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The poisonous effect of alkali adatoms on Cl2 sticking
N
surface science
ELSEVIER
Surface Science363 (1996) 11-21
The poisonous effect of alkali adatoms on C12 sticking S. Yoneda, Y. Babasaki, M. Tanaka, F.H. Geuzebroek 1, F. Koga, N. Yamazaki, A. Namiki * Department of Electrical and Electronic Engineering, Toyohashi University of Technology, Tempaku, Toyohashi 44I, Japan
Received 31 August 1995;accepted for publication 8 September 1995
Abstract
C12 sticking on alkali-covered Si(100) is activated by the incident energies, Ei, i.e., there is a potential barrier to sticking. To understand the origin of the activation barrier, scattering experiments are performed. The observed C12 scattering is predominated by direct-inelastic scattering even for Ei < 0.1 eV, suggesting that the Cs adatoms play a role in making the physisorption potential shallow, or the gas-surface interaction potential repulsive. Pauli repulsion is proposed for the origin of this repulsive interaction. Keywords: Alkali metals; Halogens;Low index crystal surfaces; Molecule-solid scattering and diffraction- inelastic; Silicon; Sticking; Surface chemical reaction
1. Introduction
Sticking of simple diatomic molecules such as H2 or 0 2 may be influenced by alkali adatoms [1]. The alkali effects on sticking appear to be either poisonous or promotive depending on the chemical nature of the molecules. Electron affinities, EA, of the molecules seems to be a crucial parameter in characterizing whether the effect is promotive or poisoning: for H2 molecules with strongly negative EA, a strong poisoning effect on dissociative sticking is recognized in various metal surfaces [ 1 - 3 ] . Non-dissociative adsorption of CO molecules which have just negative EA ( - 1.5 eV) are also observed to be poisoned by the alkali adatoms [ 1]. On the other hand, for 0 2 or N O * Corresponding author. 1Present address: Koninklijk/SheU-Laboratofium, Amsterdam, P.O. Box 3003, 1003 AA Amsterdam, The Netherlands.
molecules, whose EA are just positive, a strong promotive effect on dissociative sticking is recognized on metals [ 1 ] as well as on semiconductors [4]. Brown et al. [ 2 ] suggest that, under conditions with reduced surface work functions, Pauli repulsion plays a crucial role in the poisonous alkali effects on sticking of negative EA molecules [2]. The driving force for the alkali poisoning in this case is energy consumption of the incident molecule upon repelling the unwanted surface electron charges out of the molecular region. On the other hand, for the case of the alkali promotion, it is widely believed that the surface electron charge transfer to the affinity level of the incident molecule plays an essential role on the alkali promotion [5]. The driving force for the alkali promotion is energy gain of the incident molecule upon pulling the surface electron charges into its volume. While C12 molecules characterized with extremely large EA (adiabatic EA,d = 2.39 eV, vertical
0039-6028/96/$15.00 Copyright© 1996Elsevier ScienceB.V. All rights reserved PH S0039-6028 (96) 00081-7
12
s. Yoneda et al./Surface Science 363 (1996) 11-21
EAv = 1.01 eV) make a prototypical combination with alkali metal atoms for harpooning in the gas phase [6], the alkali effect on C12 sticking on surfaces has been less studied [7]. Recently, we reported that a weak alkali poisoning at low incident energies Ei _<0.1 eV and a promotion at high Ei >_0.4 eV take place on Si(100) surfaces at 300 K, i.e., C12 sticking on alkali-covered Si(100) is activated by E i. A feature similar to this alkali poisoning effect on the Clz molecular sticking has been reported on neat alkali metal surfaces; the ambient Clz molecules with average Ei-~ 0.05 eV, barely stick onto sodium or potassium surfaces [8]. For all by far the most favorable conditions for surface harpooning, the inefficient sticking of Cle molecules is quite unexpected. Yet this unexpected result has not been explained theoretically in terms of the gas-surface interaction potential [9]. We anticipate that the sticking reaction of C12 molecules proceeds by a common mechanism between the two systems, the alkali-covered Si(100) and the alkali metal surfaces. Generally speaking, the sticking reaction of molecules may proceed along either a direct path as the molecule collides with the surface or an indirect path via physisorption. The physisorptionmediated process may be a major route to sticking if the incident energy is small, say Ei < 0.1 eV, while for higher E i a direct sticking channel becomes dominant. The alkali adatoms change not only the surface electronic structure enhancing the density of the vital surface electrons around the Fermi level, EF, as well as reducing the surface work-function, but also the surface phonon structure makes energy dissipation easier as the molecule collides with the surface. The former might result in the acceleration of the direct sticking channel due to harpooning, and the latter might result in the enhancement of physisorption-mediated sticking. The quite unexpected result of the less efficient C12 sticking over alkali overlayers [7,8] should be explained by pursuing its sticking dynamics. Scattered molecules which failed to stick to the surface may convey direct messages in their timeof-flight (TOF) distributions as to the gas-surface interaction potentials along which the dynamics of sticking and scattering are dictated. If the gas-
surface interaction potential is repulsive, or shallow enough not to allow physisorption at the region in front of the barrier to sticking, for low Ei regime not overcoming the barrier, direct-inelastic (DI) scattering would prevail trapping-desorption (TD) scattering as the molecules collide with the surfaces. Thus, we measure TOF spectra of scattered Clz molecules, focussing our attention on what the prevailing scattering mode is. Reducing the surface temperature to 200 K is of dual importance for the C1/ scattering experiment on the cesiated Si(100) surfaces: firstly, the lifetime of the physisorbed molecules can be longer at 200 K than at 300 K, and thus the physisorption-mediated sticking becomes more facile at 200 K than at 300 K. Secondly, multi-Cs overlayers can be stably formed on the Si(100) surfaces at 200 K, and then the C12 molecular sticking on such multi-Cs overlayers can be worth comparing with Andersson's results [8] of the unexpectedly inefficient C12 sticking on alkali metal surfaces. In this paper, reviewing the recent progress regarding the C12-alkalated surface interaction in our laboratory and adding new results to it, we reveal that the DI scattering probability is increased by alkali adatoms, which suggests that the interaction potential of C12 and the alkalated surfaces is repulsive in the asymptotic region up to the top of the barrier. An alkali adatomenhanced Pauli repulsion mechanism is proposed for the origin of the Cl2-surface repulsive interaction.
2. Experimental The experimental procedure for the C12 molecular beam scattering in a ultra-high vacuum (UHV) system has been described previously [7]. The key points of the experimental procedure are as follows: the incident energies, Ei, of the chlorine beams are lowered to 0.055 eV by seeding into Kr. The energy of Ei = 0.055 eV is not enough to excite the C12 vibration (0.07 eV) by collision with the surface. TOF spectra are measured by a quadrupole mass spectrometer (QMS) at specular angle (0~ = 0z = 30°) at each stage of C12 dosing. The measured TOF spectra were analyzed by fitting with a
S. Yonedaet aL/Surface Science363 (1996) 11-21 so-called shifted Maxwell velocity distribution function (as density rather than flux), f(t)
=
C/t 4
exp[--M(L/t
-
7Vo)2/2krt],
(1)
by using a nonlinear least-mean squares program with convolution of the incident beam profile. Here, vo is the incident beam velocity, 7 the nonaccomodation factor (7 = 0 means complete accomodation with the surface), and L the flight distance (11 cm). The clean Si(100) surfaces (p-type, 10 f~.cm) were obtained by Ar + ion sputtering and successive thermal annealing at 1370 K. Cs depositions below one monolayer (1 ML: one Cs atom per one Si atom in the top layers 1-10]) were done under pressure below 5 x 10-lo Torr at room temperature using an alkali dispenser (SAES Getters); and 5 M L Cs overlayers were obtained by further Cs depositions at 200 K onto the surfaces precovered with 1 M L Cs at 300 K. Alkali coverages were monitored by Auger electron spectrometry (AES), referencing the saturation coverage at 300 K. The pressures during C12 beam exposure were below 3 x 10 -1° Torr, being enough for contamination-free dosing. The surfaces were checked by AES to be free from C or O contamination before and after C12 dosing. In order to prepare a surface with depleted electron populations around EF, oxygen gas was introduced onto the Cs(1 ML)/Si(100) through a variable leak valve. Surface temperatures, T~, were controlled by means of thermal conduction cooling with liquid nitrogen, and monitored within an accuracy of __5 K with an almel/chromel thermocouple attached to a Ta-made sample holder. The C1 uptakes could not be accurately evaluated by AES because quite efficient electronstimulated desorption took place within a few seconds. Therefore, initial sticking probabilities, So, were evaluated from scattering data as So = 1 - Ii/I:, where Ii and I : are scattered flux evaluated from T O F spectra measured at the specular angle (0~--O: = 30 °) at the initial and final stages of C12 dosing, respectively [7]. At the final stages of dosing, the surface is fully passivated with C1 and, in principle, all the incident molecules scatter back to the gas phase. This occurs at 300 K, but not at 200 K as will be seen in Section 3. Therefore, so can be evaluated only at 300 K. Irrespective of
13
the crudeness of the method employed, the values of So(0.75_0.05) obtained on the alkali-free Si(100) for E i > 0 . 0 9 e V I-7] turned out to be accurate within 5% deviation from the values recently obtained by Sullivan et al. 1-11] who employed the method of King and Wells [12]. This agreement in So between the two experiments arises from the high reactivity and highly corrugated interaction of C12 with the clean Si(100) [10]; the former results in a very small Ii and the latter in a very broad scattering lobe for the DI scattering [7].
3. Sticking probability and TOF 3.1. Results at 300 K In Fig. 1 we summarize the results of So versus El plots for the clean and 1 M L alkali-covered surfaces at 300K. For the clean Si(100), with decreasing E i a sharp decrease in So is observed for Ei = 0.076 and 0.058 eV, which was also observed more precisely by Sullivan et al. [ 11 ]. They admit another reincrease in So upon decreasing Ei further to 0.035 eV. From this behavior of so, they suggest that, at the lower incident energy, Ei < 0.045 eV, the major part of C12 sticking is mediated by
|
0.6
0.4 o.o
i
t
i
• Cs( 1ML)/Si(100) • K(1ML)/Si(100) a Clean Si(100) o Q(0.2L)/Cs(1ML) /Si(100) r t f i 0.4 o8 Ei (ev)
Fig. 1. Initial sticking probabilities, So, versus Ei for C12 on Si(100) at 300 K. Some data points are from Ref. I7].
