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Preferential neutralization in low energy helium ion scattering from the second layer of MoS2 basal plane surfaces
Ll2
Surface Science 124 (1983) L12-L18 North-Holland Publishing Company
SURFACE
SCIENCE
LETTERS
PREFERENTIAL NEUTRALIZATION SCATTERING FROM THE SECOND SURFACES S.M. DAVIS
IN LOW ENERGY HELIUM ION LAYER OF MoS, BASAL PLANE
and J.C. CARVER
Exxon Research and Development
Luboratory,
P.O. Box 2226, Baton Rouge, Louisrana 70821, USA
and A. WOLD Department Received
of Chemrstty,
Brown University, Providence,
13 July 1982; accepted
for publication
Rhode Island, USA
12 October
1982
Low energy 3He+ and 4He+ scattering along the [IOO] direction of single crystal MoS, basal plane surfaces has been investigated. The layered atomic structure of this compound was used to demonstrate that preferential neutralization accompanies scattering from the second surface layer. Preferential neutralization that takes place upon scattering at molybdenum atomic sites leads to a marked increase in the MO/S LEISS intensity ratio with increasing primary ion energy (0.38-2 keV).
Neutralization processes have been identified as a decisive factor controlling the scattered yield in low energy ion scattering spectroscopy (LEISS). For helium and neon ion scattering from nickel or yttrium surfaces, Buck and Brongersma [l] have shown that these neutralization processes appear to be adequately described using models formulated by Hagstrum [2] for direct Auger neutralization or resonance neutralization followed by Auger de-excitation. In this case, the scattered intensity, I+, is related to the incident ion flux, (I, the detection sensitivity, T, the normal I,+ > the scattering cross-section, component of scattered ion velocity, VI , and a characteristic neutralization constant, p, by
Z+=Z,+oTexp(-p/V:).
(1)
The factor exp( -p/V:) represents the probability that a scattered ion will escape the influence of the solid surface without experiencing neutralization. This probability increases as the ion-solid interaction time decreases and as the distance of the ion from the surface increases [2]. A more general form for
0039-6028/83/0000-0000/$03.00 0 1983 North-Holland
S.M. Davis et al. / Preferential
the ion escape probability,
neutralization
m LE helium ion scattering
P’ , has been proposed
P+=&Y,(l-PN)+(l-cx,)P,],
by Poelsema
L13
et al. [3], i.e., (2)
with aI = exp( -P/V:),
a2 = exp( -P/v:),
which includes neutralization probabilities during the incident trajectory, (1 a,), the collision, P,, and the outgoing trajectory, (1 - a,), where Vi and V: are the normal components of incident and scattered ion velocity, respectively. Reionization of neutrals produced along the incoming trajectory may also take place during collision with probability P,. Recent angular distribution studies by Overbury [4] and MacDonald [5] and co-workers have revealed that up to 95-98s of the total neutralization may occur during the collision event. Moreover, for neon and argon ion scattering from Cu(lOO), the reionization process also appears to become important [6]. Recent calculations for Li+ and He+ scattering from Ni( 100) [7] and Ni( 110) [8,9] indicate that the neutralization probability is strongly dependent upon the trajectory of the scattered particle. While LEISS studies have now been reported for a variety of oxides, halides, and other binary compound surfaces [lo- 151, previous studies of the neutralization process have mostly concentrated on atomically clean metals. For these metals there exists little possibility of directly distinguishing differences in scattered yield and neutralization probability that arise from scattering in the first and second atomic surface layers. In this communication, we report studies of 3He+ and 4He+ scattering from single crystal MoS, basal plane surfaces for which the scattering from the first and second atomic layers is clearly distinct. The layered atomic structure of this transition metal compound has been used to demonstrate that preferential neutralization accompanies scattering from the second surface layer. The LEISS experiments were carried out in a Leybold-Heraeus UHV apparatus (base pressure < 2 x lo-” mbar) equipped with a differentially pumped O-5 keV ion gun and 