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De-excitation in solid SiCl4 following deep-core excitation at the K-edge: relation between ion desorption and auger decay
.......~,::~i~i~ii~i~i~z~i~e~:~, -...........
surfa.ce science ELSEVIER
Surface Science 357-358 (1996) 302-306
De-excitation in solid SiC14 following deep-core excitation at the K-edge: relation between ion desorption and Auger decay Y. Baba *, K. Yoshii, T.A. Sasaki Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, lbaraki-ken, 319-I1, Japan
Received 15 August 1995; accepted for publication27 October 1995
Abstract Photon-stimulated ion desorption from solid SiC14 following the deep-core excitations at the K-edges has been investigated together with the Auger decay spectra using synchrotron radiation. The major Auger-decay channel following the ls ~ o- * resonance excitation was KLL spectator Auger decay, in which the excited electron remains in the o- * orbital during the Auger transition. The photon-energy dependence of the ion desorption yields around the C1 K-edge revealed that the C1+ ions scarcely desorb in the photon energy region where C1Kt~ normal Auger decay happens. This fact indicates that the spectator Auger decay is essential for the C1+ desorption because the existence of the spectator electron in the antibonding o-* orbital reduces the Si-C1 bonding character resulting in the fra~nentation. Among the C1 ls ~ o-* resonances, C1+ desorption yield is high at the C1 l s ~ o" ~(8al ) resonance compared to that at the C1 ls---> o-*(9t 2) resonance. The result is explained by the higher component of the antibonding C1 3p * in the 8a I orbital. Based on these results, it is concluded that the dissociation of the Si-C1 bond by the C1 ls ~ o- * resonance excitation is faster than the core hole decay, which means that the C1 atom moves during the core life time, i.e. ultrafast non-Franck-Condon-like dissociation happens. Keywords: Auger electron spectroscopy; Desorptioninduced by electronictransitions(DIET); Halides;Photon stimulateddesorption(PSD);
Silane; X-ray absorptionspectroscopy
1. Introduction Photon-stimulated desorption (PSD) induced by inner-shell excitation has attracted much attention, because core excitation is in many cases superior to valence excitation in order to desorb a specific element or site from molecular adsorbates using an energy-tunable synchrotron beam in the X-ray region. It has been established that most of the desorbing species by core excitation are positive ions, and
* Corresponding author. Fax: + 81 29 282 5927.
the mechanism of the ion desorption is well interpreted on the basis of the K n o t e k - F e i b e l m a n model (KF model) [1]. Most of the works on PSD by core excitation to date have employed photons in the vacuum ultraviolet (VUV) region ( < 1 keV) [2]. This paper reports the PSD following deep-core excitation employing a synchrotron beam in the X-ray region ( 1 . 8 - 2 . 9 keV). Here we investigate the PSD following the 1s ~ 3p * resonance excitation in SiC14, which is one of the important molecules in silicon semiconductor processing. The characteristics of PSD by deep-core excita-
003%6028/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S 0039-6028(96)001 33-1
Y. Baba et al./ Surface Science 357-358 (1996) 302-306
tion are summarized in; (i) high desorption yield due to the formation of highly multiple-charged ions by Auger cascades, and (ii) high site-selectivity for the desorption of a specific eleruent or fragment because of the large energy separation of the core levels in the deep-core region. In order to clarify the resorption mechanism, we concentrate on the relation between ion desorption and Auger decay processes.
