- Email: [email protected]
Structure sensitivity of scattered Ne+ ion fractions from a Ni{100} surface
Surface Science 256 (1991) 77-86 North-Holland
77
Structure sensitivity of scattered Ne + ion fractions from a Ni{ 100} surface C.C. Hsu and J.W. Rabalais Department of Chemistry, University of 51ouston. Houston, TX 77204-5641, USA Received 23 February 1991; accepted for publication 9 April 1991
The incident, a, and azimuthal, 8, angle dependence of the sampling depth and scattered ion fractions for 4 and 8 keV Ne + scattering from a clean Ni{100} surface have been investigated. Time-of-flight techniques are used for velocity analysi~ of both neutrals (N) and ions (I). Spectra of N + 1 and N (obtained by electrostatic deflection of I) allow determination of scattered N e " ion fractions as Y = [(N + !) - N ] / ( N + I). The results show that (i) the ion frac;' ns exhibit a strong d e p e n d e n c e on a and 8, (ii) the critical incident angle % used in strut.ture determinationa is dependent on the type of scattering spectra collected, i.e., N + I or 1 only, and (iii) layer-specific information can be obtained by a judicious choice of ot and 8.
1. Introduction The use of low-energy ( < 10 keV) ions in surface analysis is becoming increasingly important due to the extreme surface sensitivity and selectivity obtainable. These properties are duc to two factors: (i) The energies of penetrating ions are rapidly attenuated due to the large collision cross sections. (ii) Ions penetrating into subsurface layers are more efficiently neutralized than those scattering from the first atomic layer. The kinematics of (i) are accurately described by classical mechanics [1]. Although much progress has been made in understanding the dynamic charge transfer process in ion-surface collisions [2-9]. a quantitative model of the electronic transitions in (ii) is not available. Theoretical models [10-13] of ion-surface neutralization using time-dependent perturbanon methods have improved, however. they are usually complex and applied to highly simplified s'vstems. As a rc'_;ult, it is important to obtain ion- surface neutralization da~a on wel~-defined systems in order to enhance our understanding of the charge exchange mechanism. The consequences of this charge exchange are particularly important in low-energy ion scattering ,';pectrometry where two different methods are used
for analysis of the scattered particles. In one case an electrostatic analyzer (ESA) [14] is used for energy analysis of the charged particles and in the other case time-of-flight (TOF) techniques [15] are used for vch.)cit\' analysis of bo',h '.he charged and neutral particles. Ion scattering spectrometry i:, used mainly for s~rfao elemental and structural analy.',;s. When only the chargeC particles are analyzed, it is difficult t~> dis~il~guish between intensity changes caused by the elemental composition of the target, structural effects such as shadowing arid blocking, and neutralization effects. Charge exchange is also important in secondary ion mass spectrometry (SIMS) where the abundances of sputtered ions are modulated by the probability of the escaping particle being in a charged state. Most of the experimental data on ion-surface charge exchange has been imerpre~ted in :crms ~)f a model which separa,es the ion ~rajec~orv into three segments, i.e.. ~he incoming ~raiectory. the dose encoumcr, and ti~c ou~goip.g ,rajector'~.~ . . .,~.i~,a,,, ,~ the incoming and outgoing trajectori~. charge exchange is believed to involve the valence cr conduction band electrons in resonant and Auger transitions at distances of "angstr6ms between the projectile and surface atoms, t h e modds [ 6] usu-
0039-6028/ 91/$03.50 ' 1991 - Elsevier Science Publishers B.V. AI tights reserved
78
C.C. Hsu, J. W. Rabalais / Structure sensitivit7 of scattered Ne + ion fractions from Ni{lO0}
ally involve an ion survival probability, P, of the form P = exp( -,'~./,, j_ ).
