- Email: [email protected]
Study of ion desorption induced by a resonant core-level excitation of condensed NH3 using Auger-electron photo-ion coincidence (AEPICO) spectroscopy combined with synchrotron radiation
N
surface science
ELSEVIER
Surface Science 390 (1997) 102-106
Study of ion desorption induced by a resonant core-level excitation of condensed NH3 using Auger-electron photo-ion coincidence (AEPICO) spectroscopy combined with synchrotron radiation Mitsuru Nagasono *, Kazuhiko Mase, Shin-ichiro Tanaka, Tsuneo Urisu Institute for Molecular Science, Myodaiji-cho, Okazaki 444, Japan
Received 15 February 1997; accepted for publication 26 June 1997
Abstract Photostimulated ion desorption at the 4al~-N ls resonant transition of condensed NH3 was studied using electron emission spectroscopy and Auger-electron photoion coincidence (AEPICO) spectroscopy. The total ion yield divided by the Auger-electron yield exhibited a threshold peak at hv=399 eV which is ascribed to the resonant transition from the N ls to the N H antibonding 4al orbital. The electron emission spectrum at the 4aa +-N 1s resonance is decomposed into three components: a valence photoelectron emission spectrum, and normal- and resonant-Auger-electron emission spectra. We ascribe the resonant-Auger-electron emission spectrum mainly to spectator-Auger transitions on the basis of the peak assignment. A series of AEPICO spectra at the 4a~~ N l s resonance was also measured as a function of the Auger-electron kinetic energy. The electron kinetic energy dependence of the H + AEPICO yield displays a shape approximately similar to that of the mixed spectrum of normal- and spectator-Auger-electron emission spectra. Based on this result the H ÷ desorption at the 4 a ~ N ls resonance is concluded to originate from the spectatorAuger transitions and from the normal-Auger transitions following the delocalization of the excited electron. © 1997 Elsevier Science B.V. Keywords: Ammonia; Amorphous surfaces; Auger electron spectroscopy; Desorption induced by electronic transitions; Photoelectron emission; Photon stimulated desorption; Synchrotron radiation photoelectron spectroscopy
1 . Introduction I o n d e s o r p t i o n i n d u c e d by core-electron transitions is a n interesting p h e n o m e n o n in surface science [1], for which a n A u g e r stimulated i o n d e s o r p t i o n m o d e l is proposed, i.e. A u g e r t r a n s itions leaving two-hole states are responsible for the i o n d e s o r p t i o n [2]. I n order to verify the A u g e r s t i m u l a t e d i o n d e s o r p t i o n m e c h a n i s m we have developed Auger-electron p h o t o i o n coincidence * Corresponding author. Fax: (+ 81) 564 53.7327; e-mail: [email protected] 0039-6028/97/$17.00 © 1997 Elsevier ScienceB.V. All rights reserved. PII $0039-6028 (97) 00525-6
( A E P I C O ) spectroscopy, which provides the ion yield i n d u c e d by selected A u g e r transitions [3-6]. Recently, we investigated H ÷ d e s o r p t i o n i n d u c e d b y N ls i o n i z a t i o n of c o n d e n s e d NH3, a n d clarified that the N K V V A u g e r transitions leaving holes in the N - H b o n d i n g orbitals induce H ÷ d e s o r p t i o n because o f h o l e - h o l e C o u l o m b repulsion [5]. I n the present p a p e r we describe a study of ion d e s o r p t i o n i n d u c e d b y the r e s o n a n t excitation from the N 1s to the 4al orbital of c o n d e n s e d NH3 using electron emission spectroscopy a n d A E P I C O spectroscopy. Three core-decay processes are expected to take place after the 4 a l a N ls trans-
M. Nagasono et al. / Surface Science 390 (1997) 102-106
ition, i.e. spectator-, participator- and normalAuger transitions. The goal of the work is to untangle the three possible core-decay contributions to the ion yield. The main method is to investigate the correlation of the H ÷ ion yield with the kinetic energy of electrons emitted coincidentally with the ions, the latter reflecting the electronic state(s) which produce the ions. The method includes a deconvolution of the electron emission spectrum by (1) subtracting the background due to electron energy loss processes, (2) subtracting the direct valence ionization, and (3) subtracting the normal-Auger decay contribution to give a pure resonant (participator and spectator) Auger-electron emission spectrum.
