Auger electron photoion coincidence technique ...

Auger electron photoion coincidence technique combined with synchrotron radiation for the study of the ion desorption mechanism in the region of resonant transitions of condensed H2O Kazuhiko Mase, Mitsuru Nagasono, Shinichiro Tanaka, Tsuneo Urisu, Eiji Ikenaga et al. Citation: J. Chem. Phys. 108, 6550 (1998); doi: 10.1063/1.476067 View online: http://dx.doi.org/10.1063/1.476067 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v108/i16 Published by the American Institute of Physics.

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JOURNAL OF CHEMICAL PHYSICS

VOLUME 108, NUMBER 16

22 APRIL 1998

Auger electron photoion coincidence technique combined with synchrotron radiation for the study of the ion desorption mechanism in the region of resonant transitions of condensed H2O Kazuhiko Mase, Mitsuru Nagasono, Shinichiro Tanaka, and Tsuneo Urisu Institute for Molecular Science, Okazaki 444-8585, Japan

Eiji Ikenaga, Tetsuji Sekitani, and Kenichiro Tanaka Department of Materials Science, Faculty of Science, Hiroshima University, Higashi Hiroshima 739-8526, Japan

~Received 29 September 1997; accepted 17 February 1998! The Auger electron photoion coincidence ~AEPICO! technique has been applied for the study of H1 desorption induced by resonant excitations of O 1s of condensed H2O. The peak positions of the AEPICO yield spectrum at the 4a 1 ←O 1s resonance (h n 5533.4 eV! are found to correspond to spectator-Auger transitions leaving (O 2s) 22 (4a 1 ) 1 , (O 2s) 21 (O 2p) 21 (4a 1 ) 1 , and (O 2p) 22 (4a 1 ) 1 states. The H1 AEPICO yield is greatly enhanced at 4a 1 ←O 1s while it is suppressed at 3 p←O 1s (h n 5537 eV! as compared with that at the O 1s ionization (h n 5560 eV!. On the basis of these results, the ultrafast ion desorption mechanism is suggested to be favorable for the H1 desorption at 4a 1 ←O 1s, that is, the repulsive potential energy surface of the (O 1s) 21 (4a 1 ) 1 state is responsible for the H1 desorption. For H1 desorption at 3 p←O 1s, a spectator-Auger stimulated ion desorption mechanism is concluded to be probable. The suppression of the H1 AEPICO yield is ascribed to the reduction of the hole–hole repulsion due to the shield effect of the 3p electron. These results demonstrate the power of the AEPICO technique to clarify the mechanism of ion desorption induced by core–electron excitations. © 1998 American Institute of Physics. @S0021-9606~98!03016-5#

scribed elsewhere.2 Briefly, measurements were carried out in an ultrahigh vacuum chamber at a beamline with a 2 m grasshopper monochromator. A remodeled homemade coincidence analyzer which consists of a single-path cylindrical mirror analyzer ~CMA! ~solid angle51 sr! and a time-offlight ~TOF! ion mass spectrometer was used. The energy resolution of the CMA was decreased to E/DE550 because of incomplete alignment. Condensed amorphous water film was used as the sample which was prepared by exposing a substrate at 80 K to 100 L of gaseous H2O. The surface was excited by synchrotron radiation, and the electrons were energy selected and detected by the CMA while the photoions were accelerated toward the TOF spectrometer. The photoion counts were recorded by a multichannel scaler as a function of the TOF difference between the electron and the photoion by taking the electron signal as the starting trigger. The photoion desorbed in coincidence with the electron emission gives a coincidence signal at a specific TOF, while an unrelated ion increases the background. Therefore, the AEPICO spectrum corresponds to an ion mass spectrum for the desorption channels initiated by the selected Auger transitions. The CMA and the TOF spectrometer were also used for Auger electron spectroscopy ~AES! and total ion yield spectroscopy, respectively. Figure 1 shows total ion and O KVV Auger electron yield spectra ~electron kinetic energy5505 eV! and a spectrum of the total ion yield divided by the Auger electron yield for condensed H2O in the oxygen K-edge region of h n 5528– 578 eV. The monochromator resolution was esti-