2. Experimental The experiments were performed at the BL-27A station of the Photon Factory in the National Laboratory for High Energy Physics. The energy resolution of the InSb(111) double-crystal monochromator was 0.8 eV at 1.85 keV (Si K-edge) and 1.3 eV at 2.83 keY (C1 K-edge). SiC14 liquid was purified through several freezepump-thaw cycles, and dosed onto the clean surface of Cu(100) at a surface temperature of 85 K through a gas doser made of glass filter. The thickness of the adsorbate, determined by the temperature-programmed desorption (TPD) measurements, was 600 layers. The Auger electron spectra were measured by a hemispherical electron energy analyzer (VSW. CLASS-100). The polar angle of the electric vector of the incident X-rays was 35 ° and the take-off direction of the Auger electrons was surface normal. To eliminate the surface charging effect on the kinetic energy, an electron flood gun with constant electron energy was used during the measurements. The X-ray absorption spectra were measured by two modes, i.e. total electron yield (TEY) mode and Auger electron yield (AEY) mode. The former was obtained by the sample current and the latter was recorded by measuring the intensity of the KLL Auger electrons using the electron energy analyzer. The desorbed ions were measured by a quadrupole mass spectrometer (ULVAC, MSQ-1000) operating in pulse-counting mode. The polar angle of the electric vector of the incident X-rays was 7 ° and the take-off direction of the ions was 83 ° from the surface. Both electron and ion yields were normalized by the photon intensity which was monitored by the current of copper mesh located in front of the sample.
303
3. Results and discussion For the excitation around the Si K-edge, the Si +, C1 +, SiC1+ and SiCl~- ions were observed almost at comparable intensity. The photon-energy dependences of electron and ion yields around the Si K-edge are shown in the lower column of Fig. 1. The electron-yield curve essentially represents the X-ray absorption spectrum. If the KF model can be applied, the electron yield measured by the KLL Auger electron yield is proportional to the desorption yield. This is indeed the case for the yield curves of four main desorbed species, i.e. Si +, C1 ÷, SiC1÷ and SiCI~- ions. For C1 K-edge excitation, on the other hand, more than 80% of the desorbed species were C1÷ ions. The electron and ion yields around the C1 K-edge are displayed in the bottom colunm of Fig. 2. A clear dissimilarity between electron-yield and Cl+-yield curves is seen. In the electron-yield curve, mainly
Si K-edge Si KLL
$pectator Nornal
~,
--o,-~-~-T
,,
,
T4
Electron yield
.i.I
J/ '~"
CI+
,,/'k,
S i C 13+ 18~,4 18;6 18~48 1858 18~52 1854 Photon e n e r g y (eV)
..............
~:""....................
Fig. l. Photon-energy dependences of intensities of SiKLL spectator and normal Auger lines (upper column), and electron, Si--, CI +, SiC1+ and SiCI~ yields (lower column) around the Si K-edge.
Y. Baba et al. / Surface Science 357-358 (1996) 302-306
304
CI K-edge Participator 10
•
CI 2s
oo o a¢ v >~
_~x___~
0
1 CI
~o
z~z~.~--__z~___zx_
~
I
1
KLL
2p
CI
I
Spectator
100
assume the primary electronic configuration as l s - 13p* 1, the succeeding electronic configuration would be 2p-23p * 1 for KLL spectator, 2p-1 for KL participator and 2p- 1V - 13p * 1 for KLV spectator, respectively, where V denotes valence orbital (dipole-forbidden transitions concerning 2s orbital is ruled out here). The kinetic energies ( E k) of the Auger electrons are simply estimated as Ek(KLL)
.pa c //
\\
= G ( K ) - 2 g b ( L ) - U(L, L) +R(L, L),
Normal
(KLL normal)
(1)
Ek(KL ) = h v - Eb(L ) + R ( L ) ,
0 ¢o
Z
"7-
]a4
(KL participator)
o
(2)
Ek(KLL) = h ~ - 2Eb(L ) - U(L, L) + R'(L, L), Electron
.kj
(3)
ek(KLV)
...
¢o
,~
(KLL spectator)
yield
=a
= h u - E b ( L ) --Eb(V ) -- U(L, V) + R"(L, V), ........,.....,...,.....,..""
28120
'".-... -.............., ........................ .......
