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Scatter-free direct recoil spectra
Volume
152,number 4,5
SCATTER-FREE
CHEMICAL
DIRECT RECOIL
F. MASSON, S. ADURU, Dcpurltnmt
Received
q/C’hernutry,
PHYSICS LETTERS
I8 November
1988
SPECTRA
C.S. SASS and J.W. RABALAIS
Universrty q~Houston.
Houston,
TX 77204, USA
I4 June 1988; in final form 25 August I988
A method 1s described for obtaining neutral direct recoil spectra without the mterference of the scattering peak by using alkali primary ions and varying the scattering conditions. Examples of K* scattering from Si( 100) and Rb+ scattering from Pt( 1 I I ) are provided and the mechanism of producing a scatter-free spectrum IS discussed.
1. Introduction In the technique of direct recoil spectrometry (DRS), a primary ion beam is directed onto a surface at a grazing angle and surface atoms which can be recoiled into a forward angle by a single collision with a primary ion are analyzed. It has been observed that the majority of these direct recoils (DR) arc neutral particles, making energy analysis by deflection analyzers, which considers only ions, undesirable. A more sensitive method of identification is time-of-flight (TOF) spectrometry, as it allows detection of both ions and neutrals. For an incident ion of mass MI and energy E,,, the TOF of a target particle of mass M? directly recoiled into a recoil angle Q is given by [ 1] t I>R=L(M,
+MZ)/(8M,Eo)“2~~s(p,
(1)
where L is the distance from the target to the detector. The applications of DRS have been recently reviewed (see ref. [2], and references therein) and include studies such as surface elemental analysis, surface structure analysis (surface reconstruction, adsorption sites on surfaces), adsorption binding energies, and surface reaction kinetics. The most intense peak in a typical TOF spectrum corresponds to the incident particles scattered (S) from the surface; it is normally one or two orders of magnitude more intense than the (DR) peaks. This (S) peak is useful in certain applications [ 31. However, it will obscure the observation of a (DR ) peak if the corresponding (DR) particle has approxi0 009-26 14/88/$ 03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division )
mately the same velocity as the (S) primaries. In this case, it would be advantageous to suppress the intense (S ) peak. Typical primary ions used in DRS are rare gas or alkali ions. The use of alkali ions has the advantage in that their neutralization probabilities are low [ 41, therefore most of them survive the scattering process as ions. The combination of the high ion fraction of the (S) alkali particles with the low ion fraction of the (DR) atoms allows a tremendous enhancement of the (DR) / (S) ratio by deflecting the ions before they reach the detector, thus acquiring the neutralonly (N) spectra. Under certain scattering conditions, it is even possible to totally eliminate the scattering signal. The purpose of this Letter is to demonstrate how neutral DRS-TOF spectra can be obtained with a scattering peak of negligible intensity through the use of incident alkali ions and appropriate scattering conditions. A semiconductor surface, Si ( loo), and a metal surface, Pt (11 l), are investigated with K+ and Rbf, respectively, as the incident ions. The results of a more detailed study of the K+/Si( 100) system are reported elsewhere [ 5 1.
2. Experimental The instrumental requirements for DRS with TOF analysis have been previously described [ 61. To summarize, the primary ions (4 keV K+ and Rb+) arrive at the sample as a pulsed beam of period 20B.V.
