin solid curves in the 0°-spectrum are the peaks deconvoluted by a least-squares curve fitting with the Voigt functions, and the thick one is the best fitted curve. The peaks with kinetic energies of 657−671 eV shifted in parallel with an increase in the exciting photon energy, and then we assigned them to valence photoelectrons. The baseline given by a dotted line in the panel a originates in molecular Auger electrons with kinetic energies of 650−660 eV. Panel c is the asymmetry parameter (β), being deduced by use of the experimental intensities I(0) and I(90) in the 0°- and 90°-spectra, respectively:  )90(2)0()]90()0([2IIII+−=β .   (1)  When photoelectrons are ejected into isotropic directions or I(0) = I(90) in Eq. 1, we get β = 0. For a limiting case of I(0) = 0, the asymmetry parameter is β = −1, and for I(90) = 0 it is β = 2. The former case means that electrons are ejected into the direction perpendicular to the linear electronic vector of SR and the latter does that electrons are released into the parallel direction. The panel c giving 0≤ β <2 suggests that the photoelectrons from valence MOs are preferentially released into the parallel direction and the molecular Auger electrons are isotropically ejected. Table 1 gives the peak maxima or IEs in the 0°-spectra, the relative peak intensities, the β value obtained by Eq. 1, and the previously published IE levels [6, 8]. The calculated IEs, the symmetry of MO and the main character given in the sixth to eighth columns were elucidated by means of the ab initio calculations with the Gaussian 03 program [15]. The lowest 14.00 eV is the IE for the HOMO of 32a’, forming a bonding σCS occupied by two electrons. Uncertainty for the IEs in the present experimental conditions was estimated to be ±0.05 eV. The IE levels measured by three independent experiments are in good agreement up to 20 eV except the IEs of 14.699, 16.147 and 17.199 eV reported in the HeI PES [6]. These peaks are shoulders exposed by a peak fitting with the Gaussian functions [6]. When we tried to deconvolute the 0°-spectrum in Fig. 1 by use of the Gaussian functions, we needed additional peaks at 14.65, 16.34 and 17.05 eV to get a satisfying result (not shown), where the peak intensities at IE = 14.65 and 16.34 eV were 0.5 and 0.3 times that of the first one, respectively. In the HeI PES [6] too, the peak intensities at 14.699 and 16.147 eV are clearly smaller than that of the first band at 13.960 eV. We employ the results obtained by use of the Voigt  - 5 - functions since we have no reasonable understanding for the small peak intensities (see the following discussion) and the Voigt functions gave better curve fittings than those by the Gaussian ones. In the energy region above 20 eV the IEs measured by TPES [8] agree with the present values. New IEs of 22.60, 26.92 and 27.86 eV were observed in this study.  Peak intensity in PES is associated with the photoabsorption cross section σab and the β parameter [16]:  )](cos1[4)( 2 θβπσθ PI ab += ,  (2)  where P2(cosθ) = (1/2)(3cos2θ − 1) is the Legendre polynomial of the second order and θ is the angle between the ejected electron and the polarization direction. The σab increases with the number of electrons and thus the peak intensities in PES are approximately proportional to the number of electrons in valence MO(s), which are given in the parentheses in the second column of Table 1. Here the relative peak intensities are given after correction for anisotropic ejection since the 0°-spectrum depends upon β, i.e., I(0) = (σab/4π)(1 + β) from Eq. 2. The numerical values of β derived by use of Eq. 1 (see the panel c) were employed for the correction. The experimental IEs in the first column thus correspond to those for the 22 valence MOs given in the seventh column.  