Cl +
2825 28130 2835 Photon e n e r g y (eV)
Fig. 2. Photon-energydependencesof intensitiesC1KLparticipator Auger lines (topmostcolumn), intensities of C1KLL spectatorand normal Augerlines (middle column),and electronand C1+ yields (bottom column) around the C1 K-edge. three peaks are seen (numbered 2, 4 and 6), which originate from C1 ls --+ o- *(8al), C1 ls --+ o- *(9t 2) and double excitation (shake-up satellite) respectively [3,4], but the Cl + yield curve has only one maximum corresponding to peak 2. To discuss the de-excitation processes leading to the ion desorption, we will concentrate on the radiationless transition (Auger decay) as a primary decay process since radiative transition plays a minor role (the X-ray fluorescence yield of Cl ls hole is 9.7% [5]). It has been established, that the resonance excitation from core to valence unoccupied orbital is mainly followed by two types of Auger decay. The first type is a participator Auger decay (or autoionization) in which the excited electron itself decays into the core hole. The second one is a spectator Auger decay in which the excited electron remains in the excited state in the course of the Auger transition. When we
(KLV spectator)
(4)
where E b is the binding energy, U is the hole-hole interaction energy, R (R', R") is the relaxation energy for the respective shells indicated in the brackets, and hv is the photon energy. Around the K-edge excitation, Eqs. (1)-(4) show that the KLL spectator Auger peak appears near the KLL normal Auger peak, the KL participator Auger peak is observed as the enhancement of the 2p (or 2s) photoelectron peak, and the KLV spectator Auger peak shows up near the 2p (or 2s) photoelectron peak. The electron energy spectra taken at various photon energies around the C1 K-edge are displayed in Figs. 3 and 4. Fig. 3 includes the C1KL participator and C1KLv spectator Auger decays. Fig. 4 covers the C1KLL spectator and normal Auger decays. In Fig. 3, a slight enhancement of the C1 2p photoelectron peak around the C1 l s ~ o-* resonance maximum ( h u = 2823.0 eV) is observed. In the topmost column of Fig. 2, the intensities of the C1 2s and C1 2p photoelectron peaks obtained from Fig. 3 are plotted as a function of the photon energy. The increase in the C1 2p intensity is about 25% at the C1 ls--+ o-*(8al) resonance maximum (peak 2 in electron yield curve). Such enhancement of the photoelectron peak around the core resonance has been observed
Y. Baba et a l . / SurfaCe Science 357-358 (1996) 302-306
CI K-edge
for the Mo 3d and Mo 3p photoelectrons around the Mo 2p excitation, and the phenomenon was interpreted in terms of the interference between direct photoemission and the participator Auger transition
B
,.,
~-.-¢a)
/
h~ (nV)
\
::,.. ~ ~ ~ "
'~-(2)
"* ~
~'~
c::
._= ~ . e " -~
2821. T
2823. 0 (3) 2823. T (4) 2824. 8
A
"'~"~"-..--(8)2825. 0
~ ' * " "
-
~" ~ " ' ~ "
"'~"~
(6) 2829. 0
~ " ~
. . . . . .
(7)2832.4 ......
'
23r80
'
3084.4 2390 ,
Kinetic energy (eV) Fig. 4. CIKLL Auger-decay spectra in normal and spectator regions excited at various photon energies around the C1 K-edge. The number indicated in each spectrum corresponds to the ones shown in the electron yield curve of Fig. 2.
CI K-edge CI2s 1 :'..
,,,,
o. o
[61. In Fig. 4, we can clearly see the spectator line (peak B) at a slightly higher energy than that of the normal Auger line (peak A), The intensities of the KLL spectator and normal Auger peaks at the Si and C1 K-edges are plotted in the upper column of Fig. 1 and in the middle column of Fig. 2, respectively. For C1 K-edge, the plots of the intensity of three types of the Auger peaks reveal that about 98% of the C1 ls o- * resonance excitations (peak 2) is followed by the spectator Auger decays (including KLL and KLV), while the contribution of the C1 2p-derived participator decay is less than 2%. Therefore, the spectator Auger decay plays an important role in the ion desorption. The Auger intensity plots for the C1 K-edge also reveal that excitation at peak 6 is followed only by
305
CI2p Ch~*
""'~" - ~ ;
Si2s
--~-~
h~ (eV) (I)2921.7
..
-.
..
_4
i
I ~ ~--'--"'~-- C4)2924. 3
]
] ',-. ~-,--.--~
(,).,3 ,
i
•, , _ _ . , ; ~
(s) 2s25. 0
~ r (7)m2.