325
Volume 152, number 4,5
CHEMICAL PHYSICS LETTERS
40 ps and width 30-40 ns. The scattered (S) and directly recoiled (DR) particles are detected by a channeltron electron multiplier (CEM) , Ions are deflected by applying a positive voltage of = 800 V on a deflection plate besides the flight path (90 cm long at @=21” and 115 cm long at @=33”). A time-toamplitude converter produces voltage pulses of height proportional to the particle flight times which are fed to a multichannel analyzer for pulse height analysis. The base pressure in the scattering chamber is below 1 x lop9 Torr. The samples are cleaned in situ by 3 keV Ar+ bombardment and electron impact heating to 1000°C for Si( 100) and 1200°C for Pt( 111). For the K ’ /Si system, the Si (DR) and K(S) intensities are monitored as a function of incidence angle cy. For the Rb+ /Pt system, the Pt (DR) cross section is > 10 times smaller than the Rb’ (S) cross section, hence Pt (DR) is not observed. For this second system, hydrogen, carbon and sulfur are the recoiled atoms studied. Sulfur, which is a natural contaminant of Pt, was allowed to diffuse to the surface by heating the crystal for 10 min at 1200°C and carbon and hydrogen were introduced on the surface by chemisorbing 0.2 L of ethylene after the previous heating. The carbon and hydrogen coverages were evaluated by comparing the C (DR) and H(DR) signal intensities to those corresponding to a clean Pt ( 111) surface saturated in ethylene (carbon coverage: 1 monolayer, hydrogen coverage: 1.5 monolayer [ 7 ] ) , The sulfur coverage was then obtained by comparing the S (DR) and C (DR ) signal intensities. This leads to an estimated [H + C + S] coverage of 20% of a monolayer.
3. Results Fig. 1 shows the ion plus neutral (I+ N) and neutral-only (N) DRS-TOF spectra for the Rb+/ Pt( 111) system. At a recoil/scattering angle of @=8=33” and incidence angle of a=25” (with respect to the surface plane), the (S) peak is totally eliminated in the (N) spectrum while the total [H + C + S ] (DR) signal intensity decreases by only a factor of z 2 when the ions are deflected. This indicates that, for these specific scattering conditions, the neutral fraction of recoils is -0.5 while the neutral fraction of the scattered Rb + is insignificant. 326
18 November I 988
19
&OF-FLIGHT (psedFig. I. DRS-TOF ion-plus-neutral (I+N) and neutral-only (N) spectra of hydrogen, carbon and sulfur-covered Pt ( 1I I ) induced by a 4 keV Rb+ beam at an incidence angle (r=25” and a detection angle q$=33”.
In order to better understand the influence of the scattering conditions on the intensity of the (N) scattering peak, the relative recoil and scattering intensities were measured as a function of the incidence angle a, at fixed detection (recoil/scattering) angle, This is illustrated in fig. 2 for the (N) spectrum of the K+/Si system at a detection angle @=2 1 O. The Si( DR) /K( S) intensity ratio increases byafactoroflSfroma=7” toa=16”,whereanear scatter-free (DR) spectrum is obtained. This dramatic increase in the (DR) / (S) ratio occurs only in the (N) spectrum. In the (I +N) spectrum, this ratio increases only by a factor of 1.4 between LY=7” and 16”. A similar trend, although less pronounced, is observed for the Rb+/Pt ( 111) system at a detection angle @= 2 lo. The (DR) / (S) ratio for recoiling of (H + C + S) stays constant between LY= 7’ and 16’ in the (1-l-N) spectrum while it increases by a factor of 2 for the (N) spectrum as shown in fig. 3. Unlike the K+/Si system, the scattering peak could not be totally eliminated in the Rb+/Pt system at @=21’, This may be due to the very large scattering cross section of Rb* on Pt at @=21’. A detection angle @= 33” was necessary in order to obtain a scatter-free DRS spectrum, as shown in fig. 1.
Volume 152. number 4,5
CHEMICAL
PHYSlCS
LETTERS
18 November
1988
I_ I Cl=lf
a=l6”
4 8 TIME-OF-FLIGHT
12
( psec) Fig. 2. DRS-TOF neutral-only (N) spectra of clean Si ( 100) induced by a 4 keV K+ beam at increasing incidence angle oz. The detection angle is fixed at @= 2 1’. The ratio Si (DR) /K(S) is 0.2 at~=7”,0.5ata=1l”and3.0ata=16”.