180” hemispherical energy analyzer. The laboratory scattering angle and angle of incidence were fixed at 120° and 30°, respectively; i.e., scattered ions were detected at the surface normal. The analyzer was operated in a retarding factor mode with AE/E = 3. The ion gun produced a beam with an approximate spot area of 4 mm2 which was rastered over 4 mm in the direction normal to the incident ion flux. Single crystal MoS, platelets with surface areas of about 0.5 cm2 were prepared from the elements by chemical vapor transport at 1370- 1420 K using Br, as the transport agent. The single crystal surfaces were carefully cleaned using repeated cycles of brief helium or neon ion sputtering at 0.5-l keV followed by annealing one to several hours at 800-870 K until scattered intensities achieved reproducible maximum levels, and surface impurities were
S. M. Davis et al. / Preferential
Ll4
neutralization
in LE helium ion scattering
barely detectable (0, Na, and K were most tenacious). Photoemission, LEISS, and LEED studies reported separately [ 16,171 have revealed that this pretreatment produces a clean, stoichiometric, and well-ordered MoS, basal plane surface that displays no activity for oxygen chemisorption. Oxygen chemisorption is easily detected by LEGS when surface sulfide vacancies are present. Primary beam currents in the range 20-100 nA/cm2 were chosen in our studies to minimize surface sputtering while still providing reliable intensities. Even at these low currents, preferential sputtering of sulfur was apparent during a series of consecutive experiments. Fig. 1 displays the variation of the MO/S intensity ratio with total ion dose for the 4He+ scattering at 500 eV incident energy. A total ion dose as low as lOI cm-’ was adequate to produce significant changes in surface composition. The initial MO/S intensity ratio was restored upon annealing to 870 K. During intensity measurements, the samples were repeatedly annealed to insure that changes in the MO/S intensity ratio due to sputtering were minimized. The idealized atomic surface structure for the MoS,(OO 1) basal plane surface is diagrammed in fig. 2. The topmost layer consists of close packed sulfide anions with hexagonal atomic arrangement and d,_, = 3.15 A (bulk). The second layer is composed of a hexagonal array of molybdenum cations with trigonal prismatic coordination and dMo_S = 2.41 A (bulk). The [ 1001 direction parallels rows of sulfide anions and valleys of molybdenum cations that are displaced by the interlayer spacing of 1.59 A (bulk). Energy distributions for 4He+ scattered along the [loo] direction of the MoS, basal plane surface are compared at 0.5 and 2.0 keV incident energy in fig. 3. A marked increase in the MO/S LEISS intensity ratio is evident with
r
I
I
,
I
MoS2(100) 0.5 kEV
I
0
1.0
‘ttE+
I
2.0
I
3.0
4,O
ION DOSE (CM-* x 10-15)
Fig. 1. Variation of the MO/S LEISS intensity ratio with total ion dose for 4He+ scattering at 0.5 keV incident energy. Preferential sputtering of surface sulfur is evident for an ion dose as low as 10” cmm2.
S.M. Davis et al. / Preferentral neutralization
in LE helium ion scattering
0
Ll5
S (1ST LAYER)
0 No (ZND LAYER) 100 110 Y INTERLAYERSPACING1,59 ii
DNoS = 2.41 i Fig. 2. Idealized
atomic
surface
structure
for the MoS,(OOl)
basal plane surface.
increasing primary ion energy. The energy dependence of this intensity ratio is depicted more clearly in fig. 4 where data are compared for 3HeC and 4He+ scattering over the energy range 0.38-2.0 keV, in which case the intensity ratio varies by nearly two orders of magnitude. In fig. 5, relative ion escape probabilities for scattering at sulfur and molybdenum atomic sites have been calculated from experimental intensities using eq. (1) (e.g., P+ = Z'/Z,' UT) and plotted as a function of the reciprocal outgoing ion velocity (1 atomic unit = 4.571 X 10m9 s/cm). Scattering crosssections using the Moliere approximation to the Thomas-Fermi screening function in a screened Coulomb potential were calculated from the tables of Nelson [18]. Beam currents were measured with a Faraday cup, and relative detection sensitivities were estimated using multiplier response functions re-
0
0.8 002 0.4 0.6 SCATTEREDION ENERGY/INCIDENT ENERGY
Fig. 3. Energy distributions MoS, basal plane surface.