28'50 28'00 2680 27'00 Kinetic energy (eV) Fig. 3. CIKL participator Auger-decay spectra excited at various photon energies around the C1 K-edge. The number indicated in each spectrum corresponds to that shown in the electron yield curve of Fig. 2.
the normal Auger decay. The most intrinsic difference between normal and spectator Auger decays is whether or not the excited electron remains in the antibonding orbitals in the course of the Auger electron emission. For the excitation at peak 6, therefore, the excited electron is immediately delocalized before the movement of the C1 atom. Thus it is concluded that the existence of an electron in the o-* orbital (hereafter we call such an electron as 'spectator electron') is essential for the fragmentation which results in the C1 + desorption. When we compare two o- * resonances with different symmetry i.e. 8a I (peak 2) and 9t~ (peak 4), the C1+ ions desorb mainly at peak 2 rather than at peak 4, although both excitations result in the spectator Auger decay. This finding has been explained in the previous paper on the basis of molecular-orbital character [7]. Briefly, the contents of the C1 3p* orbital component is higher in the 8a 1 than in the 9t 2 [8], consequently the spectator electron in the 8a I is
306
Y. Baba et a l . / Surface Science 357-358 (1996) 302-306
more effective for the dissociation of the Si-C1 bond than the spectator electron in the 9t 2. This speculation supports the above conclusion that the spectator electron in an antibonding orbital is essential for C1 + desorption. The high desorption yield by the C1 ls o- * (8a 1) resonance indicates that the nuclear motion of the Si-C1 bond is equivalent to or faster than the core life time. This means that the Franck-Condon transition cannot be applied to the Si-C1 dissociation process. Such ultrafast non-Franck-Condonlike dissociation was reported for H + desorption by core excitation from condensed molecules such as H 2 0 [9] and benzene [10]. The present results reveal that such a non-Franck-Condon-like process exists even for the desorption of heavier atoms like chlorine. In summary, we have observed a clear dissimilarity between electron and C1 + desorption yields in solid SiC14 around the C1 K-edge excitation. The C1 ÷ ions scarcely desorb in the photon energy region where normal Auger decay happens. Among the C1 ls ~ o- * resonances, the C1 ls ~ o- *(8a 1) excitation is more effective for the C1 + desorpfion than the C1 ls ~ o-*(9t 2) excitation. Based on these results, it is concluded that ultrafast non-Franck-Condon-like process happens for the fragmentation of the Si-C1 bond following the C1 ls --* o- * excitation especially when the ~r * orbital has a high antibonding character.
Acknowledgements The authors would like to acknowledge the staff of the Photon Factory of the National Laboratory for High Energy Physics for their assistance throughout the experiments. This work was performed under the approval of the Photon Factory Program Advisory Committee (PF-PAC no. 93G316).
References [1] M.L. Knotek and P.J. Feibelrnan, Phys. Rev. Lett. 40 (1978) 964. [2] R.A. Rosenberg, S.P. Frigo and J.K. Simons, Appl. Surf. Sci. 79/80 (1994) 47. [3] J.L. Ferrer, S. Bodeur and I. Nenner, J. Electron Spectrosc. Relat. Phenom. 52 (1990)711. [4] Y. Baba, K. Yoshii, H. Yamamoto and T.A. Sasaki, J. Phys.: Condens. Matter 7 (1995) 1991. [5] M.O. Krause, J. Phys. Chem. Ref. Data 8 (1979) 307. [6] T.A. Sasaki, Y. Baba, K. Yoshii, H. Yamamoto and T. Nakatani, Phys. Rev. B 50 (1994) 15519. [7] Y. Baba, K. Yoshii and T.A. Sasaki, Surf. Sci. 341 (1995) 90. [8] H. Ishikawa, K. Fujima, H. Adachi, E. Miyanchi and T. Fujii, J. Phys. Chem. 94 (1991) 6740. [9] D. Coulman, A. Puschmann, W. Wurth, P. Feulner, H.P. Steinrfick and D. Menzel, Chem. Phys. Lett. 41 (1990) 1014. [10] D. Menzel, G. Rocker, H.P. Steinriick, D. Coulman, P.A. Heimann, W. Huber, P. Zebiseh and D.R. Lloyd, J. Phys. Chem. 96 (1992) 1724.