4. Discussion The small increase of Si (DR) /K(S) intensity ratio with the incidence angle LYin the (ISN) spectrum can be explained in terms of geometric effects. At small cy, shadowing effects from neighboring Si atoms are such that no large enough impact parameters are available for the (DR) of a given Si atom. This has been verified by trajectory simulations taking into account the probable buckled dimer-type structure of the Si( 100) surface [ 5 1. The much larger increase in the Si( DR) /K(S) ratio with increasing (Yin the (N) spectra than in the (I+N) spectra suggests that other phenomena are important in the (N) spectra. These phenomena are also responsible for the increase in the [H+C+S] (DR)/Rb(S) ratio with increasing ff in the (N) spectra whereas thiS ratio is essentially constant in the (I+N) spectra. These are electronic effects [ 81 and are due to the difference in the
Fig. 3. DRS-TOF
neutral-only (N) spectra of hydrogen, carbon Pt( 111) induced by a 4 keV Rb’ beam at increasing incidence angle (Y. The detection angle is fixed at 9-21”.Theratio [HtCtS] (DR)/Rb(S) is 1.6ata-7”, 1.5 at~=Ildand2.9ata=16”. and sulfur-covered
neutralization probabilities of the (S) and (DR) particles with incidence angle. Since the (N) fractions of (S) alkali species are much lower than those of (DR) species ( ~6% for K’ and Rb ’ versus =50°/6for [H+C+S] and >90%forSi), theseelectronic effects are much more significant to the (N) scattered flux than to the (N) recoiled flux. Therefore, the primary contribution to the increase of the (DR) / (S) ratio in the (N ) spectra is the lower neutralization probability of the scattered alkali with increasing incidence angle (Y. In order to understand the neutralization process, the scattering event may be partitioned into three sequences: the incoming trajectory, the close atomic encounter, and the outgoing trajectory. In the close atomic encounter, the neutralization probability is determined by the apsis or distance of closest approach, according to the Fano-Lichten mechanism [ 91. Experimental data for the K+/Si ( 100) system suggests that the close encounter contributes to the overall neutralization process [ 5 1, however, only the mechanisms involving electron exchange outside this region will be considered further for the explanation 327
Volume
I SZ,number 4.5
CHEMICAL
PHYSICS LETTERS
of the changes observed herein. This is because at the fixed detection angles (fixed impact parameter and apsis) employed in this work, the Fano-Lichten model requires that the degree of neutralization in the close encounter for a given projectile/target combination is independent of incident angle (Y. On the incoming and outgoing trajectories, ncutralization can occur by Auger and resonant charge exchange with the surface [ lo]. The low neutralization probability of alkali ions scattering off a metal or semiconductor surface is due to their valence orbitals lying above the metal Fermi level I&] or the top of the semiconductor valence band IET 1, respcctively, thus minimizing resonant or Auger neutralization transitions. The values of importance to this study are: ionization potential IP~4.34 eV for K, lETI =4.99 eV for Si( loo), IP=4.18 eV for Rb, and 1&]=5.70 eV for Pt(ll1). For the Rb+/ Pt ( 11 I ) system, the adsorbed atoms H, C and S, because of their low coverage, are expected, in a first approximation, not to influence the neutralization process. The limited amount of neutralization that does occur in these scattering processes is due to a broadening of the alkali valence level as it approaches the surface [ 8,11- 13 1. This broadening results in the partial overlap of alkali valence orbitals with the metal or semiconductor valence bands [ 51. The decreasing (N ) fraction of the scattered K and Rb with increasing incidence angle IY, may be explained by the model of Los et al. [ 141. This model assumes that an equilibrium