2. Experiments T h e apparatus used is described elsewhere [3, 4]. In brief, experiments were carried out in an ultrahigh vacuum chamber on line BL-2B1, which incorporates a 2-m grasshopper m o n o c h r o m a t o r , at the Ultraviolet Synchrotron Orbital Radiation facility at the Institute for Molecular Science [7]. P-polarized radiation impinged on the sample with an incidence angle of 60 ° to the surface normal. The photon energy resolution was about 1.2 eV at hv=400eV (Nls region). Electron emission spectra were measured by a double-path cylindrical mirror electron energy analyzer (CMA) with a resolution of a b o u t 3.3 eV at the path energy of 100 eV. The angle between the double-path C M A axis and surface normal was 30 °. A E P I C O spectra were measured by a remodeled electron ion coincidence analyzer consisting of a single-pass C M A without retarding grids, arranged at 30 ° f r o m the surface normal, and a time-of-flight ( T O F ) ion mass spectrometer in front of the sample [4]. The energy resolution of the single-path C M A was about 8 eV because of incomplete alignment. The T O F spectrometer collected ions desorbed into all solid angles. The sample was prepared by exposing an Au foil at 80 K to 100 L of N H 3. Total ion yield ( T I Y ) and Auger-electron yield ( A E Y ) spectra were also measured as a function of the photon energy by the electron ion coincidence analyzer.
103
3. Results and discussion Fig. 1 shows TIY, A E Y and T I Y divided by A E Y ( T I Y / A E Y ) spectra of condensed NH3 in the N K-edge region. The T I Y corresponds to the H ÷ yield from the surface NH3 because the dominant ion species is reported to be H ÷ [8]. The A E Y was measured at a fixed kinetic energy of 369 eV with a window of about 8 eV, which corresponds to the largest N K W Auger peak from condensed NH3. The A E Y shows the photoabsorption spectrum of condensed N H 3 in the first 50 layers or so. The T I Y and A E Y spectra are virtually identical to those reported by Menzel et al. [8]. The photon energy of the present study was calibrated on the basis of their report. The T I Y / A E Y spectrum displays a characteristic threshold peak at hv--399 eV, which has been assigned to the resonant transition from the N ls to the 4al orbital with an N - H antibonding character [9]. The 4al +-N 1s resonance, however, exhibits no corresponding peak in the A E Y spectrum. This result suggests that the dipole transition probability of the 4 a ~ N ls is reduced considerably in
a5
5 /17
.~c--
i'(
. . . .
d
-
\
400
TIY / AEY
405 410 415 Photon Energy (eV)
....._....._......................j,.......
420
.......
425
Fig. 1. TIY, AEY, and TIY/AEY spectra of condensed NH3. The Auger-electrons were selected at an electron kinetic energy of 369 eV with a window of about 8 eV corresponding to N KVV Auger-electrons. These spectra were normalized to the incident photon flux obtained from the current at a gold-mesh positioned in the radiation path.
104
M. Nagasono et al. / Surface Science 390 (1997) 102-106
the bulk NH3 because of band formation of the 4a 1 orbital through the hydrogen bonds. At the surface, on the other hand, the probability of the 4 a l a N ls transition is expected to be less perturbed because of the reduced coordination number. Therefore, the characteristic peak in the TIY/AEY spectrum at hv=399 eV suggests the existence of efficient H ÷ desorption channels initiated by the 4al+--N ls resonance of the surface N
' 'C;;e'l;;el 'i;nli;a'ti;n . . . . .
12' '/~
,-, [ Core level resonant excitation
' ' ' ' ' ' .... 4
A
-]
\
c =
]
H3.