surfa.ce science ELSEVIER
Surface Science 357-358 (1996) 302-306
De-excitation in solid SiC14 following deep-core excitation at the K-edge: relation between ion desorption and Auger decay Y. Baba *, K. Yoshii, T.A. Sasaki Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, lbaraki-ken, 319-I1, Japan
Received 15 August 1995; accepted for publication27 October 1995
Abstract Photon-stimulated ion desorption from solid SiC14 following the deep-core excitations at the K-edges has been investigated together with the Auger decay spectra using synchrotron radiation. The major Auger-decay channel following the ls ~ o- * resonance excitation was KLL spectator Auger decay, in which the excited electron remains in the o- * orbital during the Auger transition. The photon-energy dependence of the ion desorption yields around the C1 K-edge revealed that the C1+ ions scarcely desorb in the photon energy region where C1Kt~ normal Auger decay happens. This fact indicates that the spectator Auger decay is essential for the C1+ desorption because the existence of the spectator electron in the antibonding o-* orbital reduces the Si-C1 bonding character resulting in the fra~nentation. Among the C1 ls ~ o-* resonances, C1+ desorption yield is high at the C1 l s ~ o" ~(8al ) resonance compared to that at the C1 ls---> o-*(9t 2) resonance. The result is explained by the higher component of the antibonding C1 3p * in the 8a I orbital. Based on these results, it is concluded that the dissociation of the Si-C1 bond by the C1 ls ~ o- * resonance excitation is faster than the core hole decay, which means that the C1 atom moves during the core life time, i.e. ultrafast non-Franck-Condon-like dissociation happens. Keywords: Auger electron spectroscopy; Desorptioninduced by electronictransitions(DIET); Halides;Photon stimulateddesorption(PSD);
Silane; X-ray absorptionspectroscopy
1. Introduction Photon-stimulated desorption (PSD) induced by inner-shell excitation has attracted much attention, because core excitation is in many cases superior to valence excitation in order to desorb a specific element or site from molecular adsorbates using an energy-tunable synchrotron beam in the X-ray region. It has been established that most of the desorbing species by core excitation are positive ions, and
* Corresponding author. Fax: + 81 29 282 5927.
the mechanism of the ion desorption is well interpreted on the basis of the K n o t e k - F e i b e l m a n model (KF model) [1]. Most of the works on PSD by core excitation to date have employed photons in the vacuum ultraviolet (VUV) region ( < 1 keV) [2]. This paper reports the PSD following deep-core excitation employing a synchrotron beam in the X-ray region ( 1 . 8 - 2 . 9 keV). Here we investigate the PSD following the 1s ~ 3p * resonance excitation in SiC14, which is one of the important molecules in silicon semiconductor processing. The characteristics of PSD by deep-core excita-
003%6028/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S 0039-6028(96)001 33-1
Y. Baba et al./ Surface Science 357-358 (1996) 302-306
tion are summarized in; (i) high desorption yield due to the formation of highly multiple-charged ions by Auger cascades, and (ii) high site-selectivity for the desorption of a specific eleruent or fragment because of the large energy separation of the core levels in the deep-core region. In order to clarify the resorption mechanism, we concentrate on the relation between ion desorption and Auger decay processes.
2. Experimental The experiments were performed at the BL-27A station of the Photon Factory in the National Laboratory for High Energy Physics. The energy resolution of the InSb(111) double-crystal monochromator was 0.8 eV at 1.85 keV (Si K-edge) and 1.3 eV at 2.83 keY (C1 K-edge). SiC14 liquid was purified through several freezepump-thaw cycles, and dosed onto the clean surface of Cu(100) at a surface temperature of 85 K through a gas doser made of glass filter. The thickness of the adsorbate, determined by the temperature-programmed desorption (TPD) measurements, was 600 layers. The Auger electron spectra were measured by a hemispherical electron energy analyzer (VSW. CLASS-100). The polar angle of the electric vector of the incident X-rays was 35 ° and the take-off direction of the Auger electrons was surface normal. To eliminate the surface charging effect on the kinetic energy, an electron flood gun with constant electron energy was used during the measurements. The X-ray absorption spectra were measured by two modes, i.e. total electron yield (TEY) mode and Auger electron yield (AEY) mode. The former was obtained by the sample current and the latter was recorded by measuring the intensity of the KLL Auger electrons using the electron energy analyzer. The desorbed ions were measured by a quadrupole mass spectrometer (ULVAC, MSQ-1000) operating in pulse-counting mode. The polar angle of the electric vector of the incident X-rays was 7 ° and the take-off direction of the ions was 83 ° from the surface. Both electron and ion yields were normalized by the photon intensity which was monitored by the current of copper mesh located in front of the sample.