situation exists which governs the distribution of electrons between the alkali valence level and the surface valence band at all points along the scattering trajectory of the alkali atom (adiabatic approximation ). One manifestation of this model is then that the final charge state of the alkali is dependent only on its outgoing trajectory, i.e. the particle bears no memory of its initial charge state or excitations in the close encounter. However, this equilibrium charge state can be adjusted only up to a distances* from the surface, called the “freezing distance”, beyond which the charge transfer probability is zero and the charge distribution is frozen. The freezing distance, s*, increases as the normal component, v,, of the outgoing velocity of the alkali decreases [ 14 1. At constant incident ion energy E. and detection angle #, v, decreases with decreasing exit angle A or correspondingly increas328
18November 1988
ing incidence angle cy, since a=@--P. Thus, s* increases with increasing incidence angle. Therefore, according to the experimental results and the above model, the degree of neutralization of the scattered alkali decreases at larger freezing distance. This is in agreement with the results of Boers et al. [ 8,121 for K+ scattering from Cu( 100). In their work, v, was decreased by decreasing the alkali incident energy &. The K+/Cu( 100) system is very similar to the systems used in this study in that the valence orbitals of K lie above the Fermi level of Cu( 100): IP=4.34 eV for K, ]EF] =4.59 eV for Cu( 100). The general phenomenon observed in this study and the work of Beers et al. [8,12] - the degree of neutralization of the scattered alkali decreasing at larger freezing distance - is due to the fact that larger freezing distances allow better adjustment of the equilibrium charge distribution between the alkali and the surface and thus a negligible neutralization of the alkali is expected since its valence orbitals are above the valence band of the metal or semiconductor. A quantitative verification of the freezing distance model used in this discussion is given elsewhere [ 51 for the K+/Si( 100) system, Since the energy difference between the 5s valence orbital of Rb and the I&] of Pt( 111) (1.5 eV) is larger than that between the 4s valence orbital of K and the ]E, I of Si( 100) (0.65 eV), the degree of charge exchange close to the surface is less for the Rb+/Pt system than for the K+/Si system. Therefore, the change with cy in the degree of neutralization of the scattered alkali, and thus in the (DR)/ (S) ratio, is expected to be less in the Rb+/Pt system than in the K+/Si system; this is indeed what is observed experimentally.
5. Conclusion Neutral direct recoil spectra which are totally free of an interfering scattering peak have been produced. It has been demonstrated that their generation by dccrcasing the exit angle is essentially controlled by electronic effects rather than geometric factors. These effects are explained in terms of the freezing distance concept which predicts that low exit angles (but also low incident energies, i.e. more generally, low normal components of the alkali outgoing
Volume 152, number 4,5
CHEMlCAL
PHYSICS LETTERS
velocity) minimize the amount of neutral alkali scattering from a metal (Pt) or a semiconductor (5) surface provided that the Fermi level or the top of the valence band, respectively, lies below the valence level of the alkali. The immediate application of the overwhelming predominance of the electronic effects is that scatter-free neutral direct recoil spectra can, in principle, be generated for any type of such metal or semiconductor surfaces.
Acknowledgement This material is based upon work supported by the National Science Foundation under Grant No. DMR8610597.
References [ I ] D.P. Smith, J. Appl. Phys. 38 ( 1967) 340. [2] J.W. Rabalais, CRC Critical Reviews, to be published.