1.0
for 0.5 and 2.0 keV “He+
scattering
along the (100)
direction
of the
SM.
Daols et (11./ Preferential
neutraltration
I
I
I
1
MoS2 BASAL PLANE
0.5
in LE helium ion scattering
I
1.5 1.0 PRIMARY ION ENERGY (KEV)
2.0
Fig. 4. Dependence of the MO/S LEISS intensity (peak area) ratio on icident ion energy for ‘He+ and 4He+ scattering from MoS, basal plane surfaces.
MoS2 BASAL PLANE
'HE+-
S
QHE+ -
MO
3H~+ -
S
3HEC --t MO
I
I
!
I
25 20 15 (SCATTERED ION VELOCITY)-l(ATOMICUNITS)
10
30
sites Fig. 5. Relative escape probabilities for ‘He+ and 4He+ scattering at sulfur and molybdenum along the (100) direction of the MoS, basal plane surface are shown as a function of the outgoing ion velocity (1 atomic unit = 2.187X 10' cm/s).
S. M. Daois et al. / Preferential
neutralization
in LE helium ion scattering
Ll7
ported by Burrous et al. [19]. The error bars indicated in fig. 5 arise from the increased uncertainty of the detection sensitivity at low scattered ion energies. Several features of fig. 5 are important. First, note that the ion escape probabilities for first and second layer scattering differ drastically. The escape probabilities for He+ scattering at molybdenum atomic sites vary logarithmically with l/1/: as predicted by eq. (l), whereas the first layer scattering does not display this simple dependence. Finally, for second layer scattering, there appears to be a small unexpected isotope effect. It is clear from fig. 5 that (1) preferential neutralization takes place upon scattering from the second, molybdenum surface layer, and (2) enhanced neutralization that accompanies scattering at molybdenum atomic sites is responsible for the marked variation of the MO/S LEISS intensity ratio with increasing He+ energy. Selective neutralization is especially pronounced at low incident ion energies (5 0.6 keV) where first layer scattering accounts for 90-95s of the total intensity. It should be noted, however, that for He+ energies greater than about 1 keV, first and second layer scattering both contribute strongly to the in-plane scattered intensity. These results compare favorably with dynamical calculations by Garrison [ 171 for 600 eV He+ , Ne+ , and Ar+ scattering along the [ 1 IO] direction of Ni( 110). Garrison indicates that most of the in-plane scattering originates from binary collisions with target atoms in the second “valley” surface layer. However, after including a collision time correction to account for preferential neutralization in second layer scattering, it is found that about 90% of the scattered intensity originates from first layer collisions. The magnitude of the neutralization constants determined from fig. 5 varies from 7.0 x 10’ cm/s (7.8 X 10’ cm/s) for 4He+ (3He’) scattering at molybdenum sites to (1.5-3) X 10’ cm/s for scattering at sulfur sites. These values are similar to those reported by Englert [8] (1.8 X 10’ cm/s), Buck [l] (3 X 10’ cm/s), and MacDonald [5] (5 x 10’ cm/s) and co-workers for He+ scattering from nickel surfaces. More detailed interpretation of the neutralization constants is rendered difficult since the apparent neutralization constant may contain velocity and target mass dependent contributions to collisional neutralization [3-51. Such collisional contributions might explain the non-logarithmic dependence of escape probability on 1/VI for scattering at sulfur sites and/or the observation of a small isotope effect for 3He’ (4He’) scattering from molybdenum sites ( PM,J3He)/P,J4He) = 1.1). If it is assumed that the energy dependence of the back-reflected yield is dominated by Auger neutralization along the outgoing trajectory, then /3M~/& 2 2.5 clearly appears to reflect the longer ion-solid interaction time for scattering in the second surface layer. Angular distribution studies would be valuable to separate the relative contributions of Auger and collisional neutralization. We are grateful to G.A. Somorjai and R.R. Chianelli for useful comments and for making the single crystal samples available to us.
LIX
SM.
