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The lowest-energy spectator Auger band of the CH3F molecule observed via F and C 1s → σ* excitation
Chemical Physics Letters 413 (2005) 263–266 www.elsevier.com/locate/cplett
The lowest-energy spectator Auger band of the CH3F molecule observed via F and C 1s ! r* excitation K. Ueda a,*, X.-J. Liu a, G. Pru¨mper a, E. Kukk a,b, H. Yoshida c, D. Sasaki c, M. Kitajima d, T. Tanaka d, C. Makochekanwa d,e, M. Hoshino f, H. Tanaka d a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan b Department of Physics, University of Turku, FIN-20014 Turku, Finland c Department of Physical Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan d Department of Physics, Sophia University, Tokyo 102-8554, Japan e Graduate School of Sciences, Kyushu University, Fukuoka 812-8581, Japan f Atomic Physics Laboratory, RIKEN, Wako, Saitama 351-0198, Japan Received 11 April 2005; in final form 20 June 2005 Available online 18 August 2005
Abstract Resonant photoemission from the fluoromethane (CH3F) molecule has been measured with photon energies scanning through the resonance in which a C or F 1s electron is promoted to the lowest unoccupied molecular orbital rCF . The spectator Auger band with the lowest binding energy of 27 eV is tentatively assigned to 2e26a1. The energy-resolved electron–ion coincidence experiments revealed that this state produces predominantly CHþ 2 ions both for C 1s and F 1s excitations, providing evidence that the spectator Auger decay to this state takes place within the molecular regime, where the valence electrons are delocalized over the entire molecule. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Remarkable progress in techniques for the generation and monochromatization of synchrotron radiation in the soft X-ray region has invoked renewed interest in resonant photoemission [1–3]. The resonant photoemission, also often called resonant Auger emission, follows a promotion of a core electron to an unoccupied molecular orbital by monochromatic soft X-rays. The resonant Auger emission can be used to investigate potential surfaces of the Auger final states that cannot be explored via the direct photoemission. For that purpose, the resonant Auger spectra are often measured by changing the photon energy across the core resonance. Fine detuning experiment, which became possible by re*
Corresponding author. Fax: +81 22 217 5380. E-mail address: [email protected] (K. Ueda).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.06.125
cent progress in soft X-ray monochromators, controls the initial nuclear conformation in the core-excited state. The resonant Auger emission then takes place in competition with the nuclear motion. Because of the evolution of the nuclear motion in the core-excited state, we can explore the potential surfaces of the resonant Auger final states in the area outside the Franck–Condon region defined by the zero-point vibrational wavefunction of the ground state. In the present work, we investigate the resonant Auger emission from the fluoromethane (CH3F) molecule with photon energies scanning through the resonance in which a C or F 1s electron is promoted to the lowest unoccupied antibonding molecular orbital rCF . The resonant Auger emission can be classified into two; participator Auger decay and spectator Auger decay. In the participator Auger decay, the electron promoted to the unoccupied orbital participates in the electronic decay.