( 1)
where v is a characteristic velocity and vl is the component of velocity normal to the surface (in versely proportional to the time spent near the surface). The constant o~ has been found to be of the order of 106-107 c m / s [2-9]. In the close encounter, the projectile approaches to within tenths of an 'angstrOm of a surface atom and inner shell electron orbitals overlap. Electrons can be promoted [6] to excited and autoionizing states as a result of this overlap. The dynamical evolution of electronic states along this time-dependent trajectory is a complicated phenomenon. In this paper we present the incident and azimuthal angle dependence of the sampling depth and scattered ion fractions for Ne + scattering trom a Ni{100} surface. The purpose is to examine (i) the possibility of obtaining layer-specific information and (ii) the effect of collecting spectra of neutrals plus ions or ions only on the critical incident angles % used in structural analysis. Surface interatomic spacings are often measured by monitoring backscattering intensity as the incident angle ~ between the beam and surface is varied [17]. As ~ is increased, atoms move om of the shadow cones of their neighbors resulting in sharp rises in intensit\' which define ~L. Interatomic spacings are calculated by using % and the shape of the shadow cone. The results obtained herein show that it is possible to obtain layerspecific data and that the scattered ion fractions have significant incident and azimuthal angular dependm!ces (even for a clean metal surface) which do effect the % values.
2. Experimental A derailed description of ti,..-of-flight scattering and recoi~itlg spectnm,etry (T()F-SARS) has been Desented elsewhere [15]. The experimental parameters used herein at.: 4 and 8 keV Ne + primary ions from a Colutron source, ion pulse width ~ 30 us, pulse rate = 30 kHz, and average currc~t 3c~sit\ = 1 hA/ram:. Spectra of neutrals
plus ions (N + I) and neutrals only (N) were collected by electrostatic deflection of the ions and ion fractions (Y) were calculated as Y = [(N + I) - N ] / ( N + I). Typical base pressures of 3 × 10-~o Torr were obtained after baking the chamber. The Ni(100} crystal was prepared by polishing with consecutively finer diamond pastes, with the final polishing using 0.05/.tm alumina. The!crystal was adjusted by laser reflection such that the uncertainty in a over an azimuthal rotation of 135 ° was < 0.3 °. The surface was cleaned in the UHV chamber by several cycles of 2 keV Ar + sputtering at ---500°C followed by annealing at - - 6 0 0 ° C . No contaminants were detectable by either T O F direct recoiling or Auger electron spectroscopy (using LEED optics). A sharp (1 x 1)Ni(100} LEED pattern was obtained after annealing the clean surface. This pattern was used for an approximate azimuthal alignment of the crystal. An azimuthal alignment to < +_1 ° was ootained from the azimuthal pat,-'rn of 4 keV Ne + scattering.
3. Ezperimental results
3.1. Time-of-flight spectra An example of T O F spectra obtained from 8 keV Ne + scattering from Ni{100} along tile (100> azimuth is shown in fig. 1. Spectra of neutrals plus ions (N + I), neutrals only (N), and ions only (I) are presented. The peaks correspond to Ne ° and Ne + quasi single scattering (SS) from Ni, i.e., the projectile experiences one large angle deflection which may be preceded or followed by minor deflections. The T O F peaks are identified by application of the classical binary elastic collision model. The positions cf the peaks are independent of the incident o~ and azimuthal 8 orientations of the sample and very close to the T O F predicted by the binary elastic collision model, indicating that the major contributicm to these peaks is SS. The broad features at lower and higher T O F on both sides of the peak correspond to multiple scattering (MS) from two or more Ni atoms. The I spectrum contains much less MS contributions and, as a result, is significantly sharper than either the N + 1
C.C. Hsu, J. I44 Rabalais / Structure sensitivity of scattered Ne + ion fractions from Ni { 1O0 } ,
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Time of F'ight (fzs) Fig. 1. Example of TOF spectra for 8 keV Ne + scattering from a clean Ni{100} surface along the (100) azimuth with scattering angle O = 1 6 6 ° and incident angle t~=18 °. ( N + I ) = neutrals pros ions, ( N ) = neutrals only and ( I ) = ions only (obtained by subtraction of (N + I ) - ( N ) ) . A schematic of the Ni{100} surface is shown in the inset, where the solid circle.,, represent first-layer atoms and the open circles represent second-layer atoms.
or N spectra. This decreased MS contribution to the I spectrum is due to the higher neutralization probability of ions in MS sequences. The bcattcring intensities f(S) wore determined as the integrated counts in a 0.5 ~s window centered at the SS peak maxim'.lm folMwing background subtraction.