I8 November
1988
[3] F. Masson, C.S. Sass, J.N. Chen, H. Kangand J.W. Rabalais, to be submitted for publication. [4] H. Nichus and G. Cornsa, Surface Sci. 152/ 153 ( 1985) 93; H. Niehus, J. Vacuum Sci. Technol. A 5 ( 1987) 75 I; H. Niehus and G. Comsa, Nucl. Instr. Methods B IS ( 1986) 122. (51 S. Aduru and J.W. Rabalais, Surface Sci. 205 (1988) 269. [6] J.A. Schultz, S. Contarini and J.W. Rabalais, Surface Sci. 154 (1985) 315; J.A. Schultz. Y.S. Jo. S. Tachi and J.W. Rabalais, Nucl. Instr. Methods B 15 (1986) 134. [ 71 F. Masson. C.S. Sass and J.W. Rabalais, Surface Sci., to be submitted for publication. (81 A.J. Algra, E. van Loencn. E.P.Th.M. Suurmcycr and A.L. Bows, Rad. Eff. 60 ( 1YX2) 173. 191 U. Fano and W. Lichten, Phys. Rev. Letters 14 (1965) 627; M. Barat and W. Lichten, Phys. Rev. A 6 ( 1972) 2 1I. [ IO] H.D. Hagstrum, in: Electron and ion spectroscopy of solids, eds. L. Fiermans, J. Vennik and W. Dekeyscr (Plenum Press, NewYork, 1978). [ I I ] A.J. Algra, Ph.D. Thesis, University of Groningen ( 1981 ); D.P. Woodruff. Nucl. Instr. Methods 194 (1982) 639. [ 121 A.L. Boers, Nucl. Instr. Methods B 2 (1984) 353. [ 131 J.W. Gadzuk, Surface SCI. 6 (iY67) 133; B. Rasserand M. Remy, Surface Sci. 93 ( 1980) 223. [ 141 E.G. Overbosch, B. Rasser, AD Tennerand J. Los, Surface sci. 92 (1980) 310.
329
152,number 4,5
SCATTER-FREE
CHEMICAL
DIRECT RECOIL
F. MASSON, S. ADURU, Dcpurltnmt
Received
q/C’hernutry,
PHYSICS LETTERS
I8 November
1988
SPECTRA
C.S. SASS and J.W. RABALAIS
Universrty q~Houston.
Houston,
TX 77204, USA
I4 June 1988; in final form 25 August I988
A method 1s described for obtaining neutral direct recoil spectra without the mterference of the scattering peak by using alkali primary ions and varying the scattering conditions. Examples of K* scattering from Si( 100) and Rb+ scattering from Pt( 1 I I ) are provided and the mechanism of producing a scatter-free spectrum IS discussed.
1. Introduction In the technique of direct recoil spectrometry (DRS), a primary ion beam is directed onto a surface at a grazing angle and surface atoms which can be recoiled into a forward angle by a single collision with a primary ion are analyzed. It has been observed that the majority of these direct recoils (DR) arc neutral particles, making energy analysis by deflection analyzers, which considers only ions, undesirable. A more sensitive method of identification is time-of-flight (TOF) spectrometry, as it allows detection of both ions and neutrals. For an incident ion of mass MI and energy E,,, the TOF of a target particle of mass M? directly recoiled into a recoil angle Q is given by [ 1] t I>R=L(M,
+MZ)/(8M,Eo)“2~~s(p,
(1)
where L is the distance from the target to the detector. The applications of DRS have been recently reviewed (see ref. [2], and references therein) and include studies such as surface elemental analysis, surface structure analysis (surface reconstruction, adsorption sites on surfaces), adsorption binding energies, and surface reaction kinetics. The most intense peak in a typical TOF spectrum corresponds to the incident particles scattered (S) from the surface; it is normally one or two orders of magnitude more intense than the (DR) peaks. This (S) peak is useful in certain applications [ 31. However, it will obscure the observation of a (DR ) peak if the corresponding (DR) particle has approxi0 009-26 14/88/$ 03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division )
mately the same velocity as the (S) primaries. In this case, it would be advantageous to suppress the intense (S ) peak. Typical primary ions used in DRS are rare gas or alkali ions. The use of alkali ions has the advantage in that their neutralization probabilities are low [ 41, therefore most of them survive the scattering process as ions. The combination of the high ion fraction of the (S) alkali particles with the low ion fraction of the (DR) atoms allows a tremendous enhancement of the (DR) / (S) ratio by deflecting the ions before they reach the detector, thus acquiring the neutralonly (N) spectra. Under certain scattering conditions, it is even possible to totally eliminate the scattering signal. The purpose of this Letter is to demonstrate how neutral DRS-TOF spectra can be obtained with a scattering peak of negligible intensity through the use of incident alkali ions and appropriate scattering conditions. A semiconductor surface, Si ( loo), and a metal surface, Pt (11 l), are investigated with K+ and Rbf, respectively, as the incident ions. The results of a more detailed study of the K+/Si( 100) system are reported elsewhere [ 5 1.