K. Ueda et al. / Chemical Physics Letters 413 (2005) 263–266
The final states are valence one-hole states V1. Thus, participator Auger emission can be observed as resonant enhancement of the valence photoemission. In the spectator Auger decay, the electron promoted to the unoccupied orbital behaves as a spectator in the electronic decay. The final states are two-hole one-electron states V2V*. In the present study, we focus on the lowestenergy spectator Auger final state. 2. Experimental
-1
S 4a1
-1
5a1 /1e
-1
2e
-1
689
660
665 670 Kinetic energy (eV)
IV
IV
6a1 III
III
II
II
I
I
688 687 Photon energy (eV)
The experiment was carried out on the c branch of the soft X-ray photochemistry beamline BL27SU [4] at SPring-8, the 8-GeV synchrotron radiation facility in Japan. The emitted radiation from a figure-8 undulator is linearly polarized either in the horizontal plane of the storage ring (1st order) or in the vertical plane perpendicular to it (0.5th order) [5]. In the present measurement, we used only the horizontal polarization. The soft X-ray monochromator is a Hettrick type which consists of a varied-line-space plane grating and a focusing mirror [6]. The degree of linear polarization was measured in a separate measurement by observing Ne 2s and 2p photolines and confirmed to be larger than 0.98 with the present setting of optics [7]. The apparatus for electron spectroscopy consists of a hemispherical electron energy analyzer (Gammadata-Scienta SES-2002), a gas cell, and a differentially pumped chamber [8]. The lens axis is fixed in the horizontal direction, perpendicular to the photon beam direction. The entrance slit of the analyzer is parallel to the photon beam direction. In addition to the electron spectroscopy described above, we have performed electron-ion coincidence experiments. The apparatus, the experimental procedure and the analyzing procedure of the coincidence data are described elsewhere [9–11]. Briefly, the setup consists of the electron spectrometer (SES-2002) and an ion timeof-flight (TOF) spectrometer, the detectors of which are both microchannel plates with delay-line anodes. The sample gas of CH3F is introduced between the pusher and extractor electrodes of the ion spectrometer. Electrons pass the pusher electrode and enter the electron spectrometer. Triggered by the electron detection, rectangular high voltage pulses with opposite signs are generated by a pulse generator and are applied to the pusher and extractor electrodes. All data are recorded in 16-channel TDC modules.
Here, 1a1 and 2a1 correspond to F 1s and C 1s core orbitals, respectively. The lowest unoccupied antibonding molecular orbital, designated as rCF , is 6a1. A promotion of the 1a1 or 2a1 electron to 6a1 exhibits a broad isolated resonance in the F or C K-shell excitation region [12,13]. Figs. 1 and 2 show the electron spectra recorded at various photon energies across the 1a1 ! 6a1 and 2a1 ! 6a1 resonances, respectively. Monochromator and analyzer bandwidths were 125 and 125 meV both in Figs. 1 and 2. The resonant enhancement of the photoelectron bands from the orbitals 2e (at binding energy BE = 13.1 eV), 5a1 and 1e (BE = 17.2 eV), and 4a1 (BE = 23.4 eV) is evident. The vibrational envelopes also change dramatically for 4a1 and the overlapping band of 5a1 and 1e, reflecting that the nuclear motion is evolved in the core-excited states and as a result the resonant Auger decay occurs outside of the Franck– Condon region of the ground state. All these three photoelectron bands disperse linearly in kinetic energy as the photon energy is tuned through the resonance. At lower kinetic energy of the 4a1 band, one small band S appears both in Figs. 1 and 2 at fixed kinetic energy. In the rest of this paper, we focus on this S band. In Fig. 3a,b, we plot the electron spectra, in binding energy scale, recorded at the photon energies on top of the resonances, 687.8 and 289.9 eV, respectively. The S band appears at about the binding energy 27 eV for both the 1a1 ! 6a1 and 2a1 ! 6a1 excitations. This suggests that the S band can be attributed to the same Auger final state for both the 1a1 ! 6a1 and 2a1 ! 6a1 excitations. We tentatively assign the S band to the spectator Auger band whose final state is 2e26a1, which has the lowest binding energy among the all spectator Auger
Intensity (arb.units)
264
675
3. Results and discussion The CH3F molecule has a geometrical symmetry of C3v in the ground state and its electronic configuration is written as: 1a21 2a21 3a21 4a21 1e4 5a21 2e4 1 A1
ð1Þ
Fig. 1. Resonant Auger electron spectra, recorded at photon energies tuned across 1a1 (F 1s) ! 6a1 resonance for CH3F. The measurements were performed along the direction of the linear light polarization. The Roman numbers I, II, III and IV represent detuning of photon energy relative to the resonance: 1.1, 0.6, 0 (on resonance), and +1.0 eV , respectively. The graph on the right side shows the corresponding excitation resonance and photon energy detuning.