79
through the neighboring atoms; since ion trajectories are focused at the edges of the cones, large enhancements in I(S) are observed at the critic,d angle % where the cone edge and atomic core coincide. The peaks of figs. 2 ar~ 3 can be identified by the interpretative illustrations in the insets of fig. 3. These insets contain views of the atomic arrangements in a plane perpendicular to the surface plane and containing the azimuth indicated. The shadowing and scattering atoms are connected by lines and labeled by letters corresponding to the letters associated with the peaks in the a scans. The a~s indicated in the insets are calculated from the angle of inclination of the shadowing-scattering atom pair with the surface and the radius of the shadow cone at the appropriate distance behind the shadowing atom. Some of the outgoing trajectories at {9 = 166 ° from subsurface atoms are blocked by atoms in
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3.2. Incident an;de, ~, scans 3.2.1. Total newral plus ion intensities versus
Scattering i~ltensities I(S) at 0 = 166 ° are plotted as a function of incident angle o~ along diH'erent crystal azimuths 8 for 4 and 8 keV N e ' ~n figs. 2 and 3, respectively. Fo,- ,,uch large @, I(S) is determined b\,' the ability of incident ions to make neariy head-on collisions with the Ni atoms. At small o~, I(S) is low because atoms lie within the shadow cones of their nearest neighbors and direct colli,';ions are not possible. As ~ increases, the edge of the shadow cone moves
O
2O 4 0 60 80 INCIDENT ANGLE a
Fig. 2. IN + ]) scattering intensity, I(S). versus inodent angle. a,, for Ni{lO0} using 4 keV Ne* at O = 1 6 6 ° along three different azimuths 8. The ~ positions are ipdicaled by circles. The letters identify peaks corresponding to the inset of fig. 3. The dashed curves represent data from ref. [14] for the same system using an electrostatic analyzer to collect only ions.
80
/ Structure sensitivity of scattered N e + ion fractions from N i { l O 0 }
C.C. hrsu, J. W. Rabalais
layers above them; such cases are not included in the insets of fig. 3. The peak intensities vary considerably depending, on the degree of shadowing, blocking, or focusing for specific trajectories. Two important observations can be made from figs. 2 and 3. First, the a scans at 4 keV contain fewer peaks and are simpler than those at 8 keV. This is a result of the -- 15% larger shadow cone radius at 4 keV compared to 8 keV as will be shown below. The smaller shadowing and block-
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Fig. 3. (N + ]~) scattering intensity, I(S), versus incident angle, ~, for Ni{100} using 8 key Ne + at 6)= 166 ° a|ong four different azimuths ?,. The ~L: positions are indicated by circles. The insets illustrate the s h a d o w i n g - s c a u e r i n g atom pairs in the surface and subsurface hyers of the plane containing the azimuth. Within the insets are shown, the layer numbering, the critical angles, a~, and the labels corresponding to the peaks in the ,~ scans. Solid circles: atoms in odd-numbered layers; open circles: atoms in even-numbered layers.
C C Hsu, J. W. Rabalais / Structure sens#ivity of scattered Ne + ion fractions from Ni {100}
The three a scans at 4 keV in fig. 2 can be compared directly to those of Fauster and Metzner ( F & M ) [14] for the same system in which only the scattered ions were collected using an electrostatic analyzer (ESA). Due to preferential neutralization of ions scattering from subsurface layers, the F & M scans exhibit only low intensity, broad peaks above a = 40 ° where the subsurface peaks of fig. 2 are clearly observed. The s!~.dow cones were computed from the Ziegler-Biersack-Littmark potential [18]. The sealing factors, C, for the potential screening length were adjusted [19] to provide the best fit between the calculated cone radii, R, and experimentally measured radii at distances L behind the target atoms. Experimental measurements of a~s from first-layer shadowing and scattering pairs along different azimuths were used for this calibration. The optimum fit for 4 and 8 keV was obtained with C = 0.81 and 0.94, respectively. The optimized cones along with the experimental points are shown in fig. 4. For L > 2 A, the difference in R for the two cones is nearly constant at - 0.1 ,~.