14
s. Yoneda et al./Surface Science 363 (1996) 11-21
physisorption, while at higher Ei, above 0.1 eV, a direct sticking channel opens to be activated by Ei. For the K,Cs(1 ML)/Si(100), it is found that So increases with increasing Ei above 0.4 eV, indicating that C12 sticking on the alkalated surfaces is also activated by Ei. It is interesting to note that the activation barrier to sticking is higher on the alkalated surfaces than on the clean Si(100). Another interesting observation on the alkali effect on sticking is that So is almost unity for Ei > 0.4 eV. This should be compared with the case on the clean Si(100) where So is smaller than unity even at the high Ei of 0.86 eV. For low Ei at 0.055 eV, So is still large at about 0.6, not tending to zero. Thus we fail to observe such inefficient C12 sticking with So=0.04 as observed on the neat alkali metal surfaces with ambient gas exposures [8]. We observe a somewhat large So even on a Cs overlayer as thick as 5 M L at 200 K. Right now, we have no reasonable explanation which can reconcile this discrepancy between the two experiments. Two extra points in Fig. 1 are obtained at Ei = 0.87 and 0.058 eV on the Cs 1 ML/Si(100) which was oxidized with 0 . 2 L (L = 1 0 - 6 Torr" s) O2 exposure prior to C12 dosing to deplete the vital surface electrons [13]. We denote this surface 02(0.2 L)/Cs(1 ML)/Si(100). The 0 . 2 L 02 exposure is chosen because the surface work-function is further decreased by more than 0.5 eV from the work-function of the Cs(1 ML)/Si(100), forming a so-called negative electron affinity (NEA) surface [14]. It is reported that the dissociated oxygen atoms in this system sit in between the first Si layer and the Cs overlayer, which does not change the double layer structure for the Cs overlayer [15]. However, the adsorbed oxygen atoms sweep out the surface electrons around EF [13]. Under the condition with depleted electron density the alkali p r o m o t i o n effect on C12 sticking is anticipated to disappear. This really happens because we observe a remarkably reduced sticking with So = 0.65 at Ei = 0.87 eV, as plotted in Fig. 1. This fact clearly indicates that the observed alkali promotion at higher Ei is due to the surface electrons accumulated around EF, which in turn suggests that the origin of the activation barrier is due to the same electrons.
The scattered molecules which escaped from sticking will convey a message as to the gas-surface interaction potentials. Fig. 2 demonstrates a dramatic difference a m o n g the T O F spectra obtained on the clean Si(100) (Fig. 2a) and C s ( 1 M L ) / S i (100) (Fig. 2b) for Ei =0.076 eV. It is found that the T O F spectrum obtained on the Cs(1 ML)/Si (100) is considerably faster at the very beginning of C12 dosing than at the later stages of dosing, while such differences are small on the clean Si(100). The T O F spectra obtained at the later stages of dosing on both surfaces can be fitted with a Maxwellian velocity distribution function, Eq. (1), with a translational temperature Tt close to 300 K and an accomodation factor 7 of nearly zero. This result indicates that T D scattering governs the whole scattering process at the later stages of dosing, implying that the probability of trapping into the physisorption well is quite high for the surface covered with a considerable amount of C1 adatoms thereby no electrons at E F. On the other
i
(b)
Ei=0.076eV Cs(1ML)/Si(100)
9 (a)
0.3
: : ....... . , o
°
Ei=0.076eV Clean Si(100)
0.6 0.9 T O F T i m e (ms)
1.2
Fig. 2. TOF spectra obtained at the initial stages of C12 dosing (solid circles) and at the final stages of C1 saturation (open circles) for El = 0.076 eV. The fines are the Maxwellian curves best fitted with Eq. (1). (a) Clean Si(100). (b) Cs(1 ML)/Si(100). The fitted temperatures at the initial and final stages are, respectively, Tt = 378 and 339 K for (a) and Tt= 539 and 336 K for (b). The values of 7 are nearly zero for all the curves (from Ref. 1-16]).
S. Yoneda et aLISurface Science 363 (1996) 11-21
hand, the T O F spectrum observed at the early stages of dosing on the Cs(1 ML)/Si(100) can also be fitted with the Maxwellian with a very small y of nearly zero, but with a much higher temperature Tt = 540 K. This is also the case for E~ = 0.058 or 0.09 eV, and it turns out that the mean final energies mostly conserve Ei. Therefore, the molecules which did not stick at the very early stages of dosing scatter back to the gas phase not via physisorption but, rather, directly. Considering the fact that ~ ~ 0, we should recall an alternative possibility of T D scattering which includes translational heating as the molecule desorbs the surface. However, this idea may be rejected, since the observed T O F spectra measured for different E~ are not identical, but, rather, different among each other memorizing Ei. On the contrary, for the clean Si(100), the T O F spectra observed at the zero coverage regime consist of both T D and DI scattering components, as plotted in Fig. 2a. Although one may admit a somewhat faster component in the leading edge of the spectrum, the peak of the T O F curve at the early stages of dosing appears close to the final one. Therefore, on the clean Si(100), T D scattering prevails over DI scattering, even at the early stages of C12 dosing. Fig. 3 shows the change of the T O F spectra obtained with E i = 0 . 0 9 eV during C12 dosing nearly zero C1 coverage to the final stage of C1 saturation. Here, one should remark that each line is plotted as a T O F flux to allow a direct comparison Of the scattering intensities. The T O F flux lines are drawn by correcting the T O F density lines obtained after fitting with a single Maxwellian such as the curves fitted in Fig. 2. One can clearly see the prominent feature in the manner of occurrence of the DI scattering which appears quite different between the clean and the Cs(1 ML)/ Si(100) surfaces. For the clean Si(100) there is not a large shift in the peak position of the T O F curves to reach the 300 K Maxwellian if we compare it with the Cs(1ML)/Si(100) case. The gradual increase in the peak intensity indicates that the sticking probability monotonously decreases with C1 coverage. However, for Cs(1 ML)/Si(100) the shift in the peak position as well as in the peak intensity is very much dependent on the C1 cover-
30
15
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i
i
I( b ) [1'{ ~" ¢-
i
8
v
><
El.= 0.09 eV
/?,,', ,,
10
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Clean S i ( l O 0 )
L4 i' c
20
I-. < tO U0
Ei= 0.09 eV
I,-
j.-
10
'..\\
"-....::,,~,, "~. ~ "'.....~..~"~.........
0.4
0.6
0,8
TOF
1.0
1.2
1.4
(ms)
Fig. 3. Changes of T O F curves during the various stages of C12 dosing from the beginning to the end for E i = 0.09 eV; the C12 beam shot numbers increases in the order a, b, c, d, e, and f. Each T O F curve has been corrected to flux Ref. [17]. T~= 300 K.
age. The gradual shift in the peak position occurs rather systematically with C1 coverage, which means that DI scattering is gradually replaced by T D scattering. However, the variation in the peak intensity is complicated. The peak intensity decreases in the first stages of dosing, and then it increases after a certain C12 dosing, at which stage its T O F curve shifts closely to the curve exhibiting T D scattering. This is quite reproducible even for Ei = 0.076 eV [ 16]. This behavior suggests that the sticking probability of C12 does not monotonously decrease with C1 coverage, but first increases with C12 dosing until the sticking begins to decrease after a certain C1 coverage. It should be noted that this first increase in sticking probability is accompanied by the reduction in the DI scattering component. This fact suggests that at the early stages of dosing the activation barriers to sticking become
X Yoneda et aL /Surface Science 363 (1996) 11-21
16
lower as C1 adatoms accumulate on the surface. Alternatively, after partial C1 adsorption, the physisorption followed by sticking is opened either by creating the physisorption wells or by softening the surface phonons. 3.2. Results at 200 K
The T O F results at 300 K suggest that the occurrence of DI scattering is the key ingredient for the weak alkali poisoning at low Ei. In order to extract the skeleton of the repulsive interaction of the C12 molecules with the alkalated surfaces, we reduce the surface temperatures to 200 K. Fig. 4
Clean Si 60 A
Cs(1ML) Si
I
Ei=0.055eV
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tok,.! 20"~"--=---' x½ , ,~'-, I
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t
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Ei=0.86
,
Ei = 0.86 eV
40
2C
O0
i
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0
i
i 4000
I
0
2000
I
I
4000
NUMBER OF CIz SHOTS Fig. 4. Plots of evolutions of the scattered C12 intensity as a function of the n u m b e r of beam shots for the clean Si(100) and the Cs(1ML)/Si(100) for Ei=0.055, 0.09, and 0.86eV, at 200 K. The scattered C12 intensities in the ordinate exhibit densities (see Ref. [17]). For direct comparison, the scattering intensities as well as the n u m b e r of beam shots have been corrected so as to m a k e the incident beam flux the same for each E i.
shows the evolution curves of the scattered C12 intensities as a function of number of the C I 2 beam shots from the clean Si(100), and the Cs(1 ML)/Si(100) for Ei = 0.055, 0.09, and 0.86 eV. The influence of the Cs adatoms on the evolutional behavior of scattered C12 intensities is apparently different from the case at 300K, particularly for E i < 0 . 1 eV. There appear to be some clear differences in the scattering evolutions between the Cs-free surface and the cesiated surfaces• N o essential differences are observed between Cs(1 ML)/Si(100) and Cs(5 ML)/Si(100). Three interesting points in Fig. 4 are summarized as follows: 1. For Ei = 0.055 eV, the initial scattering intensity is much larger on the cesiated surface than on the clean surface. We recall that, at 300 K, the initial scattering intensities on both surfaces are almost the same for low Ei, as mentioned above. 2. With increasing El, the initial intensity of the s c a t t e r e d C12 flUX [17] tends to increase on the Cs-free surface, but on the cesiated surface it tends to decrease to almost zero at Ei = 0.86 eV. C12 sticking on the cesiated surfaces can be activated by Ei similarly to the case at 300 K, while on the clean surfaces it is decelerated by increasing Ei. 3. For the clean surface at low Ei, upon accumulating dose and, thus, increasing C1 coverages, the scattering intensities decrease in the first stages of dosing, reaching their minimum of nearly zero, and then sharply increase, eventually levelling off. However, for the cesiated surfaces they first increase, reach their maximum, and then exponentially decrease, tending to zero. In order to know what type of scattering modes take place at each stage of dosing, we plot the T O F data obtained at the early and the later stages of dosing for the Ei = 0.055, 0.09, and 0.86 eV beams in Figs. 5 and 6 for clean Si(100) and 1 M L Cs/Si(100), respectively• The T O F spectra are fitted to the Maxwell!an function of Eq. (1) to evaluate the translational temperature, Tt. Because of the limited number of beam shots for the highly reactive surfaces, the observed scattering intensities at the early stages of dosing are so small that the least mean squares fitting results in a large ambiguity. Nevertheless, the DI and T D scattering modes can be easily distinguished because of the long
S. Yoneda et al.tSurface Science 363 (1996) 11-21
3O
17
3O • b
20
,~ ~~ x -~1 *I
~
~.~.