303
3. Results and discussion For the excitation around the Si K-edge, the Si +, C1 +, SiC1+ and SiCl~- ions were observed almost at comparable intensity. The photon-energy dependences of electron and ion yields around the Si K-edge are shown in the lower column of Fig. 1. The electron-yield curve essentially represents the X-ray absorption spectrum. If the KF model can be applied, the electron yield measured by the KLL Auger electron yield is proportional to the desorption yield. This is indeed the case for the yield curves of four main desorbed species, i.e. Si +, C1 ÷, SiC1÷ and SiCI~- ions. For C1 K-edge excitation, on the other hand, more than 80% of the desorbed species were C1÷ ions. The electron and ion yields around the C1 K-edge are displayed in the bottom colunm of Fig. 2. A clear dissimilarity between electron-yield and Cl+-yield curves is seen. In the electron-yield curve, mainly
Si K-edge Si KLL
$pectator Nornal
~,
--o,-~-~-T
,,
,
T4
Electron yield
.i.I
J/ '~"
CI+
,,/'k,
S i C 13+ 18~,4 18;6 18~48 1858 18~52 1854 Photon e n e r g y (eV)
..............
~:""....................
Fig. l. Photon-energy dependences of intensities of SiKLL spectator and normal Auger lines (upper column), and electron, Si--, CI +, SiC1+ and SiCI~ yields (lower column) around the Si K-edge.
Y. Baba et al. / Surface Science 357-358 (1996) 302-306
304
CI K-edge Participator 10
•
CI 2s
oo o a¢ v >~
_~x___~
0
1 CI
~o
z~z~.~--__z~___zx_
~
I
1
KLL
2p
CI
I
Spectator
100
assume the primary electronic configuration as l s - 13p* 1, the succeeding electronic configuration would be 2p-23p * 1 for KLL spectator, 2p-1 for KL participator and 2p- 1V - 13p * 1 for KLV spectator, respectively, where V denotes valence orbital (dipole-forbidden transitions concerning 2s orbital is ruled out here). The kinetic energies ( E k) of the Auger electrons are simply estimated as Ek(KLL)
.pa c //
\\
= G ( K ) - 2 g b ( L ) - U(L, L) +R(L, L),
Normal
(KLL normal)
(1)
Ek(KL ) = h v - Eb(L ) + R ( L ) ,
0 ¢o
Z
"7-
]a4
(KL participator)
o
(2)
Ek(KLL) = h ~ - 2Eb(L ) - U(L, L) + R'(L, L), Electron
.kj
(3)
ek(KLV)
...
¢o
,~
(KLL spectator)
yield
=a
= h u - E b ( L ) --Eb(V ) -- U(L, V) + R"(L, V), ........,.....,...,.....,..""
28120
'".-... -.............., ........................ .......