K. Ueda et al. / Chemical Physics Letters 413 (2005) 263–266 -1
-1
4a1
-1
5a1 /1e
-1
2e
288.0
S
Intensity (arb.units)
6a1 III IV V
IV
II
III
I
II
288.5 289.0 Photon energy (eV)
I
V
3sa1
265 270 Kinetic energy (eV)
VI
260
289.5
VI 275
Fig. 2. Resonant Auger electron spectra, recorded at photon energies tuned across 2a1 (C 1s) ! 6a1 resonance for CH3F. The measurements were performed along the direction of the linear light polarization. The Roman numbers I, II, III, IV, V and VI represent detuning of photon energy relative to the resonance: 0.9, 0.6, 0.3, 0 (on resonance), +0.2, and +0.5 eV , respectively. The graph on the right side shows the corresponding excitation resonance and photon energy detuning.
a
1a1 −> 6a1
Intensity (arb.units)
2e
-1
5a1 /1e
-1
-1
4a1
b
-1
-2
2e 6a1
2 a 1 −> 6a1
265
is assigned to 1e12e16a1 or a part of the participator 4a1 band. Because of the antibonding nature of the 6a1 orbital, 1 both the 1a1 1 6a1 and 2a1 6a1 core-excited states are expected to be dissociative and the C–F stretching motion is caused in these core-excited states. A question arises on whether the electron emission is at the dissociation limit or still within the molecular regime. The fact that the S band appears at about the same binding energies for both the 1a1 ! 6a1 and 2a1 ! 6a1 excitations implies that the electron emission is still in the molecular regime. To draw a decisive conclusion, however, it is indispensable to investigate the ion production from the Auger final state. Fig. 4 compares the ion productions from the S band populated via the resonant Auger decay from the 1 1a1 1 6a1 and 2a1 6a1 states. Here, the ion TOF spectra were recorded in coincidence with the electrons in the binding energy range 26–28 eV. The branching ratios of the ion productions are very similar for both the 1a1 ! 6a1 and 2a1 ! 6a1 excitations. The main product is CHþ 2 . If the resonant Auger emission was at the dissociation limit and if the initial hole was 1a1 (F 1s), then the F+ would be produced from the core-excited F fragment. However, the F+ production is minor independent of the initial hole created. Thus, we can conclude that the resonant Auger emission corresponding to the S band takes place within the molecular region, where the valence electrons are delocalized over the entire molecule. In conclusion, we have investigated resonant photoemission from CH3F and the subsequent molecular dissociation, after promotion of a F or C 1s electron to the antibonding orbital rCF . The spectator Auger final state, 2e26a1, with the binding energy of 27 eV, tentatively assigned to 2e26a1, turned out to have a potential surface which is approximately parallel to the core-excited 15
a
1a1
CH2+
6a1
15
20 Binding energy (eV)
25
Intensity (arb.units)
10 +
CH 5
+
H
+
C
CH3+ F+
+
+
CF CHF
0 15
b
2a1
6a1
10 5
Fig. 3. Resonant Auger electron spectra for: (a) F KVV and (b) C KVV, as a function of the binding energy of the Auger final states.
0 1
final states. The non-dispersive behavior of the S band observed in Figs. 1 and 2 stems from the parallelness between potential energy surfaces for the core-excited state 1 1a1 1 6a1 or 2a1 6a1 and the Auger final state. We note that one cannot exclude the possibility that the S band
2
3 4 Time of fight (µs)
5
Fig. 4. TOF ion spectra coincident with: (a) F KVV and (b) C KVV Auger electrons, with binding energy between 26 and 28 eV. The area of the entire range of the TOF spectrum is normalized to be the same for both TOF spectra.