Ion desorption induced by core–electron transitions of surface molecules is an attractive topic in surface dynamics, because the understanding of the mechanism is far from complete. Recently, Auger electron photoion coincidence ~AEPICO! spectroscopy combined with synchrotron radiation has been developed as a novel and powerful technique in this field, which provides the ion desorption yield for selected Auger final states.1 Very recently, we investigated the H1 desorption mechanism at the 4a 1 ←O 1s resonance of condensed amorphous H2O using Auger electron and AEPICO spectroscopies, and concluded that the (O 1s) 21 (4a 1 ) 1 state and/or the spectator-Auger final states @~valence orbitals!22(4a 1 ) 1 states# are responsible for the H1 desorption, and that H1 desorption yield is greatly enhanced because of the O–H antibonding character of the 4a 1 . 2 In the case of H1 desorption induced by O 1s ionization, on the other hand, normal-Auger final states @ (valence orbitals!22 states# were concluded to be responsible for H1 desorption, and the driving force for the H1 desorption was attributed to the hole–hole Coulomb repulsion and the electron missing in the orbitals with O–H bonding character.3 To contribute to the comprehensive understanding of the ion desorption mechanism induced by core– electron excitations, the present article describes an AEPICO study in the region of resonant transitions from O 1s of condensed amorphous H2O. The results demonstrate the power of the AEPICO technique combined with synchrotron radiation for the study of the ion desorption mechanism. The apparatus and experimental conditions were de0021-9606/98/108(16)/6550/4/$15.00

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FIG. 1. ~a! Total ion, ~b! O KVV Auger electron yield spectra ~electron kinetic energy5505 eV! and ~c! a spectrum of the total ion yield divided by the Auger electron yield for condensed H2O in the region of h n 5528– 578 eV. There is no meaning below 532 eV in spectrum ~c!, because the Auger electron signal there should be zero.

mated to be about 1.5 eV.2 For H2O in the gas phase the ionization potential of O 1s is reported to be 539.7 eV,4 while the excitation energies from O 1s to 4a 1 /3s, 2b 2 /3p, 3 p, and 4 p are reported as 534.0, 535.9, 537.0, and 537.8 eV, respectively.5 Since the energy levels of the orbitals of condensed H2O are not largely shifted from those in the gas phase,6 the assignments for the gas phase are expected to be approximately valid also for condensed H2O. Besides, the perturbation is reduced on the surface because of reduced hydrogen bonding.7,8 Actually, the 4a 1 ←O 1s resonance of surface H2O of condensed water was reported to be 533.660.3 eV,9 which is approximately equal to that in the gas phase. Therefore the assignments reported for H2O in the gas phase are tentatively adopted in the present paper. Hereafter we denote the 4a 1 /3s state as 4a 1 . The spectrum of the total ion yield divided by the Auger electron yield has exhibited a characteristic threshold peak at h n 5533.361.0 eV and a broad suppression in the region of h n 5537– 545 eV in agreement with a previous report.9 These results suggest that H1 desorption probability is greatly enhanced at the 4a 1 ←O 1s resonance while suppressed at the resonance to the Rydberg states. Since the energy of the (O 1s) 21 (3a 1 ) 21 (4a 1 ) 1 state is 15.9 eV higher than the (O 1s) 21 state,10 the threshold of shakeup excitations is expected to be at h n 5555.6 eV. So, the increase of H1 at 545 eV cannot be explained by shakeup excitations. The effects of shakeup excitations are minor even above h n 5556 eV, because no characteristic structure was observed in the spectrum of the total ion yield divided by Auger electron yield in the region of h n 5545– 580 eV. This result suggests that ion desorption derived from two-hole states created by normalAuger transitions is predominant above the ionization potential of O 1s. This is in contrast with the case of H2O dissociatively chemisorbed on a Si~100! surface where ion desorption stimulated by shakeup and shakeoff excitations is reported to dominate over that by normal excitations.11,12 Series of AEPICO spectra were measured at h n 5533.4, 537, 540, and 560 eV for electron kinetic energies of 440–530 eV corresponding to the O KVV Auger transi-

FIG. 2. H1 AEPICO yield spectra at h n 5~a! 533.4, ~b! 537, ~c! 540, and ~d! 560 eV. The solid lines show typical AES. ~e! H1 AEPICO yield spectra divided by the counts of the energy-selected electron.