The electron emission spectra measured at the N ls ionization (by=419 eV) and the 4 a a ~ N ls resonance (hv=399 eV) are shown in Fig. 2 as a normal-Auger-electron emission spectrum (normal-AES) (A) and a resonant electron emission spectrum (B) respectively. The ionization potential of the N 1s level of NH 3 in the gas phase is reported to be 405.6 eV [10]. The backgrounds due to energy-loss processes are subtracted in these spectra using a deconvolution technique in which the near-elastic backscattering electron spectrum measured by a primary electron beam of 370 eV was adopted as an energy-loss function [11]. The resonant spectrum (Fig. 2B) is expected to include contributions not only from resonant-Auger decay but also from valence electron excitations and nomaal-Auger decay following delocalization of a 4aa electron due to the resonant tunneling into the 4aa unoccupied band [4]. The mixed (normal +resonant) AES (Fig. 2C) was obtained from t h e spectrum at the 4 a ~ N ls resonance (Fig. 2B) by subtracting the shifted valence photoelectron spectrum at hv=396 eV (Fig. 2D). The resonant-AES (Fig. 2E) was extracted from the mixed AES (Fig. 2C) by subtracting the scaled normal-AES (Fig. 2A). In the resonant-AES (Fig. 2E) a main and broad peak was observed at an electron kinetic energy of 376 eV. This peak is not located at the kinetic energy of photoelectrons from the valence orbital, and is shifted to a higher kinetic energy compared with the main peak in the normal-AES. The resonant-Auger decay is classified into minor participator-Auger and predominant spectator-Auger transitions [12]. In participator-Auger transitions the electron in the unoccupied orbital takes part in the Auger decay, leaving one-hole in a valence orbital. The kinetic energy of the participator-
Valence ionization ) (hi) = 396 eV)
t
Spectator-AES (hi)=399eV)
L'-,,
320
2a
i i
1
E=C.A ,
[ i
;/ ~6 ~
t
S,7,,,,
340 360 380 Electron Kinetic Energy (eV)
Fig. 2. Electron emission spectra at hv=419 (A), 399 (B), and 396 eV (D). In these spectra the backgrounds due to energyloss processes were subtracted (see the text). The mixed (normal + resonant) AES (C) was extracted from the resonantelectron emission spectrum (B) by subtracting the valence photoelectron spectrum (D) after shifting it by the photon energy difference of 3 eV. The resonant-AES (E) was obtained from the mixed AES (C) by subtracting the scaled normal-AES (A') (which is obtained from (A) multiplied by an appropriate factor of 1/15, so that the resulting spectrum is canceled for electron kinetic energies less than 350 eV). The AES (C) and (E) were smoothed.
Auger-electron is the same as that of a photoelectron from the corresponding valence orbital. In spectator-Auger transitions, on the other hand, the excited electron does not participate in the transitions, and a two-hole-one-particle state is formed. The electron emitted by a spectator-Auger transition possesses a higher kinetic energy than that from the corresponding normal-Auger transition [12]. Therefore, we conclude that the main peak in the resonant-AES (Fig. 2E) arises from spectator-Auger transitions. A series of AEPICO spectra was measured at
M. Nagasono et al. / Surface Science 390 (1997) 102-106 H +
105
hv = 399 eV L
i
i
.4
:.
F
380
200
\t
~ 390
~4.~-~
150
370
360 --F
350
5=
o
AEP1CO
g
340i2 g
,
50
330 320 310
200
300
400 500 600 700 Difference TOF (ns)
300 800 2 1 0 Electron Yield (arb.u.)
Fig: 3. The AEPICO spectra (left) and electron emission spectra (B and F ) at the 4 a ~ N ls resonance. AEPICO spectra correspond to the electron kinetic energy scale (fight axis). The electron emission spectrum ( F ) shows raw data measured using the single-pass CMA of the coincidence analyzer under the same conditions as the AEPICO measurements. The electron emission spectrum (B) is the same as that in Fig. 2. The accumulation time was 300 s for each AEPICO spectrum.
the 4 a l a N ls resonance in the N KVV Auger region, as shown in Fig. 3. The coincidence H ÷ signals were observed in the T O F difference region of 475-565 ns with a good signal to background ratio. This observation is consistent with the argument that the enhanced TIY/AEY peak at h v = 399 eV is derived from the H ÷ desorption channels initiated by the 4at~-N ls resonance. The data points of the AEPICO yield obtained from the integrated H ÷ peak above the background level were plotted as a function of the electron kinetic energy, as shown in Fig. 4. The error bars of the AEPICO yield represent standard deviations) In contrast to the traditional ion yield spectra, the AEPICO measurements provide the ion yield arising from the selected Auger transitions correspondThe error in the AEPICO yield spectrum is given by T + ~rb2k, where A + T is the integrated sum of accidental and true coincidence counts in the H + coincidence time window (475-565 ns in the present case) and abk is the standard deviation of the background counts out of the time window.
0
300
''''
.
.
.
.
.
320
.