266
K. Ueda et al. / Chemical Physics Letters 413 (2005) 263–266
1 states 1a1 1 6a1 and 2a1 6a1 and to produce predomiþ nantly CH2 ions both for C 1s and F 1s excitations. This observation indicates that the Auger decay to this state takes place within the molecular regime, before the delocalization of the valence electrons takes place due to elongation of the C–F bond.
Acknowledgements The experiment was carried out with the approval of the SPring-8 program review committee. This study was supported by Grants-in-Aid for Scientific Research from the Japanese Society for the Promotion of Science (JSPS). X.J.L. is grateful to Inoue Foundation and Tohoku University (COE program) for financial support, E.K. is grateful to Tohoku University for the hospitality and financial support and C.M. is grateful to J.S.P.S for financial support under Grant No. P0406. References [1] S.L. Sorensen, S. Svensson, J. Electron. Spectrosc. Relat. Phenom. 114–116 (2001) 1. [2] K. Ueda, J. Phys. B 36 (2003) R1.
[3] J.D. Bozek, S.E. Canton, E. Kukk, N. Berrah, Chem. Phys. 289 (2003) 149. [4] H. Ohashi, E. Ishiguro, Y. Tamenori, H. Kishimoto, M. Tanaka, M. Irie, T. Tanaka, T. Ishikawa, Nucl. Instrum. Meth. A 467–468 (2001) 529. [5] T. Tanaka, H. Kitamura, J. Synchrotron Radiat. 3 (1996) 47. [6] H. Ohashi, E. Ishiguro, Y. Tamenori, et al., Nucl. Instrum. Meth. A 467–468 (2001) 533. [7] H. Yoshida, Y. Senba, M. Morita, T. Goya, A. De Fanis, N. Saito, K. Ueda, Y. Tamenori, H. Ohashi, AIP Conf. Proc. 705 (2004) 267. [8] Y. Shimizu, H. Ohashi, Y. Tamenori, et al., J. Electron. Spectrosc. Relat. Phenom. 114-116 (2001) 63. [9] G. Pru¨mper, Y. Tamenori, A. De Fanis, U. Hergenhahn, M. Kitajima, M. Hoshino, H. Tanaka, K. Ueda, J. Phys. B 38 (2005) 1. [10] G. Pru¨mper, K. Ueda, U. Hergenhahn, A. De Fanis, Y. Tamenori, M. Kitajima, M. Hoshino, H. Tanaka, J. Electron. Spectrosc. Relat. Phenom. 144–147 (2005) 227. [11] G. Pru¨mper, K. Ueda, Y. Tamenori, M. Kitajima, N. Kuze, H. Tanaka, C. Makochekanwa, M. Hoshino, M. Oura, Phys. Rev. A 71 (2005) 052704. [12] N. Kosugi, K. Ueda, Y. Shimizu, H. Chiba, M. Okunishi, K. Ohmori, Y. Sato, E. Shigemasa, Chem. Phys. Lett. 246 (1995) 475. [13] K. Ueda, Y. Shimizu, H. Chiba, M. Okunishi, K. Ohmori, Y. Sato, E. Shigemasa, N. Kosugi, J. Electron. Spectrosc. Relat. Phenom. 79 (1996) 441.