Full geometry optimization was performed in the MO calculations. The calculated bond lengths, which depend on the position of F atom in the molecule, were 1.295−1.296, 1.875 and 1.553−1.569 Å for the C−F, C−S and S−F bonds, respectively. Bond angles were 109.0−109.6° for the CF3 group and 88.3−93.0° for SF5. These bond lengths and angles are consistent with the reported values [8, 17]. Ionization energies for the optimized geometry based on the Koopmans’ theorem are given in the sixth column of Table 1, being a little higher than the experimental ones up to 20 eV and then a little lower. Assignments in the last column are given by a combination of MOs for CF4 and SF6: The electronic configuration of the ground state of CF4 is written as (3a1)2(2t2)6 for the inner valence shells and (4a1)2(3t2)6(1e)4(4t2)6(1t1)6 for the outer valence MOs [18], and that for SF6 is (4a1g)2(3t1u)6(2eg)4 for the inner shells and (5a1g)2(4t1u)6(1t2g)6(3eg)4(1t2u)6(5t1u)6(1t1g)6 for the outer valence [19]. The notation of 4t2 + 5t1u for the 32a’, for example, means the MO constructed with the 4t2 of CF4 and the 5t1u of SF6. The assignments here given are made for  - 6 - the sake of convenience in understanding of the MO characters. The asymmetry parameters in the third column of Table 1 are a constant of β = 1.02 ± 0.04 in the region of IE ≤20 eV. The MOs from 32a’ to 27a’ are mainly composed of the two outermost shells of CF4 and three those of SF6. These five MOs for CF4 and SF6 have the nF character originating in “near-pure” F2p electrons. The np electron in atomic orbital gives the β = 1 in an np → εd ionization and does β = 0 in np → εs [16] on the assumption of dipole approximation on a single-electron model. The parameter β = 1.02 ± 0.04 observed for CF3SF5 suggests thus that the nF electron with a 2p character is probably ionized through a 2p → εd like emission though the electron angular momentum of atom is no longer valid in molecule. In the energy region above 20 eV, the asymmetry parameters lie large at β = 1.42 ± 0.10 except the β = 0.85 at 27.86 eV. The ab initio calculations show that contribution of the C2s, S3p and S3s atomic orbitals becomes larger in the MOs at IE >20 eV. Thus electron emission from the MO in IE >20 eV is complex. Ionization through an ns → εp like electron emission with β = 2 should be partly responsible for the observed β = 1.42 ± 0.10.  In concluding we measured the ionization energies for valence electrons of CF3SF5 in the 12−35 eV region by means of angle-resolved photoelectron spectroscopy. The ionization energies obtained in the present work were able to be reliably evaluated by use of ab initio MO calculations and of the nature of valence bonds for CF4 and SF6.    Acknowledgments  The authors are grateful to Dr. O. Takahashi of Hiroshima University for his evaluation of the molecular orbital calculations. They thank the members of the research team for soft X-ray photochemistry at SPring-8 for their comments. This study was carried out with the approval of the SPring-8 Program Committee (2003B0154-NSb-np) and was supported by a Grant-in-Aid for Scientific Research and by the Budget of Atomic Energy Research from the Ministry of the Education, Culture, Sports, Science and Technology of Japan. We thank the Research Center for Computational Science, Okazaki Research Facilities, the National Institutes of Natural Sciences for the use of the computers and the Library Program Gaussian 03.   - 7 - References  [1] W.T. Sturges, T.J. Wallington, M.D. Hurley, K.P. Shine, K. Sihra, A. Engel, D.E. Oram, S.A. Penkett, R. Mulvaney, C.A.M. Brenninkmeijer, Science 289 (2000) 611. [2] R.Y.L. Chim, R.A. Kennedy, R.P. Tuckett, Chem. Phys. Lett. 367 (2003) 697. [3] R.P. Tuckett, Chem. Phys. Lett. 369 (2003) 256. [4] P. Limão-Vieira, P.A. Kendall, S. Eden, N.J. Mason, J. Heinesch, M.-J. Hubin-Franskin, J. Delwiche, A. Giuliani, Radiat. Phys. Chem. 68 (2003) 193. [5] P.A. Hatherly, A.J. Flaxman, Chem. Phys. Lett. 380 (2003) 512.  [6] P. Limão-Vieira, S. Eden, P.A. Kendall, N.J. Mason, A. Giuliani, J. Heinesch, M.-J. Hubin-Franskin, J. Delwiche, S.V. Hoffmann, Int. J. Mass Spectrom. 233 (2004) 335. [7] P.A. Kendall, N.J. Mason, J. Electron Spectrosc. Rel. Phen. 120 (2001) 27. [8] R.Y.L. Chim, R.A. Kennedy, R.P. Tuckett, W. Zhou, G.K. Jarvis, D.J. Collins, P.A. Hatherly, J. Phys. Chem. A 105 (2001) 8403. [9] T. Ibuki, Y. Shimada, S. Nagaoka, A. Fujii, M. Hino, T. Kakiuchi, K. Okada, K. Tabayashi, T. Matsudo, Y. Yamana, I.H. Suzuki, Y. Tamenori, Chem. Phys. Lett. 392 (2004) 303. [10] T. Ibuki, Y. Shimada, R. Hashimoto, S. Nagaoka, M. Hino, K. Okada, I.H. Suzuki, Y. Morishita, Y. Tamenori, Chem. Phys. 314 (2005) 119. [11] H. Ohashi, E. Ishiguro, Y. Tamenori, H. Okumura, A. Hiraya, H. Yoshida, Y. Senba, K. Okada, N. Saito, I.H. Suzuki, K. Ueda, T. Ibuki, S. Nagaoka, I. Koyano, T. Ishikawa, Nucl. Instr. Meth. Phys. Res. A 467-468 (2001) 533. [12] T. Tanaka, T. Hara, M. Oura, H. Ohashi, H. Kimura, S. Goto, Y. Suzuki, H. Kitamura, Rev. Sci. Instrum. 