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81
3.2.2. Total (N + I) and component N and I intensities versus a
Scattering intensities for N + I, N, and I were collected as shown in fig. 1 for a values corresponding to the first peak of the a scans of fig. 3. The results are plotted in fig. 5 and the acs are listed in table 1. As the interatomic spacings increase, the a~s decrease as expected. The a~ values for the N + I and I scans for each ~ i m u t h are within experimental error, i.e., < 0.3 °, with the exception of the (100) azimuth where a~I is 1 . 8 ° larger than a~tN +i) . These results were reproduced several times and are well outside of the experimental uncertainty of the measurements. 3.2. 3. Scattered ion fractions versus a
The scattered ion fractions Y determined from the T O F spectra of the a scans of fig. 5 are plotted in fig. 6. The Y values and their behavior differ along the various azimuths, yore> is lowest due to the short interatomic spacings and the simultaneous scattering from both first and secend layers, y(m0> is high because scattering results from only the first layer in the low c~ range: the value increases with increasing a up to a maximum of Y = 32% at o~ = 21 °. This increase of Y with o~ is expected from the decreased ionsurface neutralization probability as the velocity of the particle perpendicular to the surface ~,.... increases. ) ' ~ ~ I decreases above ~ ~ 21 ° due to the onset of second-layer scattering (fig. 3). Intermediate ion fractions are obtained along (310) and (210). y ( 3 1 0 ) iS enhanced by the long interatomic spacings, but is reduced by the combination of both first- and second-layer scattering that is accessible at all o~. y<2m> is also enhanced by the long interatomic spacings; it increases in the range a = 7 o 12 ° due to the increase in ~'a, however it is reduced by the accessibility of secondlayer scattering beginning at a = t3 ° 3.3. Az~mudmt angle 6 scans 3.3.1. Scattering inlensiaes versus 8
Total N + 1I scattering intensities l(S) as a function of azimuthal angle 8 are plotted in t~g. 7 for different incident angles a. Beginning at low o, the o~ = 20.5 ° scan is dominated by iptense max-
C.C. ttsu, J. 14/'. Rabalais / Structure sensitwity o/scattered Ne + ion fractions front Ni (100}
82
ima at (110) with minor maxima near (310). Three factors contribute to the high intensity at (110): (i) The a ,alue corresponds to the maximum in the a scan of fig. 3. (ii) This azimuth presents the highest atomic density, e.g., there are twice as many atoms exposed to the beam compared to the (100) direction. (iii) The incident and scattered ions are focused by the (110) surface semichannels, i.e., the first and second atomic layers serve as "walls" and "base", respectively,
Table 1 Critical incident angle, a,., values for 8 keV Ne + scattering from Ni{ 100} along different azimuths Azinmth
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18.3 ° 15.3 °
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C.C. Hsu, J. IV. Rabalais ,I Structure sensith~ity of scattered Ne + ion fractions from Ni {100)
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Fig. 6. Scattered ion fractions, Y = [IN + | ) - N ] / ( N + II, versus incident angle, 0~, from the data of fig. 5. An example error bar is shown for one of the points. (100> = closed circles; {210} = open circles: (310> = open triangles; (110> = closed triangles.