0
340
360
380
Electron Kinetic Energy (eV) Fig. 4. The AEPICO yield spectrum as a function of the electron kinetic energy, The data points of the AEPICO yield (closed circles) are the integrated H ÷ peaks in Fig. 3. The AES (C and E), and the electron emission spectrum ( F ) are the same as those in Figs. 2 and 3.
ing to the kinetic energy of the electron emitted coincidentally with the ion. In other words, the AEPICO yield spectrum reflects the Auger transition probability of surface molecules multiplied by the probability of ion desorption derived from the selected Auger transitions. The shape o f the AEPICO yield spectrum is found to resemble that of the mixed (normal+ spectator) AES (Fig. 2C), except for the maximum peak at t h e electron kinetic energy of 376 eV. This result indicates that the ion desorption is induced by normal- and resonant-Auger decay. The maximum peak in the AEPICO yield spectrum at 376 eV, however, does not originate from the normal-Auger decay, because it was not observed in the AEPICO yield spectrum measured at the N ls ionization ( h v = 429 eV) [5]. A favorable explanation is that the H ÷ ion is produced by a spectator-Auger stimulated ion desorption model which is described as a three-step model; the first step is the 4 a t ~ N ls transition, the second step is spectator-Auger decay leaving a v~-iv214a~ 1 state, and the final step is H + desorption. Another candidate is an ultrafast ion desorption mechanism which is described by a four-step model [8]; the first step is the 4 a ~ N ls transition, the second step is the expansion of the N H z - H distance, the third step is
106
M. Nagasono et al. / Surface Science 390 (1997) 102-106
s p e c t a t o r - A u g e r decay, a n d the final step is H ÷ d e s o r p t i o n . B o t h m o d e l s are consistent with the result t h a t the A E P I C O yield p e a k c o r r e s p o n d s to t h a t o f the s p e c t a t o r - A E S , as s h o w n in Fig. 4. I n o r d e r to clarify the c o n t r i b u t i o n s o f the two models, high-resolution AEPICO measurements a n d t h e o r e t i c a l investigations are required. I n s u m m a r y , the H ÷ d e s o r p t i o n i n d u c e d b y a r e s o n a n t N l s e x c i t a t i o n o f c o n d e n s e d N H 3 was investigated. T h e T I Y / A E Y s p e c t r u m exhibits a t h r e s h o l d p e a k at h v = 3 9 9 eV, which is a t t r i b u t e d to the r e s o n a n t excitation f r o m the N l s to the N - H a n t i b o n d i n g 4 a l orbital. T h e electron emission s p e c t r u m at the 4al+--N ls r e s o n a n c e was d e c o m p o s e d i n t o three c o m p o n e n t s : a valence p h o t o e l e c t r o n emission spectrum, a n o r m a l - A E S , a n d a r e s o n a n t - A E S . T h e structure o f the A E P I C O yield s p e c t r u m is o b s e r v e d to r e s e m b l e the m i x e d n o r m a l - a n d r e s o n a n t - A E S expect f o r the strongest peak. T h e p o s i t i o n o f the strongest p e a k is o b s e r v e d to c o r r e s p o n d to s p e c t a t o r - A u g e r transitions. O n the basis o f these results we ascribe the H + d e s o r p t i o n at the 4 a l a N l s r e s o n a n c e to A u g e r s t i m u l a t e d i o n d e s o r p t i o n f o l l o w i n g delocali z a t i o n o f the excited electron, a n d to s p e c t a t o r Auger stimulated ion desorption and/or ultrafast ion desorption.
Acknowledgements W e wish to t h a n k the staff o f the U V S O R for v a l u a b l e s u p p o r t . This w o r k was s u p p o r t e d b y the J o i n t Studies P r o g r a m ( 1 9 9 5 - 1 9 9 6 ) o f I M S .