Clean Si(100) Ei=0.055eV 20
a : initial stage b : final stage
-
Cs(1ML)/Si(100) Ei=0.055eV
a l't ~"
a : initial stage b : final stage
b
' r.13'
,.c:l
30 1
I
r13
Clean Si(100) Ei=0.09eV 20
"'A
b 1
o ~ ~ . . ~
I,-i I~'t~' 30l | 201-]
a : initial stage
(D (D ,,,.a q...a
1
2
3
"~4
Q~
30
20
4
Cs(1ML)/Si(100) [
]/ I.%* a I~
Ei=0.09eV a,
*'.
C.) 0~-
...........................
"~"
~D .+,.a t.,.l
................................. ~
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4
: initial stage ]
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1
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0.)
20f i
Ei=0.86eV Cs(IML)/Si(IO0)
a : initial stage b : final stage
L/ a ~
a : initial stage ] b : final stage 1
o.s
'i .....................¢.~ ...................2
c13
•
.
2
Time of Flight [ms] Fig. 5. TOF spectra of scattered C12 molecules from the Cs free-surfaces (T~ =200K) at the very beginning of dosing accumulated for 20 beam shots (solid triangles), and at the late stages of dosing around 4000 beam shots (open circles). Zero TOF time on the abscissa is referenced with respect to the time when the beam passes the chopper. It is quite evident that the TOF spectra for the solid triangles are always faster than those for the open circles. The solid lines for the solid triangles are the best fitted curves with f(t) of Eq.(1) for y = 0 , and Tt= 296 K, 381K and 1280K for Ei=0.055 , 0.09, and 0.86eV, respectively. The solid lines for the open circles for (a) and (b) are the best fitted curves with f(t) for Tt = 104 and 100 K for Ei = 0.055 and 0.09 eV, respectively. The solid line with a bimodal distribution for (c) is obtained with a combined function of Eq.(1), f(t, Ttl)+f(t, Tt2), for the best fitted parameters, Ttl = 868 K and Tt2 = 95 K.
°o
'
Time of Flight [ms] Fig. 6. TOF spectra of scattered C12 molecules from Cs(1 ML)/Si(100) at the early stages (solid triangles) and late stages (open circles) of dosing at 200 K for E i = 0.055 eV (a), 0.09 eV (b), and 0.86 eV (c). Solid lines are the fitted curves with Eq. (1) for the best fitted parameters, Tt = 273 (294), 563 (400), and 1392 K (833 K) for the triangles and for Ei = 0.055, 0.09, and 0.86 eV, respectively. The values for Tt in brackets indicate the fitting parameters for the open circles. r e s i d e n c e in t h e p h y s i s o r p t i o n wells; for E i = 0.055 o r 0.09 eV, t h e p e a k o f t h e T O F c u r v e s at t h e l a t e r stages o f d o s i n g a p p e a r s in c o n s i d e r a b l y s l o w e r t i m e r e g i o n s t h a n t h e T O F c u r v e s o b t a i n e d at e a r l y stages o f d o s i n g , b e s i d e s t h e T O F c u r v e m e a s u r e d at t h e l a t e r stages o f d o s i n g for h i g h Ei = 0.86 e V s h o w s a c l e a r l y b i m o d a l d i s t r i b u t i o n
18
S. Yoneda et aL/Surface Science 363 (1996) 11-21
on the Cs-free surfaces (see Fig. 5). As a matter of fact, the slower component can be ascribed to TD scattering, with the faster one to DI scattering. From curve fitting of Eq. (1) to the T O F curves obtained on the clean Si(100) surface at the later stages of dosing we deduce "best fitted" T O F temperatures of about 100 K, independent of El, which is considerably lower than T~= 200 K. The extent of deviation of Tt from T~ becomes small as T~ increases. At T~_> 280 K, we find Tt -~ T~, and y ,--0, indicating that TD scattering takes place. We consider that the "best fitted" translational temperatures obtained by fitting to Eq. (1) do not show a true translational temperature to characterize TD scattering, but they are apparently affected by the delayed emission from the physisorption wells. In contrast to the T O F results on the Cs-free surfaces, the observed T O F spectra on the cesiated surfaces never include a slower T O F contribution even at the fairly late stages of dosing. This means that only DI scattering takes place on the cesiated surfaces. At low temperatures around 200 K the physisorbed molecule resides on the surface with a certain lifetime. During the long residence in the physisorption well the molecule can move around the surface until it finds its sticking place, thereby reducing desorption when the surface is still not passivated by C1 adatoms. The long-lived physisorption at 200 K results in almost unity sticking for low Ei ~-0.055 eV where the trapping probability is quite high. In other words, the sticking probability at 200 K can be solely counted by the probability of DI scattering. The initial DI scattering yield after correction to flux [17] really increases with El, as shown in Fig. 5. Therefore, we conclude that for the zero C1 coverage regime, no TD scattering takes place, but DI scattering does. The initial sticking probabilities are solely determined by the DI scattering probability. The initial DI scattering probability for low Ei is considerably enhanced by the Cs adatoms. This fact indicates that the fresh alkali overlayer causes a potential shallowing for the C12 physisorption wells, or the C12-alkalated surface interaction potential is repulsive all the way to the top of the activation barrier. (One should remember that such
repulsive potentials are experienced by only a fraction, ~30%, of the incident molecules with respect to the saturated scattering intensity on the clean Si(100); Fig. 3a.) Another interesting feature of the scattering curves on the cesiated surfaces is that the scattered C12 yield decreases with increasing C12 dose, or C1 coverage. This does not necessarily mean infinite adsorption due to dissociative sticking but, rather, mean physisorption, since we observe strong C12 desorption at TPD temperatures around 220 K, above which temperatures strong TD scattering is observed. This means that the lifetime of C12 physisorption is almost infinite on the Cl-saturated 1 M L Cs/Si(100) at 200K. Once the C I - - C s + ionic bonds are formed, they make the physisorption potential deeper. The manner of contribution by the Cs adatoms to the physisorption wells is quite opposite in trend between the Cl-free surfaces and the Cl-passivated surfaces.
4. Origin of the repulsive interaction In general, the physisorption potentials comprise two terms, Pauli repulsion and van der Waals attraction [18,19]. The effective range of the former decreases exponentially due to the spread of the surface electron wave-function into the vacuum, being short-ranged, while the latter is long-ranged and effective at the asymptotic region. The subtle energy balance of the two interactions results in a physisorption well of the order of 100 meV. The van der Waals interaction can be expected to increase with increasing surface dielectric constant [18-20]. This might contribute t o deepening the well rather than shallowing it when the surface becomes metallic upon Cs deposition. Therefore, such potential shallowing observed here may be achieved by shifting up Pauli repulsion. It may be adequate for understanding the activation process of molecular reactions on the alkalated surfaces to employ a method to construct an adiabatic potential from two diabatic potentials for the initial and final states [2]. Presently, the relevant initial and final states are a molecularly neutral state and an ionic state, respectively. The neutral C12 in the diabatic neutral state can be
S. Yoneda et at~Surface Science 363 (1996) 11-21
considered as a closed shell system because of the inhibition of electron insertion into the affinity level. The diabatic potential of the neutral state, V.(Z), consists of two terms, a short-range Pauli repulsion, WR(Z), and a long-range van der Waals attraction, V~ow(Z), as
E(z) = R(z) + Edw(Z).
(2)
The Pauli repulsion term VvR(Z) can be counted by integrating the increase in kinetic energies of the surface electrons [21] as, WR(Z)=
f~[ K(E)n(E,Z)dE,
(3)
where K(E) is the kinetic energy increase of the unit charge with energy E in the surface bands, and n(E,Z) is a local electron density of state at Z. The contribution to VvR(Z) at the asymptotic region comes mainly from the energetically uppermost electron charges split out to the C12 position. The wave-functions of such electrons exponentially tail off into the vacuum. We put a crude approximation of n(E,Z) = no exp [--2V~2mdp/hZZ]6(E Ev). Then, Eq. (3) can be recast as -
VVR(Z) "~ Kno exp [ -- ~v/~Z],
-
(4)
where K is a contant depending on the nature of C12, no is the surface electron charge density at Z = 0, and a unit of eV is taken for ~b and 2t for Z. On usual surfaces either with high work-function of about 5 eV or with depleted electron density at E F , the term of VpR(Z) is small at the asymptotic region, and therefore the physisorption well exists as a shallow minimum of the potential due to the subtle energy balance between the two terms at a position not far from the surface, around Z -- 5 ~t. For 0u = 1 ML, however, the surface is highly metallic [22,23], accumulating the vital electrons around EF to increase n o [24] on the average by a factor of 2 at 0 u = 1 M L for one Cs atom per one Si dangling bond. The wave-functions of these surface electrons tail off far into the vacuum due to the extreme lowering of the work-function from 4.9eV on the clean Si(100) to 1.3eV on the Cs(1 ML)/Si(100) surface. If we employ these values for no and ¢ in relative comparison of VvR(Z) between the alkali-covered surface and the alkali-
19
free surface, we find v~R(z)is larger on the cesiated surface than on the clean surface by two and five orders of magnitude at Z = 5 and 10 A, respectively. We thus expect that these are large enough for VvR(Z) to prevail over the attractive term, Edw(Z), even at the asymptotic region far from the surface beyond Z = 5 A. This results in an upward shift in the potential energy curve, yielding a totally repulsive potential for the neutral state. This situation is schematically drawn in Fig. 7. For the second diabatic state for the final state, we consider the C12 negative ion state which will be formed by a surface electron transfer as a molecular intermediate for dissociation [25,26].