Fi+, 8. Scattering intensity, IIS), for ions only tl) versus azimuthal angle, 8, for two different incidem angles, ~.
to guide the trajectories. It has been shown [15] that such semichanneling can produce sharp spatial anisotropy in the I(S) distributions. At (310),
only factor (iii) contributes to enhancement +~f intensity. As a increases, the ma-,ima at (I10) shift towards the <210> and <310> positions. This is due to the accessibility of sub,,urface }aver scattering along these azimuths (fi~:. 3) and the k ~ s s o f t h e e n h a n c e m e n l factors(i} ~;,!!iii) above as ~ increases. The scattering intensities I(S) of the ion component I are plotted in fig. 8 as a function of 8 for two of the a values shown in fig. 7. At low incidence (c~ = 20.5 o ), both the N + ~ and [ intensity patterns are similar with peaks along <110). At high incidence (~ = 4 5 °), the ~ intens, ty is independent of 8 while the N + ~ intensity exhibits a strong 8 dependence. This ]~>,,,, of 8 sensitivity in the ion intensi,'v at high a results from the
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C.C. Hsu, J. W. Rabalais / Structure sensitivity of scattered Ne + ion fractions from Ni {100}
84
3.3.2. Scattered ion fractions versus The scattered ion fractions Y are plotted as a function of 8 in fig. 9 for the same o~ values used in fig. 7. At low a, maxima are observed at <100> and (210) due to exclusively first-layer scattering and long interatomic spacings. Minima are observed at (110) and <310> due to a combination of first- and second-layer scattering. As a increases, the maximum at (100> dominates because subsurface-layer scattering becomes accessible for the other azimuths. When a = 50 o this condition no longer persists because subsurface-layer scattering is accessible along (100). For a = 55 °, subsurface scattering is readily accessible in all directions restdting in low Y and weak azimuthal dependence. The large variations in the scattered ion fractions Y (,from -- 2 to 40%) as a function of 8 (fig. 9) are mainly due to the modulation of surface and subsurface scattering as 8 is scanned. The Y values are controlled by charge exchange along the outgoing trajectories since the distances of closest approach at these energies and scattering angle are
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INCIDENT ANGLE Fig. 10. Plots of scattered ion fraction,., Y, and Y / P , where P is the ion survival probability from eq. (1), versus incident angle, a, for the (100> (circles) aria (210> (triangles) azimuths. An example error bar is shown for one of the points.
well within the threshold for reionization of Ne, as will be detailed in section 4.2. This can be demonstrated along the (100) and (210) azimuths where pure first-layer scattering is observed up to a = 23 ° and c ~ 12 °. respectively. Plotting eq. (1) as Y/exp(-vJv±) versus a with v,: = 0.60 x 107 and t.5 x 107 c m / s for the <210) and (100> azimuths, respectively, produces a constant that is independent of a as shown in fig. 10. This simple treatment is not possible along other azimuths due to combined multilayer scattering. The difference in a, for the two azimuths may arise from the dependence of the neutralization rate on interatomic spacings.
q. Discussion
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t:::i,D. o. So:tiered ion fractions, )', versus azimuthal angle 8, for
the four incident angles of fig. 7.
4. !. Effect of neutralization on ~,
!on neutralization during scattering can affect the ~ values in two ways. First, since there is a very high neutralization probability for subsurface scattering, I(S) versus a scans obtained by collec-
C.C. Hsu, J. W. Rabalais / Structure sensitwi O, of scatwre, ,¢e + ion fractions from Ni [ lO0}
tion of only ions exhibit extremely low intensity peaks at high a. It is sometimes difficult to even observe these high o~ peaks and it is usually not possible to extract accurate a~ values for secondand third-layer scattering from such scans. A good example of this is shown in fig. 2. Second, variations in the scattered ion fractions Y as a function of a can cause shifts in the a~ positions if oaly the ions are collected. Considering the second factor above, i.e., the variation of Y with a, tabl~ 1 shows that the % values determined from N + I and I only are within the experimental uncertainty of the measurement for all azimuths except (100). Fig. 6 shows that the highest Y values are obtained along (100) and that Y increases by 100% in the critical low-a region. This large increase in Y results in a shift ~f the a~ value for the I scan to 1.8 ° higher than that of the N + I scan. No such shifts are observed for the other azimuths because the Y values do not change by such large amounts in the critical low-c~ region. These results indicate that the % values obtained from measurements of I are sensitive to Y and that the structures de~ termined from them carry a corresponding uncertainty. For example, the 1.8 ° shift in % along (100) translates to a decrease in the interatomic spacings of - 0 . 4 ,~. This artifact is not encountered when N + 1 measurements arc used.