Cl +
2825 28130 2835 Photon e n e r g y (eV)
Fig. 2. Photon-energydependencesof intensitiesC1KLparticipator Auger lines (topmostcolumn), intensities of C1KLL spectatorand normal Augerlines (middle column),and electronand C1+ yields (bottom column) around the C1 K-edge. three peaks are seen (numbered 2, 4 and 6), which originate from C1 ls --+ o- *(8al), C1 ls --+ o- *(9t 2) and double excitation (shake-up satellite) respectively [3,4], but the Cl + yield curve has only one maximum corresponding to peak 2. To discuss the de-excitation processes leading to the ion desorption, we will concentrate on the radiationless transition (Auger decay) as a primary decay process since radiative transition plays a minor role (the X-ray fluorescence yield of Cl ls hole is 9.7% [5]). It has been established, that the resonance excitation from core to valence unoccupied orbital is mainly followed by two types of Auger decay. The first type is a participator Auger decay (or autoionization) in which the excited electron itself decays into the core hole. The second one is a spectator Auger decay in which the excited electron remains in the excited state in the course of the Auger transition. When we
(KLV spectator)
(4)
where E b is the binding energy, U is the hole-hole interaction energy, R (R', R") is the relaxation energy for the respective shells indicated in the brackets, and hv is the photon energy. Around the K-edge excitation, Eqs. (1)-(4) show that the KLL spectator Auger peak appears near the KLL normal Auger peak, the KL participator Auger peak is observed as the enhancement of the 2p (or 2s) photoelectron peak, and the KLV spectator Auger peak shows up near the 2p (or 2s) photoelectron peak. The electron energy spectra taken at various photon energies around the C1 K-edge are displayed in Figs. 3 and 4. Fig. 3 includes the C1KL participator and C1KLv spectator Auger decays. Fig. 4 covers the C1KLL spectator and normal Auger decays. In Fig. 3, a slight enhancement of the C1 2p photoelectron peak around the C1 l s ~ o-* resonance maximum ( h u = 2823.0 eV) is observed. In the topmost column of Fig. 2, the intensities of the C1 2s and C1 2p photoelectron peaks obtained from Fig. 3 are plotted as a function of the photon energy. The increase in the C1 2p intensity is about 25% at the C1 ls--+ o-*(8al) resonance maximum (peak 2 in electron yield curve). Such enhancement of the photoelectron peak around the core resonance has been observed
Y. Baba et a l . / SurfaCe Science 357-358 (1996) 302-306
CI K-edge
for the Mo 3d and Mo 3p photoelectrons around the Mo 2p excitation, and the phenomenon was interpreted in terms of the interference between direct photoemission and the participator Auger transition
B
,.,
~-.-¢a)
/
h~ (nV)
\
::,.. ~ ~ ~ "
'~-(2)
"* ~
~'~
c::
._= ~ . e " -~
2821. T
2823. 0 (3) 2823. T (4) 2824. 8
A
"'~"~"-..--(8)2825. 0
~ ' * " "
-
~" ~ " ' ~ "
"'~"~
(6) 2829. 0
~ " ~
. . . . . .
(7)2832.4 ......
'
23r80
'
3084.4 2390 ,
Kinetic energy (eV) Fig. 4. CIKLL Auger-decay spectra in normal and spectator regions excited at various photon energies around the C1 K-edge. The number indicated in each spectrum corresponds to the ones shown in the electron yield curve of Fig. 2.
CI K-edge CI2s 1 :'..
,,,,
o. o
[61. In Fig. 4, we can clearly see the spectator line (peak B) at a slightly higher energy than that of the normal Auger line (peak A), The intensities of the KLL spectator and normal Auger peaks at the Si and C1 K-edges are plotted in the upper column of Fig. 1 and in the middle column of Fig. 2, respectively. For C1 K-edge, the plots of the intensity of three types of the Auger peaks reveal that about 98% of the C1 ls o- * resonance excitations (peak 2) is followed by the spectator Auger decays (including KLL and KLV), while the contribution of the C1 2p-derived participator decay is less than 2%. Therefore, the spectator Auger decay plays an important role in the ion desorption. The Auger intensity plots for the C1 K-edge also reveal that excitation at peak 6 is followed only by
305
CI2p Ch~*
""'~" - ~ ;
Si2s
--~-~
h~ (eV) (I)2921.7
..
-.
..
_4
i
I ~ ~--'--"'~-- C4)2924. 3
]
] ',-. ~-,--.--~
(,).,3 ,
i
•, , _ _ . , ; ~
(s) 2s25. 0
~ r (7)m2.