2. Experimental The instrumental requirements for DRS with TOF analysis have been previously described [ 61. To summarize, the primary ions (4 keV K+ and Rb+) arrive at the sample as a pulsed beam of period 20B.V.
325
Volume 152, number 4,5
CHEMICAL PHYSICS LETTERS
40 ps and width 30-40 ns. The scattered (S) and directly recoiled (DR) particles are detected by a channeltron electron multiplier (CEM) , Ions are deflected by applying a positive voltage of = 800 V on a deflection plate besides the flight path (90 cm long at @=21” and 115 cm long at @=33”). A time-toamplitude converter produces voltage pulses of height proportional to the particle flight times which are fed to a multichannel analyzer for pulse height analysis. The base pressure in the scattering chamber is below 1 x lop9 Torr. The samples are cleaned in situ by 3 keV Ar+ bombardment and electron impact heating to 1000°C for Si( 100) and 1200°C for Pt( 111). For the K ’ /Si system, the Si (DR) and K(S) intensities are monitored as a function of incidence angle cy. For the Rb+ /Pt system, the Pt (DR) cross section is > 10 times smaller than the Rb’ (S) cross section, hence Pt (DR) is not observed. For this second system, hydrogen, carbon and sulfur are the recoiled atoms studied. Sulfur, which is a natural contaminant of Pt, was allowed to diffuse to the surface by heating the crystal for 10 min at 1200°C and carbon and hydrogen were introduced on the surface by chemisorbing 0.2 L of ethylene after the previous heating. The carbon and hydrogen coverages were evaluated by comparing the C (DR) and H(DR) signal intensities to those corresponding to a clean Pt ( 111) surface saturated in ethylene (carbon coverage: 1 monolayer, hydrogen coverage: 1.5 monolayer [ 7 ] ) , The sulfur coverage was then obtained by comparing the S (DR) and C (DR ) signal intensities. This leads to an estimated [H + C + S] coverage of 20% of a monolayer.
3. Results Fig. 1 shows the ion plus neutral (I+ N) and neutral-only (N) DRS-TOF spectra for the Rb+/ Pt( 111) system. At a recoil/scattering angle of @=8=33” and incidence angle of a=25” (with respect to the surface plane), the (S) peak is totally eliminated in the (N) spectrum while the total [H + C + S ] (DR) signal intensity decreases by only a factor of z 2 when the ions are deflected. This indicates that, for these specific scattering conditions, the neutral fraction of recoils is -0.5 while the neutral fraction of the scattered Rb + is insignificant. 326
18 November I 988
19
&OF-FLIGHT (psedFig. I. DRS-TOF ion-plus-neutral (I+N) and neutral-only (N) spectra of hydrogen, carbon and sulfur-covered Pt ( 1I I ) induced by a 4 keV Rb+ beam at an incidence angle (r=25” and a detection angle q$=33”.
In order to better understand the influence of the scattering conditions on the intensity of the (N) scattering peak, the relative recoil and scattering intensities were measured as a function of the incidence angle a, at fixed detection (recoil/scattering) angle, This is illustrated in fig. 2 for the (N) spectrum of the K+/Si system at a detection angle @=2 1 O. The Si( DR) /K( S) intensity ratio increases byafactoroflSfroma=7” toa=16”,whereanear scatter-free (DR) spectrum is obtained. This dramatic increase in the (DR) / (S) ratio occurs only in the (N) spectrum. In the (I +N) spectrum, this ratio increases only by a factor of 1.4 between LY=7” and 16”. A similar trend, although less pronounced, is observed for the Rb+/Pt ( 111) system at a detection angle @= 2 lo. The (DR) / (S) ratio for recoiling of (H + C + S) stays constant between LY= 7’ and 16’ in the (1-l-N) spectrum while it increases by a factor of 2 for the (N) spectrum as shown in fig. 3. Unlike the K+/Si system, the scattering peak could not be totally eliminated in the Rb+/Pt system at @=21’, This may be due to the very large scattering cross section of Rb* on Pt at @=21’. A detection angle @= 33” was necessary in order to obtain a scatter-free DRS spectrum, as shown in fig. 1.