Davis et al. / Preferential
neutralization
in LE helium ion scattering
References [I] [2] [3] [4] (51 [6] [7] (81 [9] [lo] [l I] [ 121 [ 131 [ 141 1151 [16] (171 [ 18) [ 191 1201
H.H. Brongersma and T.M. Buck, Nucl. Instr. Methods 132 (1976) 559. H.D. Hagstrum, Phys. Rev. 96 (1954) 336. B. Poelsema, L.K. Verheij and A.L. Boers, Surface Sci. 64 (1977) 537. S.H. Overbury, P.F. Dittner and S. Datz, Nucl. Instr. Methods 170 (1980) 543. R.J. MacDonald and P.J. Martin, Surface Sci. 111 (1981) L739. S.B. Luitjens, A.J. Algra, E.P.7h.M. Suurmeijer and A.L. Boers, Surface Sci. 99 (1980) 631. 652. D.P. Woodruff, Surface Sci. 116 (1982) L219. E.Taglauer, W. Englert, W. Heiland and D.P. Jackson, Phys. Rev. Letters 45 (1980) 740. D.P. Jackson, W. Heiland and E. Taglauer, Phys. Rev. B24 (1981) 4198. R.C. McCune, Appl. Surface Sci. 5 (1980) 275. M. Shelef, M.A.Z. Wheeler and H.C. Yao, Surface Sci. 47 (1975) 697. T.N. Taylor and W.P. Ellis, Surface Sci. 107 (1981) 249. R.C. McCune, O.E. Chelgren and M.A.Z. Wheeler, Surface Sci. 84 (1979) L5 15. C. Oshima, M. Aono, T. Tanaka, S. Kawai, S. Zaima and Y. Shibata, Surface Sci. 102 (1981) 312. R. De Gryse, J.P. Landuyt, A. Vermeire and J. Vennik, Appt. Surface Sci. 6 (1980) 430. SM. Davis, J.C. Carver and A. Weld, to be published. M. Salmeron, R.R. Chianelli, A, Wold and G.A. Somorjai, Chem. Phys. Letters, submitted. G.C. Nelson, Sandia Laboratory Report 79-0712 (1979). C.N. Burrous, A.J. Lieber and V.T. Zaviantseff, Rev. Sci. Instr. 38 (1967) 1477. B.J. Garrison, Surface Sci. 87 (1979) 683.
Surface Science 124 (1983) L12-L18 North-Holland Publishing Company
SURFACE
SCIENCE
LETTERS
PREFERENTIAL NEUTRALIZATION SCATTERING FROM THE SECOND SURFACES S.M. DAVIS
IN LOW ENERGY HELIUM ION LAYER OF MoS, BASAL PLANE
and J.C. CARVER
Exxon Research and Development
Luboratory,
P.O. Box 2226, Baton Rouge, Louisrana 70821, USA
and A. WOLD Department Received
of Chemrstty,
Brown University, Providence,
13 July 1982; accepted
for publication
Rhode Island, USA
12 October
1982
Low energy 3He+ and 4He+ scattering along the [IOO] direction of single crystal MoS, basal plane surfaces has been investigated. The layered atomic structure of this compound was used to demonstrate that preferential neutralization accompanies scattering from the second surface layer. Preferential neutralization that takes place upon scattering at molybdenum atomic sites leads to a marked increase in the MO/S LEISS intensity ratio with increasing primary ion energy (0.38-2 keV).
Neutralization processes have been identified as a decisive factor controlling the scattered yield in low energy ion scattering spectroscopy (LEISS). For helium and neon ion scattering from nickel or yttrium surfaces, Buck and Brongersma [l] have shown that these neutralization processes appear to be adequately described using models formulated by Hagstrum [2] for direct Auger neutralization or resonance neutralization followed by Auger de-excitation. In this case, the scattered intensity, I+, is related to the incident ion flux, (I, the detection sensitivity, T, the normal I,+ > the scattering cross-section, component of scattered ion velocity, VI , and a characteristic neutralization constant, p, by
Z+=Z,+oTexp(-p/V:).