Vn (.9 rr LtJ Z ILl
o_ .* =/
..J
--t-
I--
z I.tJ
/---
0 ft.
0
I
I
I
I
5
10
15
20
Fig. 7. Schematic drawing of the relevant potential energy curves of C12 for Cs(1 ML)/Si(100). The ordinate for potential energy is not scaled because of ambiguity in the absolute value for each potential energy curve. The curve V~ represents the diabatic potential energy curve for the cesiated surface, which is shifted above the potential energy curve for the clean surface by the net increase of Pauli repulsion. The thick solid line exhibiting the barrier is the adiabatic potential energy curve constructed from avoided crossing of the diabatic neutral V~ and ionic states Vio. (from Ref. 1-161).
20
X Yoneda et al./Surface Science 363 (1996) 11-21
The potential for the negative ion, V~on(Z), may be a decreasing function with decreasing Z if it follows the image potential even at the near asymptotic region. If the adiabatic expansion of the C1-C1 distance is not allowed in the course of Collision as discussed in gas-phase harpooning reactions [27], the vertical electron affinity EAr sets the potential at infinite distance. The curve of V~on(Z) will then cross the curve of V~(Z) at the asymptotic region, say Z = 7 ,-~ 10 A [28]. There is an interaction, V-~t(Z), which mixes the two diabatic states. By diagonalizing the Hamiltonian matrix the adiabatic potential, V~a(Z), can be constructed. Since V~(Z) is totally repulsive, the avoided crossing yields the activation barrier at the crossing region, in front of which no attractive well is formed beyond the asymptotic region. This feature is schematically illustrated in Fig. 7. In this way the existence of the activation barrier to Clz sticking onto the alkalated surfaces can be explained based on Pauli repulsion (see Ref.[16] for further discussion). As pointed out by Brown et al. [2], since Pauli repulsion is a quite general principle, the alkali poisoning effect on CO [1] or Nz [29,30] nondissociative molecular adsorption might also be due to alkali-enhanced Pauli repulsion. Right now, however, we fail to get any positive evidence for such alkali-enhanced Pauli repulsion in scattering experiments since no enhancement in the DI scatterings of CO, N 2 o r Xe, which have negative electron affinities, is observed at all as the Si(100) surface is covered with 1 M L Cs atoms [31]. In order to complete further understanding of the Pauli repulsion mechanism proposed above for C12, w e need further considerations of the key terms in the model, such as the factor K in Eq. (4), the interaction energy Vim(Z) between the ionic state and the covalent state, the recoiling effect by the adatoms upon collision, and the influence of C1z vibrational excitation.
Acknowledgements This work was financially supported by a Grantin-Aid for Scientific Research from the Ministry of
Education, Science and Culture, in the priority area "free radical science".
References [1] M.P. Kisldnova, in: Poisoning and Promotion in Catalysis Based on Surface Science Concepts and Experiments, Studies in Surface Science and Catalysis, Vol. 70, Eds. B. Delmon and J.T. Yates (Elsevier Science Publishing Company, Amsterdam, 1992) Chapter 6. [2] J.K. Brown, A.C. Luntz and P.A. Schultz, J. Chem. Phys. 95 (1991) 3767. [3] C. Resch, V. Zhukov, A. Lugstein, H.F. Berger, A. Winkler and K.D. Rendulic, Chem. Phys. 177 (1993) 421. [4] L. Surnev, in: Physics and Chemistry of Alkali Metal Adsorption, Eds. H.P. Bonzel, A.M. Bradshow and G. Ertl (Elsevier, New York, 1989) p. 173. R. Miranda, ibid, p. 425, and references therein. P. Soukiassian and H.I. Stanberg, ibid, p. 449, and references therein. H.J. Ernst and M.L. Yu, Phys. Rev. B 41 (1989) 12953. A. Namiki, S. Suzuki, H. Kato, Y. Babasaki, M. Tanaka, T. Nakamura and T. Suza!d, J. Chem. Phys. 97 (1992) 3781. [5] E.G. Michel, J.E. Ortega, E.M. Oellig, M.C. Asensio, J. Feron and R. Miranda, Phys. Rev. B 38 (1988) 13399. J.K. Norskov, S. Holloway and N.D. Lang, Surf. Sci. 137 (1984) 65. [6] R.D. Levine and R.B. Bernstein, in: Molecular Reaction Dynamics (Oxford University Press, Oxford, 1987). [7] F.H. Geuzebroek, Y. Babasaki, M. Tanaka, T. Nakamura and A. Namiki, Surf. Sci. 297 (1993) 141. [8] D. Andersson, B. Kasemo and L. Wallden, Surf. SCI. 152/153 (1985) 576. D. Andersson, PhD thesis, Department of Physics, University of Gothenberg, 1989. [9] B.I. Lundqvist, Surf. Sci. 242 (1991) 365. [10] T. Abukawa and S. Kono, Phys. Rev. B 37 (1988) 9097. [11] D.J.D. Sullivan, H.C. Flaum and A.C. Kummel, J. Phys. Chem. 97 (1993) 12051. [12] D.A. King and M.G. Wells, Surf. Sci. 29 (1972) 454. [13] S. Nishigaki, T. Sasaki, S. Matsuda, N. Kawanishi, H. Takeda and K. Yamada, Surf. Sci. 242 (1991) 358. K. Yamada and S. Nishigaki, Surf. Sci. 293 (1993) L893. [14] B. Goldstein, Surf. SCI. 35 (1973) 227. J.D. Levine, Surf. Sci. 34 (1973) 90. R. Miranda, in: Physics and Chemistry of Alkali Metal Adsorption, Eds. H.P. Bonzel, A.M. Bradshow and G. Ertl (Elsevier, New York, 1989) p. 425. [15] T. Abukawa, Y. Enta, T. Kashiwakura, S. Suzuki, S. Kono and T. Sakamoto, J. Vac. Sci. Technol. A 8 (1990) 3205. [16] A. Namiki, H. Hayashi, Y. Ukai and F.H. Geuzebroek, J. Chem. Phys. 102 (1995) 9689. [17] The scattering intensity in the ordinate of Fig. 1 exhibits the density of the scattered molecules rather than their
S. Yoneda et aL/Surface Science 363 (1996) 11-21
[18] [ 19] [20] [21] [22] [23]
flux due to electron-impacted ionization of the molecules in the QMS. The corrected flux intensity can be obtained by multiplying velocities to the TOF spectra deconvoluted from the raw TOF data of Figs. 2 or 3. This correction is important for E i = 0.86 eV because the velocity of the DI scattered molecules is much higher than E i = 0.055 or 0.09 eV. For comparison of the initial scattered flux with respect to the scattering intensity for Ei = 0.055 eV in Fig. 1 we need a multiplication factor of 1.2 and 3.0 to the initial scattering intensity for E i = 0.09 and 0.86 eV, respectively. M.C. Desjonqneres and D. Spanjaard, in: Concepts in Surface Physics (Springer, Berlin, 1993). A. Zangwill, in: Physics at Surfaces (Cambridge University Press, Cambridge, 1988). Y.S. Barash and V.L. Ginzburg, Sov. Phys. Usp. 27 (1984) 467. A. Liebsch, J. Harris, B. Salanon and J. Lapujoulade, Surf. Sci. t23 (1982) 338. L.S.O. Johansson and B. Reihl, Phys. Rev. Lett. 67 (1991) 2191; Surf. Sci. 287/288 (1993) 524. U.A. Effner, D. Badt, J. Binder, T. Bertrams, A. Brodde, Ch. Lunau, N. Neddermeyer and M. Hanbiicken, Surf.
[24] [25] [26] [27] [28]
[29] [30]
[31]
21
Sci. 277 (1992) 207. A. Brodde, T. Bertrams and H. Neddermeyer, Phys. Rev. B 47 (1993) 4508. H. Ishida and K. Terakura, Phys. Rev. B 40 (1989) 11519. J.K. Norskov, D.M. Newns and B.I. Lundqvist, Surf. Sci. 80 (1979) 179. J.W. Gadzuk and S. Holloway, Chem. Phys. Lett. 114 (1985) 314; J. Chem. Phys. 84 (1986) 3502. A.W. Kleyn, J. Los and E.A. Gislason, Phys. Rep. 90 (1982) 1. The crossing point can, in principle, be estimated by solving the equation, ~b+ Eb = EA,, + eZ/4z, where Eb is the binding energy of transferring electron measured from Er. Rough estimation of the crossing point can be performed by approximating Eb =0. Crossing occurs multiply depending on the distribution of Eb. R.A. de Paola, F.M. Hoffmann, D. Heskett and E.W. Plummer, Phys. Rev. B 35 (1987) 4236. E. Umbach, in: Physics and Chemistry of Alkali Metal Adsorption, Eds. H.P. Bonzel, A.M. Bradshow and G. Ertl (Elsevier, New York, 1989) p. 241. A. Namiki, F. Koga, N. Yamazaki and H. Dohshita, unpublished results.