References [1] R.A. Rosenberg, V. Rehn, in: R.Z. Bachrach (Ed.), Synchrotron Research: Advances in Surface and Interface Science, Vol. 2, Issues and Technology, Plenum, New York, 1992, p. 267, and references cited therein. [2] D.E. Ramaker, in: W. Brenig, D. Menzel (Eds.), Desorption Induced by Electronic Transitions, DIET-II, Springer Series in Surface Sciences, Vol. 4, Springer, Berlin, 1985, p. 10. [3] K. Mase, M. Nagasono, S. Tanaka, T. Urisu, Y. Murata, Rev. Sci. Instrum. 68 (1997) 1703. [4] K. Mase, M. Nagasono, S. Tanaka, T. Urisu, E. Ikenaga, T. Sekitani, K. Tanaka, Surf. Sci. 390 (1997) 97, this issue. [5] M. Nagasono, K. Mase, S. Tanaka, T. Urisu, Surf. Sci. 377-379 (1997) 380. [6] M. Nagasono, K. Mase, S. Tanaka, T. Urisu, in preparation. [7] S. Tanaka, UVSOR Activity Rep. 24 (1997) 14. [8] D. Menzel, G. Rocker, D. Conlman, P. Feulner, W. Wurth, Phys. Scr. 41 (1990) 588. [9] J. Schirmer, A.B. Trofunov, K.J. Randall, J. Feldhaus, A.M. Bradshaw, Y. Ma, C.T. Chen, F. Sette, Phys. Rev. A 47 (1993) 1136. [10] G.R. Wight, C.E. Brion, J. Electron Spectrosc. Relat. Phenom. 4 (1974) 25. [11] R.R. Rye, T.E. Madey, J.E. Houston, P.H. Holloway, J. Chem. Phys. 69 (1978) 1504. [12] H. Aksela, S. Aksela, A. Naves de Brito, G.M. Bancroft, K.H. Tan, Phys. Rev. A 45 (1992) 7948.
surface science
ELSEVIER
Surface Science 390 (1997) 102-106
Study of ion desorption induced by a resonant core-level excitation of condensed NH3 using Auger-electron photo-ion coincidence (AEPICO) spectroscopy combined with synchrotron radiation Mitsuru Nagasono *, Kazuhiko Mase, Shin-ichiro Tanaka, Tsuneo Urisu Institute for Molecular Science, Myodaiji-cho, Okazaki 444, Japan
Received 15 February 1997; accepted for publication 26 June 1997
Abstract Photostimulated ion desorption at the 4al~-N ls resonant transition of condensed NH3 was studied using electron emission spectroscopy and Auger-electron photoion coincidence (AEPICO) spectroscopy. The total ion yield divided by the Auger-electron yield exhibited a threshold peak at hv=399 eV which is ascribed to the resonant transition from the N ls to the N H antibonding 4al orbital. The electron emission spectrum at the 4aa +-N 1s resonance is decomposed into three components: a valence photoelectron emission spectrum, and normal- and resonant-Auger-electron emission spectra. We ascribe the resonant-Auger-electron emission spectrum mainly to spectator-Auger transitions on the basis of the peak assignment. A series of AEPICO spectra at the 4a~~ N l s resonance was also measured as a function of the Auger-electron kinetic energy. The electron kinetic energy dependence of the H + AEPICO yield displays a shape approximately similar to that of the mixed spectrum of normal- and spectator-Auger-electron emission spectra. Based on this result the H ÷ desorption at the 4 a ~ N ls resonance is concluded to originate from the spectatorAuger transitions and from the normal-Auger transitions following the delocalization of the excited electron. © 1997 Elsevier Science B.V. Keywords: Ammonia; Amorphous surfaces; Auger electron spectroscopy; Desorption induced by electronic transitions; Photoelectron emission; Photon stimulated desorption; Synchrotron radiation photoelectron spectroscopy
1 . Introduction I o n d e s o r p t i o n i n d u c e d by core-electron transitions is a n interesting p h e n o m e n o n in surface science [1], for which a n A u g e r stimulated i o n d e s o r p t i o n m o d e l is proposed, i.e. A u g e r t r a n s itions leaving two-hole states are responsible for the i o n d e s o r p t i o n [2]. I n order to verify the A u g e r s t i m u l a t e d i o n d e s o r p t i o n m e c h a n i s m we have developed Auger-electron p h o t o i o n coincidence * Corresponding author. Fax: (+ 81) 564 53.7327; e-mail: [email protected] 0039-6028/97/$17.00 © 1997 Elsevier ScienceB.V. All rights reserved. PII $0039-6028 (97) 00525-6
( A E P I C O ) spectroscopy, which provides the ion yield i n d u c e d by selected A u g e r transitions [3-6]. Recently, we investigated H ÷ d e s o r p t i o n i n d u c e d b y N ls i o n i z a t i o n of c o n d e n s e d NH3, a n d clarified that the N K V V A u g e r transitions leaving holes in the N - H b o n d i n g orbitals induce H ÷ d e s o r p t i o n because o f h o l e - h o l e C o u l o m b repulsion [5]. I n the present p a p e r we describe a study of ion d e s o r p t i o n i n d u c e d b y the r e s o n a n t excitation from the N 1s to the 4al orbital of c o n d e n s e d NH3 using electron emission spectroscopy a n d A E P I C O spectroscopy. Three core-decay processes are expected to take place after the 4 a l a N ls trans-
M. Nagasono et al. / Surface Science 390 (1997) 102-106
ition, i.e. spectator-, participator- and normalAuger transitions. The goal of the work is to untangle the three possible core-decay contributions to the ion yield. The main method is to investigate the correlation of the H ÷ ion yield with the kinetic energy of electrons emitted coincidentally with the ions, the latter reflecting the electronic state(s) which produce the ions. The method includes a deconvolution of the electron emission spectrum by (1) subtracting the background due to electron energy loss processes, (2) subtracting the direct valence ionization, and (3) subtracting the normal-Auger decay contribution to give a pure resonant (participator and spectator) Auger-electron emission spectrum.