Volume 152. number 4,5
CHEMICAL
PHYSlCS
LETTERS
18 November
1988
I_ I Cl=lf
a=l6”
4 8 TIME-OF-FLIGHT
12
( psec) Fig. 2. DRS-TOF neutral-only (N) spectra of clean Si ( 100) induced by a 4 keV K+ beam at increasing incidence angle oz. The detection angle is fixed at @= 2 1’. The ratio Si (DR) /K(S) is 0.2 at~=7”,0.5ata=1l”and3.0ata=16”.
4. Discussion The small increase of Si (DR) /K(S) intensity ratio with the incidence angle LYin the (ISN) spectrum can be explained in terms of geometric effects. At small cy, shadowing effects from neighboring Si atoms are such that no large enough impact parameters are available for the (DR) of a given Si atom. This has been verified by trajectory simulations taking into account the probable buckled dimer-type structure of the Si( 100) surface [ 5 1. The much larger increase in the Si( DR) /K(S) ratio with increasing (Yin the (N) spectra than in the (I+N) spectra suggests that other phenomena are important in the (N) spectra. These phenomena are also responsible for the increase in the [H+C+S] (DR)/Rb(S) ratio with increasing ff in the (N) spectra whereas thiS ratio is essentially constant in the (I+N) spectra. These are electronic effects [ 81 and are due to the difference in the
Fig. 3. DRS-TOF
neutral-only (N) spectra of hydrogen, carbon Pt( 111) induced by a 4 keV Rb’ beam at increasing incidence angle (Y. The detection angle is fixed at 9-21”.Theratio [HtCtS] (DR)/Rb(S) is 1.6ata-7”, 1.5 at~=Ildand2.9ata=16”. and sulfur-covered
neutralization probabilities of the (S) and (DR) particles with incidence angle. Since the (N) fractions of (S) alkali species are much lower than those of (DR) species ( ~6% for K’ and Rb ’ versus =50°/6for [H+C+S] and >90%forSi), theseelectronic effects are much more significant to the (N) scattered flux than to the (N) recoiled flux. Therefore, the primary contribution to the increase of the (DR) / (S) ratio in the (N ) spectra is the lower neutralization probability of the scattered alkali with increasing incidence angle (Y. In order to understand the neutralization process, the scattering event may be partitioned into three sequences: the incoming trajectory, the close atomic encounter, and the outgoing trajectory. In the close atomic encounter, the neutralization probability is determined by the apsis or distance of closest approach, according to the Fano-Lichten mechanism [ 91. Experimental data for the K+/Si ( 100) system suggests that the close encounter contributes to the overall neutralization process [ 5 1, however, only the mechanisms involving electron exchange outside this region will be considered further for the explanation 327
Volume
I SZ,number 4.5
CHEMICAL
PHYSICS LETTERS
of the changes observed herein. This is because at the fixed detection angles (fixed impact parameter and apsis) employed in this work, the Fano-Lichten model requires that the degree of neutralization in the close encounter for a given projectile/target combination is independent of incident angle (Y. On the incoming and outgoing trajectories, ncutralization can occur by Auger and resonant charge exchange with the surface [ lo]. The low neutralization probability of alkali ions scattering off a metal or semiconductor surface is due to their valence orbitals lying above the metal Fermi level I&] or the top of the semiconductor valence band IET 1, respcctively, thus minimizing resonant or Auger neutralization transitions. The values of importance to this study are: ionization potential IP~4.34 eV for K, lETI =4.99 eV for Si( loo), IP=4.18 eV for Rb, and 1&]=5.70 eV for Pt(ll1). For the Rb+/ Pt ( 11 I ) system, the adsorbed atoms H, C and S, because of their low coverage, are expected, in a first approximation, not to influence the neutralization process. The limited amount of neutralization that does occur in these scattering processes is due to a broadening of the alkali valence level as it approaches the surface [ 8,11- 13 1. This broadening results in the partial overlap of alkali valence orbitals with the metal or semiconductor valence bands [ 51. The decreasing (N ) fraction of the scattered