surface science
ELSEVIER
Surface Science363 (1996) 11-21
The poisonous effect of alkali adatoms on C12 sticking S. Yoneda, Y. Babasaki, M. Tanaka, F.H. Geuzebroek 1, F. Koga, N. Yamazaki, A. Namiki * Department of Electrical and Electronic Engineering, Toyohashi University of Technology, Tempaku, Toyohashi 44I, Japan
Received 31 August 1995;accepted for publication 8 September 1995
Abstract
C12 sticking on alkali-covered Si(100) is activated by the incident energies, Ei, i.e., there is a potential barrier to sticking. To understand the origin of the activation barrier, scattering experiments are performed. The observed C12 scattering is predominated by direct-inelastic scattering even for Ei < 0.1 eV, suggesting that the Cs adatoms play a role in making the physisorption potential shallow, or the gas-surface interaction potential repulsive. Pauli repulsion is proposed for the origin of this repulsive interaction. Keywords: Alkali metals; Halogens;Low index crystal surfaces; Molecule-solid scattering and diffraction- inelastic; Silicon; Sticking; Surface chemical reaction
1. Introduction
Sticking of simple diatomic molecules such as H2 or 0 2 may be influenced by alkali adatoms [1]. The alkali effects on sticking appear to be either poisonous or promotive depending on the chemical nature of the molecules. Electron affinities, EA, of the molecules seems to be a crucial parameter in characterizing whether the effect is promotive or poisoning: for H2 molecules with strongly negative EA, a strong poisoning effect on dissociative sticking is recognized in various metal surfaces [ 1 - 3 ] . Non-dissociative adsorption of CO molecules which have just negative EA ( - 1.5 eV) are also observed to be poisoned by the alkali adatoms [ 1]. On the other hand, for 0 2 or N O * Corresponding author. 1Present address: Koninklijk/SheU-Laboratofium, Amsterdam, P.O. Box 3003, 1003 AA Amsterdam, The Netherlands.
molecules, whose EA are just positive, a strong promotive effect on dissociative sticking is recognized on metals [ 1 ] as well as on semiconductors [4]. Brown et al. [ 2 ] suggest that, under conditions with reduced surface work functions, Pauli repulsion plays a crucial role in the poisonous alkali effects on sticking of negative EA molecules [2]. The driving force for the alkali poisoning in this case is energy consumption of the incident molecule upon repelling the unwanted surface electron charges out of the molecular region. On the other hand, for the case of the alkali promotion, it is widely believed that the surface electron charge transfer to the affinity level of the incident molecule plays an essential role on the alkali promotion [5]. The driving force for the alkali promotion is energy gain of the incident molecule upon pulling the surface electron charges into its volume. While C12 molecules characterized with extremely large EA (adiabatic EA,d = 2.39 eV, vertical
0039-6028/96/$15.00 Copyright© 1996Elsevier ScienceB.V. All rights reserved PH S0039-6028 (96) 00081-7
12
s. Yoneda et al./Surface Science 363 (1996) 11-21
EAv = 1.01 eV) make a prototypical combination with alkali metal atoms for harpooning in the gas phase [6], the alkali effect on C12 sticking on surfaces has been less studied [7]. Recently, we reported that a weak alkali poisoning at low incident energies Ei _<0.1 eV and a promotion at high Ei >_0.4 eV take place on Si(100) surfaces at 300 K, i.e., C12 sticking on alkali-covered Si(100) is activated by E i. A feature similar to this alkali poisoning effect on the Clz molecular sticking has been reported on neat alkali metal surfaces; the ambient Clz molecules with average Ei-~ 0.05 eV, barely stick onto sodium or potassium surfaces [8]. For all by far the most favorable conditions for surface harpooning, the inefficient sticking of Cle molecules is quite unexpected. Yet this unexpected result has not been explained theoretically in terms of the gas-surface interaction potential [9]. We anticipate that the sticking reaction of C12 molecules proceeds by a common mechanism between the two systems, the alkali-covered Si(100) and the alkali metal surfaces. Generally speaking, the sticking reaction of molecules may proceed along either a direct path as the molecule collides with the surface or an indirect path via physisorption. The physisorptionmediated process may be a major route to sticking if the incident energy is small, say Ei < 0.1 eV, while for higher E i a direct sticking channel becomes dominant. The alkali adatoms change not only the surface electronic structure enhancing the density of the vital surface electrons around the Fermi level, EF, as well as reducing the surface work-function, but also the surface phonon structure makes energy dissipation easier as the molecule collides with the surface. The former might result in the acceleration of the direct sticking channel due to harpooning, and the latter might result in the enhancement of physisorption-mediated sticking. The quite unexpected result of the less efficient C12 sticking over alkali overlayers [7,8] should be explained by pursuing its sticking dynamics. Scattered molecules which failed to stick to the surface may convey direct messages in their timeof-flight (TOF) distributions as to the gas-surface interaction potentials along which the dynamics of sticking and scattering are dictated. If the gas-
surface interaction potential is repulsive, or shallow enough not to allow physisorption at the region in front of the barrier to sticking, for low Ei regime not overcoming the barrier, direct-inelastic (DI) scattering would prevail trapping-desorption (TD) scattering as the molecules collide with the surfaces. Thus, we measure TOF spectra of scattered Clz molecules, focussing our attention on what the prevailing scattering mode is. Reducing the surface temperature to 200 K is of dual importance for the C1/ scattering experiment on the cesiated Si(100) surfaces: firstly, the lifetime of the physisorbed molecules can be longer at 200 K than at 300 K, and thus the physisorption-mediated sticking becomes more facile at 200 K than at 300 K. Secondly, multi-Cs overlayers can be stably formed on the Si(100) surfaces at 200 K, and then the C12 molecular sticking on such multi-Cs overlayers can be worth comparing with Andersson's results [8] of the unexpectedly inefficient C12 sticking on alkali metal surfaces. In this paper, reviewing the recent progress regarding the C12-alkalated surface interaction in our laboratory and adding new results to it, we reveal that the DI scattering probability is increased by alkali adatoms, which suggests that the interaction potential of C12 and the alkalated surfaces is repulsive in the asymptotic region up to the top of the barrier. An alkali adatomenhanced Pauli repulsion mechanism is proposed for the origin of the Cl2-surface repulsive interaction.
2. Experimental The experimental procedure for the C12 molecular beam scattering in a ultra-high vacuum (UHV) system has been described previously [7]. The key points of the experimental procedure are as follows: the incident energies, Ei, of the chlorine beams are lowered to 0.055 eV by seeding into Kr. The energy of Ei = 0.055 eV is not enough to excite the C12 vibration (0.07 eV) by collision with the surface. TOF spectra are measured by a quadrupole mass spectrometer (QMS) at specular angle (0~ = 0z = 30°) at each stage of C12 dosing. The measured TOF spectra were analyzed by fitting with a
S. Yonedaet aL/Surface Science363 (1996) 11-21 so-called shifted Maxwell velocity distribution function (as density rather than flux), f(t)
=
C/t 4
exp[--M(L/t
-
7Vo)2/2krt],
(1)
by using a nonlinear least-mean squares program with convolution of the incident beam profile. Here, vo is the incident beam velocity, 7 the nonaccomodation factor (7 = 0 means complete accomodation with the surface), and L the flight distance (11 cm). The clean Si(100) surfaces (p-type, 10 f~.cm) were obtained by Ar + ion sputtering and successive thermal annealing at 1370 K. Cs depositions below one monolayer (1 ML: one Cs atom per one Si atom in the top layers 1-10]) were done under pressure below 5 x 10-lo Torr at room temperature using an alkali dispenser (SAES Getters); and 5 M L Cs overlayers were obtained by further Cs depositions at 200 K onto the surfaces precovered with 1 M L Cs at 300 K. Alkali coverages were monitored by Auger electron spectrometry (AES), referencing the saturation coverage at 300 K. The pressures during C12 beam exposure were below 3 x 10 -1° Torr, being enough for contamination-free dosing. The surfaces were checked by AES to be free from C or O contamination before and after C12 dosing. In order to prepare a surface with depleted electron populations around EF, oxygen gas was introduced onto the Cs(1 ML)/Si(100) through a variable leak valve. Surface temperatures, T~, were controlled by means of thermal conduction cooling with liquid nitrogen, and monitored within an accuracy of __5 K with an almel/chromel thermocouple attached to a Ta-made sample holder. The C1 uptakes could not be accurately evaluated by AES because quite efficient electronstimulated desorption took place within a few seconds. Therefore, initial sticking probabilities, So, were evaluated from scattering data as So = 1 - Ii/I:, where Ii and I : are scattered flux evaluated from T O F spectra measured at the specular angle (0~--O: = 30 °) at the initial and final stages of C12 dosing, respectively [7]. At the final stages of dosing, the surface is fully passivated with C1 and, in principle, all the incident molecules scatter back to the gas phase. This occurs at 300 K, but not at 200 K as will be seen in Section 3. Therefore, so can be evaluated only at 300 K. Irrespective of
13
the crudeness of the method employed, the values of So(0.75_0.05) obtained on the alkali-free Si(100) for E i > 0 . 0 9 e V I-7] turned out to be accurate within 5% deviation from the values recently obtained by Sullivan et al. 1-11] who employed the method of King and Wells [12]. This agreement in So between the two experiments arises from the high reactivity and highly corrugated interaction of C12 with the clean Si(100) [10]; the former results in a very small Ii and the latter in a very broad scattering lobe for the DI scattering [7].
3. Sticking probability and TOF 3.1. Results at 300 K In Fig. 1 we summarize the results of So versus El plots for the clean and 1 M L alkali-covered surfaces at 300K. For the clean Si(100), with decreasing E i a sharp decrease in So is observed for Ei = 0.076 and 0.058 eV, which was also observed more precisely by Sullivan et al. [ 11 ]. They admit another reincrease in So upon decreasing Ei further to 0.035 eV. From this behavior of so, they suggest that, at the lower incident energy, Ei < 0.045 eV, the major part of C12 sticking is mediated by
|
0.6
0.4 o.o
i
t
i
• Cs( 1ML)/Si(100) • K(1ML)/Si(100) a Clean Si(100) o Q(0.2L)/Cs(1ML) /Si(100) r t f i 0.4 o8 Ei (ev)
Fig. 1. Initial sticking probabilities, So, versus Ei for C12 on Si(100) at 300 K. Some data points are from Ref. I7].