The lowest-energy spectator Auger band of the CH3F molecule observed via F and C 1s ! r* excitation K. Ueda a,*, X.-J. Liu a, G. Pru¨mper a, E. Kukk a,b, H. Yoshida c, D. Sasaki c, M. Kitajima d, T. Tanaka d, C. Makochekanwa d,e, M. Hoshino f, H. Tanaka d a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan b Department of Physics, University of Turku, FIN-20014 Turku, Finland c Department of Physical Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan d Department of Physics, Sophia University, Tokyo 102-8554, Japan e Graduate School of Sciences, Kyushu University, Fukuoka 812-8581, Japan f Atomic Physics Laboratory, RIKEN, Wako, Saitama 351-0198, Japan Received 11 April 2005; in final form 20 June 2005 Available online 18 August 2005
Abstract Resonant photoemission from the fluoromethane (CH3F) molecule has been measured with photon energies scanning through the resonance in which a C or F 1s electron is promoted to the lowest unoccupied molecular orbital rCF . The spectator Auger band with the lowest binding energy of 27 eV is tentatively assigned to 2e26a1. The energy-resolved electron–ion coincidence experiments revealed that this state produces predominantly CHþ 2 ions both for C 1s and F 1s excitations, providing evidence that the spectator Auger decay to this state takes place within the molecular regime, where the valence electrons are delocalized over the entire molecule. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Remarkable progress in techniques for the generation and monochromatization of synchrotron radiation in the soft X-ray region has invoked renewed interest in resonant photoemission [1–3]. The resonant photoemission, also often called resonant Auger emission, follows a promotion of a core electron to an unoccupied molecular orbital by monochromatic soft X-rays. The resonant Auger emission can be used to investigate potential surfaces of the Auger final states that cannot be explored via the direct photoemission. For that purpose, the resonant Auger spectra are often measured by changing the photon energy across the core resonance. Fine detuning experiment, which became possible by re*
Corresponding author. Fax: +81 22 217 5380. E-mail address: [email protected] (K. Ueda).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.06.125
cent progress in soft X-ray monochromators, controls the initial nuclear conformation in the core-excited state. The resonant Auger emission then takes place in competition with the nuclear motion. Because of the evolution of the nuclear motion in the core-excited state, we can explore the potential surfaces of the resonant Auger final states in the area outside the Franck–Condon region defined by the zero-point vibrational wavefunction of the ground state. In the present work, we investigate the resonant Auger emission from the fluoromethane (CH3F) molecule with photon energies scanning through the resonance in which a C or F 1s electron is promoted to the lowest unoccupied antibonding molecular orbital rCF . The resonant Auger emission can be classified into two; participator Auger decay and spectator Auger decay. In the participator Auger decay, the electron promoted to the unoccupied orbital participates in the electronic decay.
K. Ueda et al. / Chemical Physics Letters 413 (2005) 263–266
The final states are valence one-hole states V1. Thus, participator Auger emission can be observed as resonant enhancement of the valence photoemission. In the spectator Auger decay, the electron promoted to the unoccupied orbital behaves as a spectator in the electronic decay. The final states are two-hole one-electron states V2V*. In the present study, we focus on the lowestenergy spectator Auger final state. 2. Experimental
-1
S 4a1
-1
5a1 /1e
-1
2e
-1
689
660
665 670 Kinetic energy (eV)
IV
IV
6a1 III
III
II
II
I
I
688 687 Photon energy (eV)
The experiment was carried out on the c branch of the soft X-ray photochemistry beamline BL27SU [4] at SPring-8, the 8-GeV synchrotron radiation facility in Japan. The emitted radiation from a figure-8 undulator is linearly polarized either in the horizontal plane of the storage ring (1st order) or in the vertical plane perpendicular to it (0.5th order) [5]. In the present measurement, we used only the horizontal polarization. The soft X-ray monochromator is a Hettrick type which consists of a varied-line-space plane grating and a focusing mirror [6]. The degree of linear polarization was measured in a separate measurement by observing Ne 2s and 2p photolines and confirmed to be larger than 0.98 with the present setting of optics [7]. The apparatus for electron spectroscopy consists of a hemispherical electron energy analyzer (Gammadata-Scienta SES-2002), a gas cell, and a differentially pumped chamber [8]. The lens axis is fixed in the horizontal direction, perpendicular to the photon beam direction. The entrance slit of the analyzer is parallel to the photon beam direction. In addition to the electron spectroscopy described above, we have performed electron-ion coincidence experiments. The apparatus, the experimental procedure and the analyzing procedure of the coincidence data are described elsewhere [9–11]. Briefly, the setup consists of the electron spectrometer (SES-2002) and an ion timeof-flight (TOF) spectrometer, the detectors of which are both microchannel plates with delay-line anodes. The sample gas of CH3F is introduced between the pusher and extractor electrodes of the ion spectrometer. Electrons pass the pusher electrode and enter the electron spectrometer. Triggered by the electron detection, rectangular high voltage pulses with opposite signs are generated by a pulse generator and are applied to the pusher and extractor electrodes. All data are recorded in 16-channel TDC modules.