tions. H1 was found to be the only ion species desorbed in all the cases. Figures 2~a!–2~d! show nonderivative AES and H1 AEPICO signal intensities as a function of electron kinetic energy ~AEPICO yield spectra! at h n 5533.4, 537, 540, and 560 eV. Figure 2~e! shows H1 AEPICO yield spectra normalized to the counts of the energy-selected electron. The AEPICO yield was greatly enhanced at 4a 1 ←O 1s (h n 5533.4 eV! while it was suppressed at h n5537 and 540 eV as compared with that at h n 5560 eV. These results are consistent with the spectrum of the total ion yield divided by the Auger electron yield shown in Fig. 1~c!. An AES reflects Auger transition probabilities, while an AEPICO yield spectrum reflects the Auger transition probabilities multiplied by the ion desorption probability initiated by the selected Auger transitions. So, the remarkable dependence of the AEPICO yield spectra on the photon energy indicates that the ion desorption mechanism is different for different excitations of O 1s. The AEPICO yield spectrum at h n 5560 eV was found to be virtually identical to that at h n 5640 eV which was ascribed to the Auger-stimulated ion desorption mechanism derived from two-hole states created by normal-Auger transitions.13


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The AEPICO yield spectrum at 4a 1 ←O 1s @ h n 5533.4 eV, Fig. 2~a!# exhibited major, medium, and minor peaks at electron kinetic energies of 510, 490, and 465 eV, respectively. The peak positions were found to be a few electron volts higher than the three normal-AES peaks of 505, 485, and 465 eV observed at h n 5560 eV @Fig. 2~b!#, which correspond to Auger final states of (O 2s) 22 , (O 2s) 21 (O 2p) 21 , and (O 2 p) 22 states,13 where O 2s denotes 2a 1 orbital and O 2 p denotes 1b 1 , 3a 1 , and 1b 2 orbitals. After a transition into a level below the ionization potential, spectator-Auger transitions are known to occur, which leave excited states with two holes in the valence orbitals and one electron in the unoccupied level.14–16 Participator-Auger transitions leaving one-valence-hole states are known to be minor processes.14,15 Besides, H1 desorption derived from one-valence-hole states is negligible.17 Since the kinetic energy of the electron emitted in a spectator-Auger transition ~spectator-Auger electron! is a few electron volts larger than that of the corresponding normal-Auger electron,14–16 the three AEPICO yield peaks at 4a 1 ←O 1s were assigned to the spectator-Auger transitions leaving (O 2s) 22 (4a 1 ) 1 , (O 2s) 21 (O 2p) 21 (4a 1 ) 1 , and (O 2 p) 22 (4a 1 ) 1 states, respectively. This result indicates that the lifetime of the 4a 1 electron on the surface is long enough to cause spectator-Auger transitions to some extent because of the reduced hydrogen bond.2 There are two precedents for this explanation. In a photostimulated ion desorption study of condensed D2O, Rosenberg et al. reported that the excited states involving the 4a 1 electron are long-lived on the surface because of reduction of hydrogen bonding.7 In a recent electron-stimulated ion desorption study of condensed D2O, Sieger et al. suggested that a reduction of surface hydrogen bonding increases the lifetime of the excited states responsible for ion desorption, and that these lifetime effects are strongest for excited states involving a 1 bands.8 A discussion on the ion desorption model at 4a 1 ←O 1s is provided elsewhere.2 Briefly, two ion desorption mechanisms were proposed. One is the spectator-Auger-stimulated ion desorption model described as a sequence of three steps:6,14 ~a! the 4a 1 ←O 1s transition, ~2! a spectatorAuger transition leaving a (valence orbitals!22(4a 1 ) 1 state, and ~3! H1 desorption. The repulsive potential energy surfaces of the (valence orbitals!22(4a 1 ) 1 states stimulate the H1 desorption in this case. The other is the ultrafast ion desorption model described as a sequence of four steps:9 ~a! the 4a 1 ←O 1s transition, ~2! expansion of the HO–H distance, ~3! a spectator-Auger transition, and ~4! H1 desorption. The repulsive potential energy surface of the (O 1s) 21 (4a 2 ) 1 state is responsible for the H1 desorption in the latter case. The two models are extreme cases of ion desorption related to spectator-Auger transitions. An intermediate model is another candidate, where both the (O 1s) 21 (4a 1 ) 1 and ~valence orbitals!22(4a 1 ) 1 states are responsible for H1 desorption. In any case, the remarkable enhancement of the H1 yield at 4a 1 ←O 1s is derived from the O–H antibonding character of the 4a 1 orbital.2 The mechanism can be verified by AEPICO yield spectroscopy, in principle. In the spectator-Auger-stimulated ion