14
s. Yoneda et al./Surface Science 363 (1996) 11-21
physisorption, while at higher Ei, above 0.1 eV, a direct sticking channel opens to be activated by Ei. For the K,Cs(1 ML)/Si(100), it is found that So increases with increasing Ei above 0.4 eV, indicating that C12 sticking on the alkalated surfaces is also activated by Ei. It is interesting to note that the activation barrier to sticking is higher on the alkalated surfaces than on the clean Si(100). Another interesting observation on the alkali effect on sticking is that So is almost unity for Ei > 0.4 eV. This should be compared with the case on the clean Si(100) where So is smaller than unity even at the high Ei of 0.86 eV. For low Ei at 0.055 eV, So is still large at about 0.6, not tending to zero. Thus we fail to observe such inefficient C12 sticking with So=0.04 as observed on the neat alkali metal surfaces with ambient gas exposures [8]. We observe a somewhat large So even on a Cs overlayer as thick as 5 M L at 200 K. Right now, we have no reasonable explanation which can reconcile this discrepancy between the two experiments. Two extra points in Fig. 1 are obtained at Ei = 0.87 and 0.058 eV on the Cs 1 ML/Si(100) which was oxidized with 0 . 2 L (L = 1 0 - 6 Torr" s) O2 exposure prior to C12 dosing to deplete the vital surface electrons [13]. We denote this surface 02(0.2 L)/Cs(1 ML)/Si(100). The 0 . 2 L 02 exposure is chosen because the surface work-function is further decreased by more than 0.5 eV from the work-function of the Cs(1 ML)/Si(100), forming a so-called negative electron affinity (NEA) surface [14]. It is reported that the dissociated oxygen atoms in this system sit in between the first Si layer and the Cs overlayer, which does not change the double layer structure for the Cs overlayer [15]. However, the adsorbed oxygen atoms sweep out the surface electrons around EF [13]. Under the condition with depleted electron density the alkali p r o m o t i o n effect on C12 sticking is anticipated to disappear. This really happens because we observe a remarkably reduced sticking with So = 0.65 at Ei = 0.87 eV, as plotted in Fig. 1. This fact clearly indicates that the observed alkali promotion at higher Ei is due to the surface electrons accumulated around EF, which in turn suggests that the origin of the activation barrier is due to the same electrons.
The scattered molecules which escaped from sticking will convey a message as to the gas-surface interaction potentials. Fig. 2 demonstrates a dramatic difference a m o n g the T O F spectra obtained on the clean Si(100) (Fig. 2a) and C s ( 1 M L ) / S i (100) (Fig. 2b) for Ei =0.076 eV. It is found that the T O F spectrum obtained on the Cs(1 ML)/Si (100) is considerably faster at the very beginning of C12 dosing than at the later stages of dosing, while such differences are small on the clean Si(100). The T O F spectra obtained at the later stages of dosing on both surfaces can be fitted with a Maxwellian velocity distribution function, Eq. (1), with a translational temperature Tt close to 300 K and an accomodation factor 7 of nearly zero. This result indicates that T D scattering governs the whole scattering process at the later stages of dosing, implying that the probability of trapping into the physisorption well is quite high for the surface covered with a considerable amount of C1 adatoms thereby no electrons at E F. On the other
i
(b)
Ei=0.076eV Cs(1ML)/Si(100)
9 (a)
0.3
: : ....... . , o
°
Ei=0.076eV Clean Si(100)
0.6 0.9 T O F T i m e (ms)
1.2
Fig. 2. TOF spectra obtained at the initial stages of C12 dosing (solid circles) and at the final stages of C1 saturation (open circles) for El = 0.076 eV. The fines are the Maxwellian curves best fitted with Eq. (1). (a) Clean Si(100). (b) Cs(1 ML)/Si(100). The fitted temperatures at the initial and final stages are, respectively, Tt = 378 and 339 K for (a) and Tt= 539 and 336 K for (b). The values of 7 are nearly zero for all the curves (from Ref. 1-16]).
S. Yoneda et aLISurface Science 363 (1996) 11-21
hand, the T O F spectrum observed at the early stages of dosing on the Cs(1 ML)/Si(100) can also be fitted with the Maxwellian with a very small y of nearly zero, but with a much higher temperature Tt = 540 K. This is also the case for E~ = 0.058 or 0.09 eV, and it turns out that the mean final energies mostly conserve Ei. Therefore, the molecules which did not stick at the very early stages of dosing scatter back to the gas phase not via physisorption but, rather, directly. Considering the fact that ~ ~ 0, we should recall an alternative possibility of T D scattering which includes translational heating as the molecule desorbs the surface. However, this idea may be rejected, since the observed T O F spectra measured for different E~ are not identical, but, rather, different among each other memorizing Ei. On the contrary, for the clean Si(100), the T O F spectra observed at the zero coverage regime consist of both T D and DI scattering components, as plotted in Fig. 2a. Although one may admit a somewhat faster component in the leading edge of the spectrum, the peak of the T O F curve at the early stages of dosing appears close to the final one. Therefore, on the clean Si(100), T D scattering prevails over DI scattering, even at the early stages of C12 dosing. Fig. 3 shows the change of the T O F spectra obtained with E i = 0 . 0 9 eV during C12 dosing nearly zero C1 coverage to the final stage of C1 saturation. Here, one should remark that each line is plotted as a T O F flux to allow a direct comparison Of the scattering intensities. The T O F flux lines are drawn by correcting the T O F density lines obtained after fitting with a single Maxwellian such as the curves fitted in Fig. 2. One can clearly see the prominent feature in the manner of occurrence of the DI scattering which appears quite different between the clean and the Cs(1 ML)/ Si(100) surfaces. For the clean Si(100) there is not a large shift in the peak position of the T O F curves to reach the 300 K Maxwellian if we compare it with the Cs(1ML)/Si(100) case. The gradual increase in the peak intensity indicates that the sticking probability monotonously decreases with C1 coverage. However, for Cs(1 ML)/Si(100) the shift in the peak position as well as in the peak intensity is very much dependent on the C1 cover-
30
15
i
i
i
I( b ) [1'{ ~" ¢-
i
8
v
><
El.= 0.09 eV
/?,,', ,,
10
"/d'%, ',), /
\
.....
,o
".\.
if
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0
".?:..
~'1
I
~ . . ,
-.-.:.-'-=.i,,"M,.,~
.
.
.
.
,"-, e
(a) / \
¢-~ Ld ¢w
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\
•
"I
i
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2C
.Q"
i
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L4 i' c
20
I-. < tO U0
Ei= 0.09 eV
I,-
j.-
10
'..\\
"-....::,,~,, "~. ~ "'.....~..~"~.........
0.4
0.6
0,8
TOF
1.0
1.2
1.4
(ms)
Fig. 3. Changes of T O F curves during the various stages of C12 dosing from the beginning to the end for E i = 0.09 eV; the C12 beam shot numbers increases in the order a, b, c, d, e, and f. Each T O F curve has been corrected to flux Ref. [17]. T~= 300 K.
age. The gradual shift in the peak position occurs rather systematically with C1 coverage, which means that DI scattering is gradually replaced by T D scattering. However, the variation in the peak intensity is complicated. The peak intensity decreases in the first stages of dosing, and then it increases after a certain C12 dosing, at which stage its T O F curve shifts closely to the curve exhibiting T D scattering. This is quite reproducible even for Ei = 0.076 eV [ 16]. This behavior suggests that the sticking probability of C12 does not monotonously decrease with C1 coverage, but first increases with C12 dosing until the sticking begins to decrease after a certain C1 coverage. It should be noted that this first increase in sticking probability is accompanied by the reduction in the DI scattering component. This fact suggests that at the early stages of dosing the activation barriers to sticking become
X Yoneda et aL /Surface Science 363 (1996) 11-21
16
lower as C1 adatoms accumulate on the surface. Alternatively, after partial C1 adsorption, the physisorption followed by sticking is opened either by creating the physisorption wells or by softening the surface phonons. 3.2. Results at 200 K
The T O F results at 300 K suggest that the occurrence of DI scattering is the key ingredient for the weak alkali poisoning at low Ei. In order to extract the skeleton of the repulsive interaction of the C12 molecules with the alkalated surfaces, we reduce the surface temperatures to 200 K. Fig. 4
Clean Si 60 A
Cs(1ML) Si
I
Ei=0.055eV
ul ¢-
.d >I-u3 Z ILl I-Z
2o
0
j
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I
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•
~
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60
Ei = 0.09eV
El=0.09 eV
(,.J
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•
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t
60
Ei=0.86
,
Ei = 0.86 eV
40
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i
2
OiO
0
i
i 4000
I
0
2000
I
I
4000
NUMBER OF CIz SHOTS Fig. 4. Plots of evolutions of the scattered C12 intensity as a function of the n u m b e r of beam shots for the clean Si(100) and the Cs(1ML)/Si(100) for Ei=0.055, 0.09, and 0.86eV, at 200 K. The scattered C12 intensities in the ordinate exhibit densities (see Ref. [17]). For direct comparison, the scattering intensities as well as the n u m b e r of beam shots have been corrected so as to m a k e the incident beam flux the same for each E i.
shows the evolution curves of the scattered C12 intensities as a function of number of the C I 2 beam shots from the clean Si(100), and the Cs(1 ML)/Si(100) for Ei = 0.055, 0.09, and 0.86 eV. The influence of the Cs adatoms on the evolutional behavior of scattered C12 intensities is apparently different from the case at 300K, particularly for E i < 0 . 1 eV. There appear to be some clear differences in the scattering evolutions between the Cs-free surface and the cesiated surfaces• N o essential differences are observed between Cs(1 ML)/Si(100) and Cs(5 ML)/Si(100). Three interesting points in Fig. 4 are summarized as follows: 1. For Ei = 0.055 eV, the initial scattering intensity is much larger on the cesiated surface than on the clean surface. We recall that, at 300 K, the initial scattering intensities on both surfaces are almost the same for low Ei, as mentioned above. 2. With increasing El, the initial intensity of the s c a t t e r e d C12 flUX [17] tends to increase on the Cs-free surface, but on the cesiated surface it tends to decrease to almost zero at Ei = 0.86 eV. C12 sticking on the cesiated surfaces can be activated by Ei similarly to the case at 300 K, while on the clean surfaces it is decelerated by increasing Ei. 3. For the clean surface at low Ei, upon accumulating dose and, thus, increasing C1 coverages, the scattering intensities decrease in the first stages of dosing, reaching their minimum of nearly zero, and then sharply increase, eventually levelling off. However, for the cesiated surfaces they first increase, reach their maximum, and then exponentially decrease, tending to zero. In order to know what type of scattering modes take place at each stage of dosing, we plot the T O F data obtained at the early and the later stages of dosing for the Ei = 0.055, 0.09, and 0.86 eV beams in Figs. 5 and 6 for clean Si(100) and 1 M L Cs/Si(100), respectively• The T O F spectra are fitted to the Maxwell!an function of Eq. (1) to evaluate the translational temperature, Tt. Because of the limited number of beam shots for the highly reactive surfaces, the observed scattering intensities at the early stages of dosing are so small that the least mean squares fitting results in a large ambiguity. Nevertheless, the DI and T D scattering modes can be easily distinguished because of the long
S. Yoneda et al.tSurface Science 363 (1996) 11-21
3O
17
3O • b
20
,~ ~~ x -~1 *I
~
~.~.