2. Experiments T h e apparatus used is described elsewhere [3, 4]. In brief, experiments were carried out in an ultrahigh vacuum chamber on line BL-2B1, which incorporates a 2-m grasshopper m o n o c h r o m a t o r , at the Ultraviolet Synchrotron Orbital Radiation facility at the Institute for Molecular Science [7]. P-polarized radiation impinged on the sample with an incidence angle of 60 ° to the surface normal. The photon energy resolution was about 1.2 eV at hv=400eV (Nls region). Electron emission spectra were measured by a double-path cylindrical mirror electron energy analyzer (CMA) with a resolution of a b o u t 3.3 eV at the path energy of 100 eV. The angle between the double-path C M A axis and surface normal was 30 °. A E P I C O spectra were measured by a remodeled electron ion coincidence analyzer consisting of a single-pass C M A without retarding grids, arranged at 30 ° f r o m the surface normal, and a time-of-flight ( T O F ) ion mass spectrometer in front of the sample [4]. The energy resolution of the single-path C M A was about 8 eV because of incomplete alignment. The T O F spectrometer collected ions desorbed into all solid angles. The sample was prepared by exposing an Au foil at 80 K to 100 L of N H 3. Total ion yield ( T I Y ) and Auger-electron yield ( A E Y ) spectra were also measured as a function of the photon energy by the electron ion coincidence analyzer.
103
3. Results and discussion Fig. 1 shows TIY, A E Y and T I Y divided by A E Y ( T I Y / A E Y ) spectra of condensed NH3 in the N K-edge region. The T I Y corresponds to the H ÷ yield from the surface NH3 because the dominant ion species is reported to be H ÷ [8]. The A E Y was measured at a fixed kinetic energy of 369 eV with a window of about 8 eV, which corresponds to the largest N K W Auger peak from condensed NH3. The A E Y shows the photoabsorption spectrum of condensed N H 3 in the first 50 layers or so. The T I Y and A E Y spectra are virtually identical to those reported by Menzel et al. [8]. The photon energy of the present study was calibrated on the basis of their report. The T I Y / A E Y spectrum displays a characteristic threshold peak at hv--399 eV, which has been assigned to the resonant transition from the N ls to the 4al orbital with an N - H antibonding character [9]. The 4al +-N 1s resonance, however, exhibits no corresponding peak in the A E Y spectrum. This result suggests that the dipole transition probability of the 4 a ~ N ls is reduced considerably in
a5
5 /17
.~c--
i'(
. . . .
d
-
\
400
TIY / AEY
405 410 415 Photon Energy (eV)
....._....._......................j,.......
420
.......
425
Fig. 1. TIY, AEY, and TIY/AEY spectra of condensed NH3. The Auger-electrons were selected at an electron kinetic energy of 369 eV with a window of about 8 eV corresponding to N KVV Auger-electrons. These spectra were normalized to the incident photon flux obtained from the current at a gold-mesh positioned in the radiation path.
104
M. Nagasono et al. / Surface Science 390 (1997) 102-106
the bulk NH3 because of band formation of the 4a 1 orbital through the hydrogen bonds. At the surface, on the other hand, the probability of the 4 a l a N ls transition is expected to be less perturbed because of the reduced coordination number. Therefore, the characteristic peak in the TIY/AEY spectrum at hv=399 eV suggests the existence of efficient H ÷ desorption channels initiated by the 4al+--N ls resonance of the surface N
' 'C;;e'l;;el 'i;nli;a'ti;n . . . . .
12' '/~
,-, [ Core level resonant excitation
' ' ' ' ' ' .... 4
A
-]
\
c =
]
H3.