(1)
The factor exp( -p/V:) represents the probability that a scattered ion will escape the influence of the solid surface without experiencing neutralization. This probability increases as the ion-solid interaction time decreases and as the distance of the ion from the surface increases [2]. A more general form for
0039-6028/83/0000-0000/$03.00 0 1983 North-Holland
S.M. Davis et al. / Preferential
the ion escape probability,
neutralization
m LE helium ion scattering
P’ , has been proposed
P+=&Y,(l-PN)+(l-cx,)P,],
by Poelsema
L13
et al. [3], i.e., (2)
with aI = exp( -P/V:),
a2 = exp( -P/v:),
which includes neutralization probabilities during the incident trajectory, (1 a,), the collision, P,, and the outgoing trajectory, (1 - a,), where Vi and V: are the normal components of incident and scattered ion velocity, respectively. Reionization of neutrals produced along the incoming trajectory may also take place during collision with probability P,. Recent angular distribution studies by Overbury [4] and MacDonald [5] and co-workers have revealed that up to 95-98s of the total neutralization may occur during the collision event. Moreover, for neon and argon ion scattering from Cu(lOO), the reionization process also appears to become important [6]. Recent calculations for Li+ and He+ scattering from Ni( 100) [7] and Ni( 110) [8,9] indicate that the neutralization probability is strongly dependent upon the trajectory of the scattered particle. While LEISS studies have now been reported for a variety of oxides, halides, and other binary compound surfaces [lo- 151, previous studies of the neutralization process have mostly concentrated on atomically clean metals. For these metals there exists little possibility of directly distinguishing differences in scattered yield and neutralization probability that arise from scattering in the first and second atomic surface layers. In this communication, we report studies of 3He+ and 4He+ scattering from single crystal MoS, basal plane surfaces for which the scattering from the first and second atomic layers is clearly distinct. The layered atomic structure of this transition metal compound has been used to demonstrate that preferential neutralization accompanies scattering from the second surface layer. The LEISS experiments were carried out in a Leybold-Heraeus UHV apparatus (base pressure < 2 x lo-” mbar) equipped with a differentially pumped O-5 keV ion gun and 180” hemispherical energy analyzer. The laboratory scattering angle and angle of incidence were fixed at 120° and 30°, respectively; i.e., scattered ions were detected at the surface normal. The analyzer was operated in a retarding factor mode with AE/E = 3. The ion gun produced a beam with an approximate spot area of 4 mm2 which was rastered over 4 mm in the direction normal to the incident ion flux. Single crystal MoS, platelets with surface areas of about 0.5 cm2 were prepared from the elements by chemical vapor transport at 1370- 1420 K using Br, as the transport agent. The single crystal surfaces were carefully cleaned using repeated cycles of brief helium or neon ion sputtering at 0.5-l keV followed by annealing one to several hours at 800-870 K until scattered intensities achieved reproducible maximum levels, and surface impurities were
S. M. Davis et al. / Preferential
Ll4
neutralization
in LE helium ion scattering
barely detectable (0, Na, and K were most tenacious). Photoemission, LEISS, and LEED studies reported separately [ 16,171 have revealed that this pretreatment produces a clean, stoichiometric, and well-ordered MoS, basal plane surface that displays no activity for oxygen chemisorption. Oxygen chemisorption is easily detected by LEGS when surface sulfide vacancies are present. Primary beam currents in the range 20-100 nA/cm2 were chosen in our studies to minimize surface sputtering while still providing reliable intensities. Even at these low currents, preferential sputtering of sulfur was apparent during a series of consecutive experiments. Fig. 1 displays the variation of the MO/S intensity ratio with total ion dose for the 4He+ scattering at 500 eV incident energy. A total ion dose as low as lOI cm-’ was adequate to produce significant changes in surface composition. The initial MO/S intensity ratio was restored upon annealing to 870 K. During intensity measurements, the samples were repeatedly annealed to insure that changes in the MO/S intensity ratio due to sputtering were minimized. The idealized atomic surface structure for the MoS,(OO 1) basal plane surface is diagrammed in fig. 2. The topmost layer consists of close packed sulfide anions with hexagonal atomic arrangement and d,_, = 3.15 A (bulk). The second layer is composed of a hexagonal array of molybdenum cations with trigonal prismatic coordination and dMo_S = 2.41 A (bulk). The [ 1001 direction parallels rows of sulfide anions and valleys of molybdenum cations that are displaced by the interlayer spacing of 1.59 A (bulk). Energy distributions for 4He+ scattered along the [loo] direction of the MoS, basal plane surface are compared at 0.5 and 2.0 keV incident energy in fig. 3. A marked increase in the MO/S LEISS intensity ratio is evident with
r
I
I
,
I
MoS2(100) 0.5 kEV
I
0
1.0
‘ttE+
I
2.0
I
3.0
4,O
ION DOSE (CM-* x 10-15)
Fig. 1. Variation of the MO/S LEISS intensity ratio with total ion dose for 4He+ scattering at 0.5 keV incident energy. Preferential sputtering of surface sulfur is evident for an ion dose as low as 10” cmm2.