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desorption mechanism, the AEPICO yield spectrum is expected to reflect the spectator-Auger transition probabilities multiplied by the ion desorption probability initiated by the individual ~valence orbitals!22(4a 1 ) 1 states. In the ultrafast ion desorption mechanism, on the other hand, the AEPICO yield spectrum is expected to be governed by the spectatorAuger transition probabilities, because the ion desorption probability does not depend on the ~valence orbitals!22(4a 1 ) 1 states. Since the shape of the AEPICO yield spectrum was found to resemble that of the spectatorAuger electron spectrum at the 4a 1 ←O 1s resonance,2 the ultrafast ion desorption model is favorable in this case. Highresolution measurements of spectator-Auger electron and AEPICO yield spectra are required for further discussion. The total ion yield divided by the Auger electron yield @Fig. 1~c!# was observed to be suppressed at the O 2 p ←O 1s transition (h n 5537 eV! in comparison with that at 4a 1 ←O 1s (h n 5533.4 eV! and that at the O 1s ionization (h n 5560 eV!. This result suggests that H1 desorption is partially suppressed by the O 3 p electron. Actually, the AEPICO yield spectrum at O 3 p←O 1s displayed a remarkable suppression in a kinetic energy region of 490–520 eV in contrast with the case at 4a 1 ←O 1s (h n 5533.4 eV! and that at O 1s ionization (h n 5560 eV! as shown in Fig. 2~e!. The ultrafast ion desorption model can be excluded at O 3 p←O 1s, because the potential energy surface of the (O 1s) 21 (3p) 1 state is not expected to be repulsive due to the O–H nonbonding character of the 3p orbital. Therefore, a most probable mechanism is the spectator-Augerstimulated ion desorption model, that is, the intermediate step is a spectator-Auger transition leaving a ~valence orbitals!22 (3p) 1 state, then H1 is desorbed along the repulsive potential energy surface of the ~valence orbitals!22 (3p) 1 state. On the analogy of the case at 4a 1 ←O 1s, electron kinetic energies of 505–510, 485–490, and 460–465 eV at the O 3 p←O 1s transition @ h n 5537 eV, Fig. 2~b!# were assigned to the spectator-Auger transitions leaving (O 2s) 22 (3p) 1 , (O 2s) 21 (O 2p) 21 (3p) 1 , and (O 2p) 22 (3p) 1 states, respectively. The H1 AEPICO yield was observed to be suppressed in a kinetic energy region of 490–520 eV corresponding to (O 2s) 21 (O 2p) 21 (3p) 1 and (O 2 p) 22 (3p) 1 states. A most probable explanation for this result is that the 3p electron reduces the hole–hole Coulomb repulsion, and eases the repulsiveness of the potential energy surfaces of the (O 2s) 21 (O 2p) 21 (3p) 1 and (O 2p) 22 (3p) 1 states. The reduction of hole–hole repulsion also explained the suppression of H1 desorption in the * ←C 1s transition of condensed CH3CN18 and cases of p CN p * (e 2u )←C 1s transition of condensed C6H6. 19 The H1 AEPICO yield at the electron kinetic energies of 460–480 eV corresponding to (O 2s) 22 (3p) 1 state was found to be less suppressed. There are two probable explanations for this result. One is that (O 2s) 21 (O 2p) 22 (3p) 1 or (O 2p) 24 (3p) 1 states created by the nonradiative decay of the O 2s hole are responsible for H1 desorption. The other is



that the (O 2s) 22 (3p) 1 state is considerably repulsive because of the O–H bonding character of the O 2s(2a 1 ) orbital. The former explanation is proposed for the H1 desorption from H2O dissociatively chemisorbed on a Si~100! surface.12 Since the ionization potential of O 1s of gaseous H2O is reported to be 539.7 eV,4 Auger electron kinetic energies of 505, 485, and 460 eV at h n 5540 eV correspond to normalAuger transitions leaving (O 2s) 22 , (O 2s) 21 (O 2 p) 21 , and (O 2p) 22 states with a slow photoelectron, respectively. The AEPICO yield spectrum at h n 5540 eV @Fig. 2~c!# displayed a slight suppression in a kinetic energy region of 490–520 eV in comparison with the case at h n 5560 eV @Fig. 2~d!#. The results are reasonable because the reduction of the hole–hole repulsion due to the slow photoelectron is expected to be not negligible, but smaller than that of the 3p electron. The authors wish to thank the staff of the synchrotron radiation facility. The authors are grateful to T. Sugahara, K. Sunayama, and Y. Tamenori for valuable technical support. This work was supported by the Joint Studies Program ~1995–1996! of the Institute for Molecular Science. This work was partly defrayed by a Grant-in-Aid for Scientific Research ~No. 09740253! from the Ministry of Education, Science, Sports, and Culture, Japan.

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