The electron emission spectra measured at the N ls ionization (by=419 eV) and the 4 a a ~ N ls resonance (hv=399 eV) are shown in Fig. 2 as a normal-Auger-electron emission spectrum (normal-AES) (A) and a resonant electron emission spectrum (B) respectively. The ionization potential of the N 1s level of NH 3 in the gas phase is reported to be 405.6 eV [10]. The backgrounds due to energy-loss processes are subtracted in these spectra using a deconvolution technique in which the near-elastic backscattering electron spectrum measured by a primary electron beam of 370 eV was adopted as an energy-loss function [11]. The resonant spectrum (Fig. 2B) is expected to include contributions not only from resonant-Auger decay but also from valence electron excitations and nomaal-Auger decay following delocalization of a 4aa electron due to the resonant tunneling into the 4aa unoccupied band [4]. The mixed (normal +resonant) AES (Fig. 2C) was obtained from t h e spectrum at the 4 a ~ N ls resonance (Fig. 2B) by subtracting the shifted valence photoelectron spectrum at hv=396 eV (Fig. 2D). The resonant-AES (Fig. 2E) was extracted from the mixed AES (Fig. 2C) by subtracting the scaled normal-AES (Fig. 2A). In the resonant-AES (Fig. 2E) a main and broad peak was observed at an electron kinetic energy of 376 eV. This peak is not located at the kinetic energy of photoelectrons from the valence orbital, and is shifted to a higher kinetic energy compared with the main peak in the normal-AES. The resonant-Auger decay is classified into minor participator-Auger and predominant spectator-Auger transitions [12]. In participator-Auger transitions the electron in the unoccupied orbital takes part in the Auger decay, leaving one-hole in a valence orbital. The kinetic energy of the participator-
Valence ionization ) (hi) = 396 eV)
t
Spectator-AES (hi)=399eV)
L'-,,
320
2a
i i
1
E=C.A ,
[ i
;/ ~6 ~
t
S,7,,,,
340 360 380 Electron Kinetic Energy (eV)
Fig. 2. Electron emission spectra at hv=419 (A), 399 (B), and 396 eV (D). In these spectra the backgrounds due to energyloss processes were subtracted (see the text). The mixed (normal + resonant) AES (C) was extracted from the resonantelectron emission spectrum (B) by subtracting the valence photoelectron spectrum (D) after shifting it by the photon energy difference of 3 eV. The resonant-AES (E) was obtained from the mixed AES (C) by subtracting the scaled normal-AES (A') (which is obtained from (A) multiplied by an appropriate factor of 1/15, so that the resulting spectrum is canceled for electron kinetic energies less than 350 eV). The AES (C) and (E) were smoothed.
Auger-electron is the same as that of a photoelectron from the corresponding valence orbital. In spectator-Auger transitions, on the other hand, the excited electron does not participate in the transitions, and a two-hole-one-particle state is formed. The electron emitted by a spectator-Auger transition possesses a higher kinetic energy than that from the corresponding normal-Auger transition [12]. Therefore, we conclude that the main peak in the resonant-AES (Fig. 2E) arises from spectator-Auger transitions. A series of AEPICO spectra was measured at
M. Nagasono et al. / Surface Science 390 (1997) 102-106 H +
105
hv = 399 eV L
i
i
.4
:.
F
380
200
\t
~ 390
~4.~-~
150
370
360 --F
350
5=
o
AEP1CO
g
340i2 g
,
50
330 320 310
200
300
400 500 600 700 Difference TOF (ns)
300 800 2 1 0 Electron Yield (arb.u.)
Fig: 3. The AEPICO spectra (left) and electron emission spectra (B and F ) at the 4 a ~ N ls resonance. AEPICO spectra correspond to the electron kinetic energy scale (fight axis). The electron emission spectrum ( F ) shows raw data measured using the single-pass CMA of the coincidence analyzer under the same conditions as the AEPICO measurements. The electron emission spectrum (B) is the same as that in Fig. 2. The accumulation time was 300 s for each AEPICO spectrum.
the 4 a l a N ls resonance in the N KVV Auger region, as shown in Fig. 3. The coincidence H ÷ signals were observed in the T O F difference region of 475-565 ns with a good signal to background ratio. This observation is consistent with the argument that the enhanced TIY/AEY peak at h v = 399 eV is derived from the H ÷ desorption channels initiated by the 4at~-N ls resonance. The data points of the AEPICO yield obtained from the integrated H ÷ peak above the background level were plotted as a function of the electron kinetic energy, as shown in Fig. 4. The error bars of the AEPICO yield represent standard deviations) In contrast to the traditional ion yield spectra, the AEPICO measurements provide the ion yield arising from the selected Auger transitions correspondThe error in the AEPICO yield spectrum is given by T + ~rb2k, where A + T is the integrated sum of accidental and true coincidence counts in the H + coincidence time window (475-565 ns in the present case) and abk is the standard deviation of the background counts out of the time window.