Here, 1a1 and 2a1 correspond to F 1s and C 1s core orbitals, respectively. The lowest unoccupied antibonding molecular orbital, designated as rCF , is 6a1. A promotion of the 1a1 or 2a1 electron to 6a1 exhibits a broad isolated resonance in the F or C K-shell excitation region [12,13]. Figs. 1 and 2 show the electron spectra recorded at various photon energies across the 1a1 ! 6a1 and 2a1 ! 6a1 resonances, respectively. Monochromator and analyzer bandwidths were 125 and 125 meV both in Figs. 1 and 2. The resonant enhancement of the photoelectron bands from the orbitals 2e (at binding energy BE = 13.1 eV), 5a1 and 1e (BE = 17.2 eV), and 4a1 (BE = 23.4 eV) is evident. The vibrational envelopes also change dramatically for 4a1 and the overlapping band of 5a1 and 1e, reflecting that the nuclear motion is evolved in the core-excited states and as a result the resonant Auger decay occurs outside of the Franck– Condon region of the ground state. All these three photoelectron bands disperse linearly in kinetic energy as the photon energy is tuned through the resonance. At lower kinetic energy of the 4a1 band, one small band S appears both in Figs. 1 and 2 at fixed kinetic energy. In the rest of this paper, we focus on this S band. In Fig. 3a,b, we plot the electron spectra, in binding energy scale, recorded at the photon energies on top of the resonances, 687.8 and 289.9 eV, respectively. The S band appears at about the binding energy 27 eV for both the 1a1 ! 6a1 and 2a1 ! 6a1 excitations. This suggests that the S band can be attributed to the same Auger final state for both the 1a1 ! 6a1 and 2a1 ! 6a1 excitations. We tentatively assign the S band to the spectator Auger band whose final state is 2e26a1, which has the lowest binding energy among the all spectator Auger
Intensity (arb.units)
264
675
3. Results and discussion The CH3F molecule has a geometrical symmetry of C3v in the ground state and its electronic configuration is written as: 1a21 2a21 3a21 4a21 1e4 5a21 2e4 1 A1
ð1Þ
Fig. 1. Resonant Auger electron spectra, recorded at photon energies tuned across 1a1 (F 1s) ! 6a1 resonance for CH3F. The measurements were performed along the direction of the linear light polarization. The Roman numbers I, II, III and IV represent detuning of photon energy relative to the resonance: 1.1, 0.6, 0 (on resonance), and +1.0 eV , respectively. The graph on the right side shows the corresponding excitation resonance and photon energy detuning.
K. Ueda et al. / Chemical Physics Letters 413 (2005) 263–266 -1
-1
4a1
-1
5a1 /1e
-1
2e
288.0
S
Intensity (arb.units)
6a1 III IV V
IV
II
III
I
II
288.5 289.0 Photon energy (eV)
I
V
3sa1
265 270 Kinetic energy (eV)
VI
260
289.5
VI 275
Fig. 2. Resonant Auger electron spectra, recorded at photon energies tuned across 2a1 (C 1s) ! 6a1 resonance for CH3F. The measurements were performed along the direction of the linear light polarization. The Roman numbers I, II, III, IV, V and VI represent detuning of photon energy relative to the resonance: 0.9, 0.6, 0.3, 0 (on resonance), +0.2, and +0.5 eV , respectively. The graph on the right side shows the corresponding excitation resonance and photon energy detuning.