S.M. Davis et al. / Preferentral neutralization
in LE helium ion scattering
0
Ll5
S (1ST LAYER)
0 No (ZND LAYER) 100 110 Y INTERLAYERSPACING1,59 ii
DNoS = 2.41 i Fig. 2. Idealized
atomic
surface
structure
for the MoS,(OOl)
basal plane surface.
increasing primary ion energy. The energy dependence of this intensity ratio is depicted more clearly in fig. 4 where data are compared for 3HeC and 4He+ scattering over the energy range 0.38-2.0 keV, in which case the intensity ratio varies by nearly two orders of magnitude. In fig. 5, relative ion escape probabilities for scattering at sulfur and molybdenum atomic sites have been calculated from experimental intensities using eq. (1) (e.g., P+ = Z'/Z,' UT) and plotted as a function of the reciprocal outgoing ion velocity (1 atomic unit = 4.571 X 10m9 s/cm). Scattering crosssections using the Moliere approximation to the Thomas-Fermi screening function in a screened Coulomb potential were calculated from the tables of Nelson [18]. Beam currents were measured with a Faraday cup, and relative detection sensitivities were estimated using multiplier response functions re-
0
0.8 002 0.4 0.6 SCATTEREDION ENERGY/INCIDENT ENERGY
Fig. 3. Energy distributions MoS, basal plane surface.
1.0
for 0.5 and 2.0 keV “He+
scattering
along the (100)
direction
of the
SM.
Daols et (11./ Preferential
neutraltration
I
I
I
1
MoS2 BASAL PLANE
0.5
in LE helium ion scattering
I
1.5 1.0 PRIMARY ION ENERGY (KEV)
2.0
Fig. 4. Dependence of the MO/S LEISS intensity (peak area) ratio on icident ion energy for ‘He+ and 4He+ scattering from MoS, basal plane surfaces.
MoS2 BASAL PLANE
'HE+-
S
QHE+ -
MO
3H~+ -
S
3HEC --t MO
I
I
!
I
25 20 15 (SCATTERED ION VELOCITY)-l(ATOMICUNITS)
10
30
sites Fig. 5. Relative escape probabilities for ‘He+ and 4He+ scattering at sulfur and molybdenum along the (100) direction of the MoS, basal plane surface are shown as a function of the outgoing ion velocity (1 atomic unit = 2.187X 10' cm/s).