0
300
''''
.
.
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.
.
320
.
0
340
360
380
Electron Kinetic Energy (eV) Fig. 4. The AEPICO yield spectrum as a function of the electron kinetic energy, The data points of the AEPICO yield (closed circles) are the integrated H ÷ peaks in Fig. 3. The AES (C and E), and the electron emission spectrum ( F ) are the same as those in Figs. 2 and 3.
ing to the kinetic energy of the electron emitted coincidentally with the ion. In other words, the AEPICO yield spectrum reflects the Auger transition probability of surface molecules multiplied by the probability of ion desorption derived from the selected Auger transitions. The shape o f the AEPICO yield spectrum is found to resemble that of the mixed (normal+ spectator) AES (Fig. 2C), except for the maximum peak at t h e electron kinetic energy of 376 eV. This result indicates that the ion desorption is induced by normal- and resonant-Auger decay. The maximum peak in the AEPICO yield spectrum at 376 eV, however, does not originate from the normal-Auger decay, because it was not observed in the AEPICO yield spectrum measured at the N ls ionization ( h v = 429 eV) [5]. A favorable explanation is that the H ÷ ion is produced by a spectator-Auger stimulated ion desorption model which is described as a three-step model; the first step is the 4 a t ~ N ls transition, the second step is spectator-Auger decay leaving a v~-iv214a~ 1 state, and the final step is H + desorption. Another candidate is an ultrafast ion desorption mechanism which is described by a four-step model [8]; the first step is the 4 a ~ N ls transition, the second step is the expansion of the N H z - H distance, the third step is
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M. Nagasono et al. / Surface Science 390 (1997) 102-106
s p e c t a t o r - A u g e r decay, a n d the final step is H ÷ d e s o r p t i o n . B o t h m o d e l s are consistent with the result t h a t the A E P I C O yield p e a k c o r r e s p o n d s to t h a t o f the s p e c t a t o r - A E S , as s h o w n in Fig. 4. I n o r d e r to clarify the c o n t r i b u t i o n s o f the two models, high-resolution AEPICO measurements a n d t h e o r e t i c a l investigations are required. I n s u m m a r y , the H ÷ d e s o r p t i o n i n d u c e d b y a r e s o n a n t N l s e x c i t a t i o n o f c o n d e n s e d N H 3 was investigated. T h e T I Y / A E Y s p e c t r u m exhibits a t h r e s h o l d p e a k at h v = 3 9 9 eV, which is a t t r i b u t e d to the r e s o n a n t excitation f r o m the N l s to the N - H a n t i b o n d i n g 4 a l orbital. T h e electron emission s p e c t r u m at the 4al+--N ls r e s o n a n c e was d e c o m p o s e d i n t o three c o m p o n e n t s : a valence p h o t o e l e c t r o n emission spectrum, a n o r m a l - A E S , a n d a r e s o n a n t - A E S . T h e structure o f the A E P I C O yield s p e c t r u m is o b s e r v e d to r e s e m b l e the m i x e d n o r m a l - a n d r e s o n a n t - A E S expect f o r the strongest peak. T h e p o s i t i o n o f the strongest p e a k is o b s e r v e d to c o r r e s p o n d to s p e c t a t o r - A u g e r transitions. O n the basis o f these results we ascribe the H + d e s o r p t i o n at the 4 a l a N l s r e s o n a n c e to A u g e r s t i m u l a t e d i o n d e s o r p t i o n f o l l o w i n g delocali z a t i o n o f the excited electron, a n d to s p e c t a t o r Auger stimulated ion desorption and/or ultrafast ion desorption.
Acknowledgements W e wish to t h a n k the staff o f the U V S O R for v a l u a b l e s u p p o r t . This w o r k was s u p p o r t e d b y the J o i n t Studies P r o g r a m ( 1 9 9 5 - 1 9 9 6 ) o f I M S .
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