K and Rb with increasing incidence angle IY, may be explained by the model of Los et al. [ 141. This model assumes that an equilibrium situation exists which governs the distribution of electrons between the alkali valence level and the surface valence band at all points along the scattering trajectory of the alkali atom (adiabatic approximation ). One manifestation of this model is then that the final charge state of the alkali is dependent only on its outgoing trajectory, i.e. the particle bears no memory of its initial charge state or excitations in the close encounter. However, this equilibrium charge state can be adjusted only up to a distances* from the surface, called the “freezing distance”, beyond which the charge transfer probability is zero and the charge distribution is frozen. The freezing distance, s*, increases as the normal component, v,, of the outgoing velocity of the alkali decreases [ 14 1. At constant incident ion energy E. and detection angle #, v, decreases with decreasing exit angle A or correspondingly increas328
18November 1988
ing incidence angle cy, since a=@--P. Thus, s* increases with increasing incidence angle. Therefore, according to the experimental results and the above model, the degree of neutralization of the scattered alkali decreases at larger freezing distance. This is in agreement with the results of Boers et al. [ 8,121 for K+ scattering from Cu( 100). In their work, v, was decreased by decreasing the alkali incident energy &. The K+/Cu( 100) system is very similar to the systems used in this study in that the valence orbitals of K lie above the Fermi level of Cu( 100): IP=4.34 eV for K, ]EF] =4.59 eV for Cu( 100). The general phenomenon observed in this study and the work of Beers et al. [8,12] - the degree of neutralization of the scattered alkali decreasing at larger freezing distance - is due to the fact that larger freezing distances allow better adjustment of the equilibrium charge distribution between the alkali and the surface and thus a negligible neutralization of the alkali is expected since its valence orbitals are above the valence band of the metal or semiconductor. A quantitative verification of the freezing distance model used in this discussion is given elsewhere [ 51 for the K+/Si( 100) system, Since the energy difference between the 5s valence orbital of Rb and the I&] of Pt( 111) (1.5 eV) is larger than that between the 4s valence orbital of K and the ]E, I of Si( 100) (0.65 eV), the degree of charge exchange close to the surface is less for the Rb+/Pt system than for the K+/Si system. Therefore, the change with cy in the degree of neutralization of the scattered alkali, and thus in the (DR)/ (S) ratio, is expected to be less in the Rb+/Pt system than in the K+/Si system; this is indeed what is observed experimentally.
5. Conclusion Neutral direct recoil spectra which are totally free of an interfering scattering peak have been produced. It has been demonstrated that their generation by dccrcasing the exit angle is essentially controlled by electronic effects rather than geometric factors. These effects are explained in terms of the freezing distance concept which predicts that low exit angles (but also low incident energies, i.e. more generally, low normal components of the alkali outgoing
Volume 152, number 4,5
CHEMlCAL
PHYSICS LETTERS
velocity) minimize the amount of neutral alkali scattering from a metal (Pt) or a semiconductor (5) surface provided that the Fermi level or the top of the valence band, respectively, lies below the valence level of the alkali. The immediate application of the overwhelming predominance of the electronic effects is that scatter-free neutral direct recoil spectra can, in principle, be generated for any type of such metal or semiconductor surfaces.
Acknowledgement This material is based upon work supported by the National Science Foundation under Grant No. DMR8610597.
References [ I ] D.P. Smith, J. Appl. Phys. 38 ( 1967) 340. [2] J.W. Rabalais, CRC Critical Reviews, to be published.
I8 November
1988
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