28'50 28'00 2680 27'00 Kinetic energy (eV) Fig. 3. CIKL participator Auger-decay spectra excited at various photon energies around the C1 K-edge. The number indicated in each spectrum corresponds to that shown in the electron yield curve of Fig. 2.
the normal Auger decay. The most intrinsic difference between normal and spectator Auger decays is whether or not the excited electron remains in the antibonding orbitals in the course of the Auger electron emission. For the excitation at peak 6, therefore, the excited electron is immediately delocalized before the movement of the C1 atom. Thus it is concluded that the existence of an electron in the o-* orbital (hereafter we call such an electron as 'spectator electron') is essential for the fragmentation which results in the C1 + desorption. When we compare two o- * resonances with different symmetry i.e. 8a I (peak 2) and 9t~ (peak 4), the C1+ ions desorb mainly at peak 2 rather than at peak 4, although both excitations result in the spectator Auger decay. This finding has been explained in the previous paper on the basis of molecular-orbital character [7]. Briefly, the contents of the C1 3p* orbital component is higher in the 8a 1 than in the 9t 2 [8], consequently the spectator electron in the 8a I is
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more effective for the dissociation of the Si-C1 bond than the spectator electron in the 9t 2. This speculation supports the above conclusion that the spectator electron in an antibonding orbital is essential for C1 + desorption. The high desorption yield by the C1 ls o- * (8a 1) resonance indicates that the nuclear motion of the Si-C1 bond is equivalent to or faster than the core life time. This means that the Franck-Condon transition cannot be applied to the Si-C1 dissociation process. Such ultrafast non-Franck-Condonlike dissociation was reported for H + desorption by core excitation from condensed molecules such as H 2 0 [9] and benzene [10]. The present results reveal that such a non-Franck-Condon-like process exists even for the desorption of heavier atoms like chlorine. In summary, we have observed a clear dissimilarity between electron and C1 + desorption yields in solid SiC14 around the C1 K-edge excitation. The C1 ÷ ions scarcely desorb in the photon energy region where normal Auger decay happens. Among the C1 ls ~ o- * resonances, the C1 ls ~ o- *(8a 1) excitation is more effective for the C1 + desorpfion than the C1 ls ~ o-*(9t 2) excitation. Based on these results, it is concluded that ultrafast non-Franck-Condon-like process happens for the fragmentation of the Si-C1 bond following the C1 ls --* o- * excitation especially when the ~r * orbital has a high antibonding character.
Acknowledgements The authors would like to acknowledge the staff of the Photon Factory of the National Laboratory for High Energy Physics for their assistance throughout the experiments. This work was performed under the approval of the Photon Factory Program Advisory Committee (PF-PAC no. 93G316).
References [1] M.L. Knotek and P.J. Feibelrnan, Phys. Rev. Lett. 40 (1978) 964. [2] R.A. Rosenberg, S.P. Frigo and J.K. Simons, Appl. Surf. Sci. 79/80 (1994) 47. [3] J.L. Ferrer, S. Bodeur and I. Nenner, J. Electron Spectrosc. Relat. Phenom. 52 (1990)711. [4] Y. Baba, K. Yoshii, H. Yamamoto and T.A. Sasaki, J. Phys.: Condens. Matter 7 (1995) 1991. [5] M.O. Krause, J. Phys. Chem. Ref. Data 8 (1979) 307. [6] T.A. Sasaki, Y. Baba, K. Yoshii, H. Yamamoto and T. Nakatani, Phys. Rev. B 50 (1994) 15519. [7] Y. Baba, K. Yoshii and T.A. Sasaki, Surf. Sci. 341 (1995) 90. [8] H. Ishikawa, K. Fujima, H. Adachi, E. Miyanchi and T. Fujii, J. Phys. Chem. 94 (1991) 6740. [9] D. Coulman, A. Puschmann, W. Wurth, P. Feulner, H.P. Steinrfick and D. Menzel, Chem. Phys. Lett. 41 (1990) 1014. [10] D. Menzel, G. Rocker, H.P. Steinriick, D. Coulman, P.A. Heimann, W. Huber, P. Zebiseh and D.R. Lloyd, J. Phys. Chem. 96 (1992) 1724.