S. M. Daois et al. / Preferential
neutralization
in LE helium ion scattering
Ll7
ported by Burrous et al. [19]. The error bars indicated in fig. 5 arise from the increased uncertainty of the detection sensitivity at low scattered ion energies. Several features of fig. 5 are important. First, note that the ion escape probabilities for first and second layer scattering differ drastically. The escape probabilities for He+ scattering at molybdenum atomic sites vary logarithmically with l/1/: as predicted by eq. (l), whereas the first layer scattering does not display this simple dependence. Finally, for second layer scattering, there appears to be a small unexpected isotope effect. It is clear from fig. 5 that (1) preferential neutralization takes place upon scattering from the second, molybdenum surface layer, and (2) enhanced neutralization that accompanies scattering at molybdenum atomic sites is responsible for the marked variation of the MO/S LEISS intensity ratio with increasing He+ energy. Selective neutralization is especially pronounced at low incident ion energies (5 0.6 keV) where first layer scattering accounts for 90-95s of the total intensity. It should be noted, however, that for He+ energies greater than about 1 keV, first and second layer scattering both contribute strongly to the in-plane scattered intensity. These results compare favorably with dynamical calculations by Garrison [ 171 for 600 eV He+ , Ne+ , and Ar+ scattering along the [ 1 IO] direction of Ni( 110). Garrison indicates that most of the in-plane scattering originates from binary collisions with target atoms in the second “valley” surface layer. However, after including a collision time correction to account for preferential neutralization in second layer scattering, it is found that about 90% of the scattered intensity originates from first layer collisions. The magnitude of the neutralization constants determined from fig. 5 varies from 7.0 x 10’ cm/s (7.8 X 10’ cm/s) for 4He+ (3He’) scattering at molybdenum sites to (1.5-3) X 10’ cm/s for scattering at sulfur sites. These values are similar to those reported by Englert [8] (1.8 X 10’ cm/s), Buck [l] (3 X 10’ cm/s), and MacDonald [5] (5 x 10’ cm/s) and co-workers for He+ scattering from nickel surfaces. More detailed interpretation of the neutralization constants is rendered difficult since the apparent neutralization constant may contain velocity and target mass dependent contributions to collisional neutralization [3-51. Such collisional contributions might explain the non-logarithmic dependence of escape probability on 1/VI for scattering at sulfur sites and/or the observation of a small isotope effect for 3He’ (4He’) scattering from molybdenum sites ( PM,J3He)/P,J4He) = 1.1). If it is assumed that the energy dependence of the back-reflected yield is dominated by Auger neutralization along the outgoing trajectory, then /3M~/& 2 2.5 clearly appears to reflect the longer ion-solid interaction time for scattering in the second surface layer. Angular distribution studies would be valuable to separate the relative contributions of Auger and collisional neutralization. We are grateful to G.A. Somorjai and R.R. Chianelli for useful comments and for making the single crystal samples available to us.
LIX
SM.
Davis et al. / Preferential
neutralization
in LE helium ion scattering
References [I] [2] [3] [4] (51 [6] [7] (81 [9] [lo] [l I] [ 121 [ 131 [ 141 1151 [16] (171 [ 18) [ 191 1201
H.H. Brongersma and T.M. Buck, Nucl. Instr. Methods 132 (1976) 559. H.D. Hagstrum, Phys. Rev. 96 (1954) 336. B. Poelsema, L.K. Verheij and A.L. Boers, Surface Sci. 64 (1977) 537. S.H. Overbury, P.F. Dittner and S. Datz, Nucl. Instr. Methods 170 (1980) 543. R.J. MacDonald and P.J. Martin, Surface Sci. 111 (1981) L739. S.B. Luitjens, A.J. Algra, E.P.7h.M. Suurmeijer and A.L. Boers, Surface Sci. 99 (1980) 631. 652. D.P. Woodruff, Surface Sci. 116 (1982) L219. E.Taglauer, W. Englert, W. Heiland and D.P. Jackson, Phys. Rev. Letters 45 (1980) 740. D.P. Jackson, W. Heiland and E. Taglauer, Phys. Rev. B24 (1981) 4198. R.C. McCune, Appl. Surface Sci. 5 (1980) 275. M. Shelef, M.A.Z. Wheeler and H.C. Yao, Surface Sci. 47 (1975) 697. T.N. Taylor and W.P. Ellis, Surface Sci. 107 (1981) 249. R.C. McCune, O.E. Chelgren and M.A.Z. Wheeler, Surface Sci. 84 (1979) L5 15. C. Oshima, M. Aono, T. Tanaka, S. Kawai, S. Zaima and Y. Shibata, Surface Sci. 102 (1981) 312. R. De Gryse, J.P. Landuyt, A. Vermeire and J. Vennik, Appt. Surface Sci. 6 (1980) 430. SM. Davis, J.C. Carver and A. Weld, to be published. M. Salmeron, R.R. Chianelli, A, Wold and G.A. Somorjai, Chem. Phys. Letters, submitted. G.C. Nelson, Sandia Laboratory Report 79-0712 (1979). C.N. Burrous, A.J. Lieber and V.T. Zaviantseff, Rev. Sci. Instr. 38 (1967) 1477. B.J. Garrison, Surface Sci. 87 (1979) 683.