Clean Si(100) Ei=0.055eV 20
a : initial stage b : final stage
-
Cs(1ML)/Si(100) Ei=0.055eV
a l't ~"
a : initial stage b : final stage
b
' r.13'
,.c:l
30 1
I
r13
Clean Si(100) Ei=0.09eV 20
"'A
b 1
o ~ ~ . . ~
I,-i I~'t~' 30l | 201-]
a : initial stage
(D (D ,,,.a q...a
1
2
3
"~4
Q~
30
20
4
Cs(1ML)/Si(100) [
]/ I.%* a I~
Ei=0.09eV a,
*'.
C.) 0~-
...........................
"~"
~D .+,.a t.,.l
................................. ~
O~
4
: initial stage ]
b
1
]
1
2
3
J 4
30 Clean Si(100) Ei=0.86eV
1~9 a~ it
0.)
20f i
Ei=0.86eV Cs(IML)/Si(IO0)
a : initial stage b : final stage
L/ a ~
a : initial stage ] b : final stage 1
o.s
'i .....................¢.~ ...................2
c13
•
.
2
Time of Flight [ms] Fig. 5. TOF spectra of scattered C12 molecules from the Cs free-surfaces (T~ =200K) at the very beginning of dosing accumulated for 20 beam shots (solid triangles), and at the late stages of dosing around 4000 beam shots (open circles). Zero TOF time on the abscissa is referenced with respect to the time when the beam passes the chopper. It is quite evident that the TOF spectra for the solid triangles are always faster than those for the open circles. The solid lines for the solid triangles are the best fitted curves with f(t) of Eq.(1) for y = 0 , and Tt= 296 K, 381K and 1280K for Ei=0.055 , 0.09, and 0.86eV, respectively. The solid lines for the open circles for (a) and (b) are the best fitted curves with f(t) for Tt = 104 and 100 K for Ei = 0.055 and 0.09 eV, respectively. The solid line with a bimodal distribution for (c) is obtained with a combined function of Eq.(1), f(t, Ttl)+f(t, Tt2), for the best fitted parameters, Ttl = 868 K and Tt2 = 95 K.
°o
'
Time of Flight [ms] Fig. 6. TOF spectra of scattered C12 molecules from Cs(1 ML)/Si(100) at the early stages (solid triangles) and late stages (open circles) of dosing at 200 K for E i = 0.055 eV (a), 0.09 eV (b), and 0.86 eV (c). Solid lines are the fitted curves with Eq. (1) for the best fitted parameters, Tt = 273 (294), 563 (400), and 1392 K (833 K) for the triangles and for Ei = 0.055, 0.09, and 0.86 eV, respectively. The values for Tt in brackets indicate the fitting parameters for the open circles. r e s i d e n c e in t h e p h y s i s o r p t i o n wells; for E i = 0.055 o r 0.09 eV, t h e p e a k o f t h e T O F c u r v e s at t h e l a t e r stages o f d o s i n g a p p e a r s in c o n s i d e r a b l y s l o w e r t i m e r e g i o n s t h a n t h e T O F c u r v e s o b t a i n e d at e a r l y stages o f d o s i n g , b e s i d e s t h e T O F c u r v e m e a s u r e d at t h e l a t e r stages o f d o s i n g for h i g h Ei = 0.86 e V s h o w s a c l e a r l y b i m o d a l d i s t r i b u t i o n
18
S. Yoneda et aL/Surface Science 363 (1996) 11-21
on the Cs-free surfaces (see Fig. 5). As a matter of fact, the slower component can be ascribed to TD scattering, with the faster one to DI scattering. From curve fitting of Eq. (1) to the T O F curves obtained on the clean Si(100) surface at the later stages of dosing we deduce "best fitted" T O F temperatures of about 100 K, independent of El, which is considerably lower than T~= 200 K. The extent of deviation of Tt from T~ becomes small as T~ increases. At T~_> 280 K, we find Tt -~ T~, and y ,--0, indicating that TD scattering takes place. We consider that the "best fitted" translational temperatures obtained by fitting to Eq. (1) do not show a true translational temperature to characterize TD scattering, but they are apparently affected by the delayed emission from the physisorption wells. In contrast to the T O F results on the Cs-free surfaces, the observed T O F spectra on the cesiated surfaces never include a slower T O F contribution even at the fairly late stages of dosing. This means that only DI scattering takes place on the cesiated surfaces. At low temperatures around 200 K the physisorbed molecule resides on the surface with a certain lifetime. During the long residence in the physisorption well the molecule can move around the surface until it finds its sticking place, thereby reducing desorption when the surface is still not passivated by C1 adatoms. The long-lived physisorption at 200 K results in almost unity sticking for low Ei ~-0.055 eV where the trapping probability is quite high. In other words, the sticking probability at 200 K can be solely counted by the probability of DI scattering. The initial DI scattering yield after correction to flux [17] really increases with El, as shown in Fig. 5. Therefore, we conclude that for the zero C1 coverage regime, no TD scattering takes place, but DI scattering does. The initial sticking probabilities are solely determined by the DI scattering probability. The initial DI scattering probability for low Ei is considerably enhanced by the Cs adatoms. This fact indicates that the fresh alkali overlayer causes a potential shallowing for the C12 physisorption wells, or the C12-alkalated surface interaction potential is repulsive all the way to the top of the activation barrier. (One should remember that such
repulsive potentials are experienced by only a fraction, ~30%, of the incident molecules with respect to the saturated scattering intensity on the clean Si(100); Fig. 3a.) Another interesting feature of the scattering curves on the cesiated surfaces is that the scattered C12 yield decreases with increasing C12 dose, or C1 coverage. This does not necessarily mean infinite adsorption due to dissociative sticking but, rather, mean physisorption, since we observe strong C12 desorption at TPD temperatures around 220 K, above which temperatures strong TD scattering is observed. This means that the lifetime of C12 physisorption is almost infinite on the Cl-saturated 1 M L Cs/Si(100) at 200K. Once the C I - - C s + ionic bonds are formed, they make the physisorption potential deeper. The manner of contribution by the Cs adatoms to the physisorption wells is quite opposite in trend between the Cl-free surfaces and the Cl-passivated surfaces.
4. Origin of the repulsive interaction In general, the physisorption potentials comprise two terms, Pauli repulsion and van der Waals attraction [18,19]. The effective range of the former decreases exponentially due to the spread of the surface electron wave-function into the vacuum, being short-ranged, while the latter is long-ranged and effective at the asymptotic region. The subtle energy balance of the two interactions results in a physisorption well of the order of 100 meV. The van der Waals interaction can be expected to increase with increasing surface dielectric constant [18-20]. This might contribute t o deepening the well rather than shallowing it when the surface becomes metallic upon Cs deposition. Therefore, such potential shallowing observed here may be achieved by shifting up Pauli repulsion. It may be adequate for understanding the activation process of molecular reactions on the alkalated surfaces to employ a method to construct an adiabatic potential from two diabatic potentials for the initial and final states [2]. Presently, the relevant initial and final states are a molecularly neutral state and an ionic state, respectively. The neutral C12 in the diabatic neutral state can be
S. Yoneda et at~Surface Science 363 (1996) 11-21
considered as a closed shell system because of the inhibition of electron insertion into the affinity level. The diabatic potential of the neutral state, V.(Z), consists of two terms, a short-range Pauli repulsion, WR(Z), and a long-range van der Waals attraction, V~ow(Z), as
E(z) = R(z) + Edw(Z).
(2)
The Pauli repulsion term VvR(Z) can be counted by integrating the increase in kinetic energies of the surface electrons [21] as, WR(Z)=
f~[ K(E)n(E,Z)dE,
(3)
where K(E) is the kinetic energy increase of the unit charge with energy E in the surface bands, and n(E,Z) is a local electron density of state at Z. The contribution to VvR(Z) at the asymptotic region comes mainly from the energetically uppermost electron charges split out to the C12 position. The wave-functions of such electrons exponentially tail off into the vacuum. We put a crude approximation of n(E,Z) = no exp [--2V~2mdp/hZZ]6(E Ev). Then, Eq. (3) can be recast as -
VVR(Z) "~ Kno exp [ -- ~v/~Z],
-
(4)
where K is a contant depending on the nature of C12, no is the surface electron charge density at Z = 0, and a unit of eV is taken for ~b and 2t for Z. On usual surfaces either with high work-function of about 5 eV or with depleted electron density at E F , the term of VpR(Z) is small at the asymptotic region, and therefore the physisorption well exists as a shallow minimum of the potential due to the subtle energy balance between the two terms at a position not far from the surface, around Z -- 5 ~t. For 0u = 1 ML, however, the surface is highly metallic [22,23], accumulating the vital electrons around EF to increase n o [24] on the average by a factor of 2 at 0 u = 1 M L for one Cs atom per one Si dangling bond. The wave-functions of these surface electrons tail off far into the vacuum due to the extreme lowering of the work-function from 4.9eV on the clean Si(100) to 1.3eV on the Cs(1 ML)/Si(100) surface. If we employ these values for no and ¢ in relative comparison of VvR(Z) between the alkali-covered surface and the alkali-
19
free surface, we find v~R(z)is larger on the cesiated surface than on the clean surface by two and five orders of magnitude at Z = 5 and 10 A, respectively. We thus expect that these are large enough for VvR(Z) to prevail over the attractive term, Edw(Z), even at the asymptotic region far from the surface beyond Z = 5 A. This results in an upward shift in the potential energy curve, yielding a totally repulsive potential for the neutral state. This situation is schematically drawn in Fig. 7. For the second diabatic state for the final state, we consider the C12 negative ion state which will be formed by a surface electron transfer as a molecular intermediate for dissociation [25,26].