a
1a1 −> 6a1
Intensity (arb.units)
2e
-1
5a1 /1e
-1
-1
4a1
b
-1
-2
2e 6a1
2 a 1 −> 6a1
265
is assigned to 1e12e16a1 or a part of the participator 4a1 band. Because of the antibonding nature of the 6a1 orbital, 1 both the 1a1 1 6a1 and 2a1 6a1 core-excited states are expected to be dissociative and the C–F stretching motion is caused in these core-excited states. A question arises on whether the electron emission is at the dissociation limit or still within the molecular regime. The fact that the S band appears at about the same binding energies for both the 1a1 ! 6a1 and 2a1 ! 6a1 excitations implies that the electron emission is still in the molecular regime. To draw a decisive conclusion, however, it is indispensable to investigate the ion production from the Auger final state. Fig. 4 compares the ion productions from the S band populated via the resonant Auger decay from the 1 1a1 1 6a1 and 2a1 6a1 states. Here, the ion TOF spectra were recorded in coincidence with the electrons in the binding energy range 26–28 eV. The branching ratios of the ion productions are very similar for both the 1a1 ! 6a1 and 2a1 ! 6a1 excitations. The main product is CHþ 2 . If the resonant Auger emission was at the dissociation limit and if the initial hole was 1a1 (F 1s), then the F+ would be produced from the core-excited F fragment. However, the F+ production is minor independent of the initial hole created. Thus, we can conclude that the resonant Auger emission corresponding to the S band takes place within the molecular region, where the valence electrons are delocalized over the entire molecule. In conclusion, we have investigated resonant photoemission from CH3F and the subsequent molecular dissociation, after promotion of a F or C 1s electron to the antibonding orbital rCF . The spectator Auger final state, 2e26a1, with the binding energy of 27 eV, tentatively assigned to 2e26a1, turned out to have a potential surface which is approximately parallel to the core-excited 15
a
1a1
CH2+
6a1
15
20 Binding energy (eV)
25
Intensity (arb.units)
10 +
CH 5
+
H
+
C
CH3+ F+
+
+
CF CHF
0 15
b
2a1
6a1
10 5
Fig. 3. Resonant Auger electron spectra for: (a) F KVV and (b) C KVV, as a function of the binding energy of the Auger final states.
0 1
final states. The non-dispersive behavior of the S band observed in Figs. 1 and 2 stems from the parallelness between potential energy surfaces for the core-excited state 1 1a1 1 6a1 or 2a1 6a1 and the Auger final state. We note that one cannot exclude the possibility that the S band
2
3 4 Time of fight (µs)
5
Fig. 4. TOF ion spectra coincident with: (a) F KVV and (b) C KVV Auger electrons, with binding energy between 26 and 28 eV. The area of the entire range of the TOF spectrum is normalized to be the same for both TOF spectra.
266
K. Ueda et al. / Chemical Physics Letters 413 (2005) 263–266
1 states 1a1 1 6a1 and 2a1 6a1 and to produce predomiþ nantly CH2 ions both for C 1s and F 1s excitations. This observation indicates that the Auger decay to this state takes place within the molecular regime, before the delocalization of the valence electrons takes place due to elongation of the C–F bond.
Acknowledgements The experiment was carried out with the approval of the SPring-8 program review committee. This study was supported by Grants-in-Aid for Scientific Research from the Japanese Society for the Promotion of Science (JSPS). X.J.L. is grateful to Inoue Foundation and Tohoku University (COE program) for financial support, E.K. is grateful to Tohoku University for the hospitality and financial support and C.M. is grateful to J.S.P.S for financial support under Grant No. P0406. References [1] S.L. Sorensen, S. Svensson, J. Electron. Spectrosc. Relat. Phenom. 114–116 (2001) 1. [2] K. Ueda, J. Phys. B 36 (2003) R1.
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