70 (1999) 4153. [13] Y. Shimizu, H. Ohashi, Y. Tamenori, Y. Muramatsu, H. Yoshida, K. Okada, N. Saito, H. Tanaka, I. Koyano, S. Shin, K. Ueda, J. Electron Spectrosc. Rel. Phen. 114-116 (2001) 63. [14] L.O. Werme, T. Bergmark, K. Siegbahn, Phys. Script. 6 (1972) 141. [15] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, C.  - 8 - Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian 03, (Revision C.02), Gaussian, Wallingford CT, 2004. [16] P.K. Ghosh, Introduction to photoelectron spectroscopy, in P.J. Elving and J.D. Winefordner (Eds.), Chemical Analysis, Vol. 67, Wiley, New York, 1983, Chapter 7. [17] C.J. Masden, D. Christen, H. Oberhammer, J. Mol. Struct. 131 (1985) 299. [18] D.M.P. Holland, A.W. Potts, A.B. Trofimov, J. Breidbach, J. Schirmer, R. Feifel, T. Richter, K. Godehusen, M. Martins, A. Tutay, M. Yalcinkaya, M. Al-Hada, S. Eriksson, L. Karlsson, Chem. Phys. 308 (2005) 43.  [19] J.T. Francis, C. Turci, T. Tyliszczak, G.G.B. de Souza, N. Kosugi, A.P. Hitchcock, Phys. Rev. A 52 (1995) 4665.     - 9 - Table 1. Ionization energies, relative peak intensities, the asymmetry parameter β and assignments for the valence orbitals of CF3SF5.   PES a 684.7 eV Relative Intensity b β value PES c HeI  21.22 eVTPES d IE calcd MO e Characterf Assignment g14.00 1.0 (2) 1.02 13.960 14.13 15.90 32a’ σCS 4t2 + 5t1u    (14.699)      15.61 1.8 (4) 1.05 15.632 15.68 18.57 15a” nF 1t1g    (16.147)  18.68 14a” nF 1t1 + 1t1g 16.55 1.9 (4) 1.00 16.461  18.72 31a’ nF 1t1 + 1t1g      19.04 13a” nF 1t1 17.01 1.8 (4) 1.02 16.845 16.94 19.48 30a’ nF 4t2 + 5t1u    (17.199)  19.59 29a’ nF 1t1 + 5t1u 17.93 1.8 (4) 1.05 17.894 17.86 19.63 (2) 28a’/12a” nF 5t1u /1e – 1t2u or 4t2 + 1t2u18.88 1.1 (2) 0.94 18.721  19.68 11a” nF 4t2 + 5t1u 19.54 1.0 (2) 1.03 19.365 19.44 19.75 27a’ nF 4t2 + 1t2u 21.23 1.2 (2) 1.40  21.34 20.08 26a’ nF 3eg 22.04 3.6 (8) 1.43  22.01 20.71 10a” nF 1e      20.96 25a’ nF 1e + 1t2u      22.06 (2) 24a’/9a” nF 1e + 1t2g / 3t2 – 1t2g 22.60 0.9 (2) 1.26   22.23 23a’ nF 3t2 – 4t1u or 3t2 + 5t1u 24.64 2.0 (4) 1.53  24.67 22.74 8a” nF 1t2g      24.39 7a” nF 3t2 + 1t2g 26.92 2.2 (4) 1.48   24.42 22a’ nF 3t2 + 1t2g      25.11 21a’ nF 3t2 + 4t1u 27.86 1.1 (2) 0.85   25.12 6a” nF 3t2 + 4t1u   a Estimated uncertainty is ±0.05 eV. b Relative peak area corrected for the β value. Number of electrons is given in the parentheses.  c Values in the parentheses are the IEs for shoulder peaks exposed by a curve fitting with the Gaussian functions [6].   - 10 - d Threshold photoelectron spectrum from Ref. [8]. e Molecular orbitals from ab initio calculations. The HOMO is 32a’, meaning that it is the 32th MO belonging to the symmetry a'.  f Main character of MO is expressed with conventional symbol of σCS for the sigma C−S bond or nF for nonbonding electrons on F atom.   g Assignments based on the MOs of CF4 and SF6. See the text for details.   - 11 - Figure caption  Fig. 1. Photoelectron spectra and the asymmetry parameter β. Panel a was measured by detecting photoelectrons in the direction parallel (0°) to the electric vector of synchrotron radiation. Thick dots are the experimental data, thin solid curves are the peaks deconvoluted by the Voigt functions and the thick curve is the sum of the thin ones. Panel b was measured at the perpendicular direction (90°).  - 12 -                   670 665 660 655 65015 20 25 30 35012Kinetic Energy / eV β valueIonization Energy / eV(c)010002000  Relative Intensities(b) 90 deg020004000600080001000012000  hν = 684.7 eV (a) 0 degFig. 1. Ibuki et al  

On the ionization energy of CF3SF5 in the valence region measured by angle-resolved photoelectron spectroscopy

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On the ionization energy of CF3SF5 in the valence region measured by angle-resolved photoelectron spectroscopy

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