Vn (.9 rr LtJ Z ILl
o_ .* =/
..J
--t-
I--
z I.tJ
/---
0 ft.
0
I
I
I
I
5
10
15
20
Fig. 7. Schematic drawing of the relevant potential energy curves of C12 for Cs(1 ML)/Si(100). The ordinate for potential energy is not scaled because of ambiguity in the absolute value for each potential energy curve. The curve V~ represents the diabatic potential energy curve for the cesiated surface, which is shifted above the potential energy curve for the clean surface by the net increase of Pauli repulsion. The thick solid line exhibiting the barrier is the adiabatic potential energy curve constructed from avoided crossing of the diabatic neutral V~ and ionic states Vio. (from Ref. 1-161).
20
X Yoneda et al./Surface Science 363 (1996) 11-21
The potential for the negative ion, V~on(Z), may be a decreasing function with decreasing Z if it follows the image potential even at the near asymptotic region. If the adiabatic expansion of the C1-C1 distance is not allowed in the course of Collision as discussed in gas-phase harpooning reactions [27], the vertical electron affinity EAr sets the potential at infinite distance. The curve of V~on(Z) will then cross the curve of V~(Z) at the asymptotic region, say Z = 7 ,-~ 10 A [28]. There is an interaction, V-~t(Z), which mixes the two diabatic states. By diagonalizing the Hamiltonian matrix the adiabatic potential, V~a(Z), can be constructed. Since V~(Z) is totally repulsive, the avoided crossing yields the activation barrier at the crossing region, in front of which no attractive well is formed beyond the asymptotic region. This feature is schematically illustrated in Fig. 7. In this way the existence of the activation barrier to Clz sticking onto the alkalated surfaces can be explained based on Pauli repulsion (see Ref.[16] for further discussion). As pointed out by Brown et al. [2], since Pauli repulsion is a quite general principle, the alkali poisoning effect on CO [1] or Nz [29,30] nondissociative molecular adsorption might also be due to alkali-enhanced Pauli repulsion. Right now, however, we fail to get any positive evidence for such alkali-enhanced Pauli repulsion in scattering experiments since no enhancement in the DI scatterings of CO, N 2 o r Xe, which have negative electron affinities, is observed at all as the Si(100) surface is covered with 1 M L Cs atoms [31]. In order to complete further understanding of the Pauli repulsion mechanism proposed above for C12, w e need further considerations of the key terms in the model, such as the factor K in Eq. (4), the interaction energy Vim(Z) between the ionic state and the covalent state, the recoiling effect by the adatoms upon collision, and the influence of C1z vibrational excitation.
Acknowledgements This work was financially supported by a Grantin-Aid for Scientific Research from the Ministry of
Education, Science and Culture, in the priority area "free radical science".
References [1] M.P. Kisldnova, in: Poisoning and Promotion in Catalysis Based on Surface Science Concepts and Experiments, Studies in Surface Science and Catalysis, Vol. 70, Eds. B. Delmon and J.T. Yates (Elsevier Science Publishing Company, Amsterdam, 1992) Chapter 6. [2] J.K. Brown, A.C. Luntz and P.A. Schultz, J. Chem. Phys. 95 (1991) 3767. [3] C. Resch, V. Zhukov, A. Lugstein, H.F. Berger, A. Winkler and K.D. Rendulic, Chem. Phys. 177 (1993) 421. [4] L. Surnev, in: Physics and Chemistry of Alkali Metal Adsorption, Eds. H.P. Bonzel, A.M. Bradshow and G. Ertl (Elsevier, New York, 1989) p. 173. R. Miranda, ibid, p. 425, and references therein. P. Soukiassian and H.I. Stanberg, ibid, p. 449, and references therein. H.J. Ernst and M.L. Yu, Phys. Rev. B 41 (1989) 12953. A. Namiki, S. Suzuki, H. Kato, Y. Babasaki, M. Tanaka, T. Nakamura and T. Suza!d, J. Chem. Phys. 97 (1992) 3781. [5] E.G. Michel, J.E. Ortega, E.M. Oellig, M.C. Asensio, J. Feron and R. Miranda, Phys. Rev. B 38 (1988) 13399. J.K. Norskov, S. Holloway and N.D. Lang, Surf. Sci. 137 (1984) 65. [6] R.D. Levine and R.B. Bernstein, in: Molecular Reaction Dynamics (Oxford University Press, Oxford, 1987). [7] F.H. Geuzebroek, Y. Babasaki, M. Tanaka, T. Nakamura and A. Namiki, Surf. Sci. 297 (1993) 141. [8] D. Andersson, B. Kasemo and L. Wallden, Surf. SCI. 152/153 (1985) 576. D. Andersson, PhD thesis, Department of Physics, University of Gothenberg, 1989. [9] B.I. Lundqvist, Surf. Sci. 242 (1991) 365. [10] T. Abukawa and S. Kono, Phys. Rev. B 37 (1988) 9097. [11] D.J.D. Sullivan, H.C. Flaum and A.C. Kummel, J. Phys. Chem. 97 (1993) 12051. [12] D.A. King and M.G. Wells, Surf. Sci. 29 (1972) 454. [13] S. Nishigaki, T. Sasaki, S. Matsuda, N. Kawanishi, H. Takeda and K. Yamada, Surf. Sci. 242 (1991) 358. K. Yamada and S. Nishigaki, Surf. Sci. 293 (1993) L893. [14] B. Goldstein, Surf. SCI. 35 (1973) 227. J.D. Levine, Surf. Sci. 34 (1973) 90. R. Miranda, in: Physics and Chemistry of Alkali Metal Adsorption, Eds. H.P. Bonzel, A.M. Bradshow and G. Ertl (Elsevier, New York, 1989) p. 425. [15] T. Abukawa, Y. Enta, T. Kashiwakura, S. Suzuki, S. Kono and T. Sakamoto, J. Vac. Sci. Technol. A 8 (1990) 3205. [16] A. Namiki, H. Hayashi, Y. Ukai and F.H. Geuzebroek, J. Chem. Phys. 102 (1995) 9689. [17] The scattering intensity in the ordinate of Fig. 1 exhibits the density of the scattered molecules rather than their
S. Yoneda et aL/Surface Science 363 (1996) 11-21
[18] [ 19] [20] [21] [22] [23]
flux due to electron-impacted ionization of the molecules in the QMS. The corrected flux intensity can be obtained by multiplying velocities to the TOF spectra deconvoluted from the raw TOF data of Figs. 2 or 3. This correction is important for E i = 0.86 eV because the velocity of the DI scattered molecules is much higher than E i = 0.055 or 0.09 eV. For comparison of the initial scattered flux with respect to the scattering intensity for Ei = 0.055 eV in Fig. 1 we need a multiplication factor of 1.2 and 3.0 to the initial scattering intensity for E i = 0.09 and 0.86 eV, respectively. M.C. Desjonqneres and D. Spanjaard, in: Concepts in Surface Physics (Springer, Berlin, 1993). A. Zangwill, in: Physics at Surfaces (Cambridge University Press, Cambridge, 1988). Y.S. Barash and V.L. Ginzburg, Sov. Phys. Usp. 27 (1984) 467. A. Liebsch, J. Harris, B. Salanon and J. Lapujoulade, Surf. Sci. t23 (1982) 338. L.S.O. Johansson and B. Reihl, Phys. Rev. Lett. 67 (1991) 2191; Surf. Sci. 287/288 (1993) 524. U.A. Effner, D. Badt, J. Binder, T. Bertrams, A. Brodde, Ch. Lunau, N. Neddermeyer and M. Hanbiicken, Surf.
[24] [25] [26] [27] [28]
[29] [30]
[31]
21
Sci. 277 (1992) 207. A. Brodde, T. Bertrams and H. Neddermeyer, Phys. Rev. B 47 (1993) 4508. H. Ishida and K. Terakura, Phys. Rev. B 40 (1989) 11519. J.K. Norskov, D.M. Newns and B.I. Lundqvist, Surf. Sci. 80 (1979) 179. J.W. Gadzuk and S. Holloway, Chem. Phys. Lett. 114 (1985) 314; J. Chem. Phys. 84 (1986) 3502. A.W. Kleyn, J. Los and E.A. Gislason, Phys. Rep. 90 (1982) 1. The crossing point can, in principle, be estimated by solving the equation, ~b+ Eb = EA,, + eZ/4z, where Eb is the binding energy of transferring electron measured from Er. Rough estimation of the crossing point can be performed by approximating Eb =0. Crossing occurs multiply depending on the distribution of Eb. R.A. de Paola, F.M. Hoffmann, D. Heskett and E.W. Plummer, Phys. Rev. B 35 (1987) 4236. E. Umbach, in: Physics and Chemistry of Alkali Metal Adsorption, Eds. H.P. Bonzel, A.M. Bradshow and G. Ertl (Elsevier, New York, 1989) p. 241. A. Namiki, F. Koga, N. Yamazaki and H. Dohshita, unpublished results.