4.2. Mechanism for production of large Ne + ion fractions The mechanism for production of large Ne + ion fractions in scattering from light metal surfaces has been identified [6]. It involves the evolution of molecular orbitals (MOs) of a transient diatomic molecule formed from the overlap of inner shell atomic orbitals (AOs) of the individual atoms [20]. Due to the availability of electronic sta~e~ a., coastrained by the Pauti principle and symmetr 5 ru~c~, electr3ns can be promoted into excited and autoionizing states. As a result, ions that are neutralized along the incoming trajectory can be efficiently rdonized in the close encounter if the distance of closest approach ( d ) is small enough to allow inner shell electronic orbitals to overlap.
85
For t: e case of Ne + and Ni, the radii of the N e L and N i M shells are both --0.3 A. For 4 and 8 keV Ne + scattering from Ni at O = 166 °, d - 0.27 and 0.19 A, respectively. For such close approaches, two N e 2 p electrons can be promoted into the highly excited 4fo MO, which leads to formation of the doubly excited autoionizing electronic state, N e * * (2p43s2), as the atoms recede from each other [61. Due to the long lifetime (--- 1.5 x 10-~4 s) of this state, autoionization to Ne+(2p s) occurs along the exit trajectory when the ion is away from the surface and the neutralization probability is low. As a result, scattering from the surface layer yields high ion fractions. Scattering from subsurface layers yields low ion fractions because autoionization occurs along the exit trajectory before the N e * * can escaoe the lattice. o
5. Conclusions Collection of scattered neutrals plus ions b\, TOF techniques as a func~_ior~ oi inciden~ ~, and azimuthal, 8, angles allows one to sample the surface as well +~,, specific subsurface layers do'+ r, to at toast nine atomic laye~+~. Thi> i~tu~uatc~ tt;c utility of T O F scattering techniques for obtaining layer-specific information about interfaces and very thin films. The scattered Ne + ion fractions are large (up to - 4 0 % ) and have a strong ~ and 8 dependence, even on a clean metal surface such as Ni. For such cases, scattering intensity versus ~x or 8 plots, which are used heavily in structural analysis, can be distorted by e|ectronic effects when only ions are collected rather than neutrals plus ions. Such an effect has been found in ~he determination of the c~iticai incideat angles ~ along various azimuths. The behavior of the ion fra,:~ion~ i~ consistent with a model in which Ne is excited into autoionizing states during the close atomic encounter which decay along the outgoing trajectory. The surviving ion fraction depends heavily on the neutralization probability of ~hese escaping nascent ions.
86
C.C. Hsu. J. ~1":Rahalais / Structure sen,~itieiO' of scattered Ne + ion fractions from Ni{lO0}
Acknowledgements This material is based on work supported by the National Science Foundation under Grant No. DMR-8914608. The authors thank H. Bu and R. Trehan for assistance with the experiments and interpretations.
I81 T.M. Buck, G.H. Wheatlev and L.K. Verheij, Surf. Sci. 90 (1979) 635.
L'--'l I.S.T. Tson 8, N.H. Tolk, T.M. Buck, J.S. Krano, T.R. Pian and R. Kelly, Nucl. Instrum. Methods 194 (1982) 655.
11Ol W. Bloss and D. Hone, Surf. Sci. 72 (1978) 277. 1111 S. Muda and T. Hanawa, Surf. Sci. 97 (1980) 283. [121 R. Hentschke, K.J. Snowdon, P. Hertel and W. Heiland,
[13]
References [1] E.A. Mashkova and V.A. Molchanov, Medium Energy Ion Reflection From Solids (North-Holland, Amsterdam, 1985). [2] D..I. O'Connor, Y.G. Shen, J.M. ~'ilson and R.J. MacDonald, Surf. Sci. 197 (1988) 277; R.J. MacDonald, D.J. O'Connor, J.M. Wilson and Y.G. Shen, Nuci. Instrum. Methods B 33 (1988) 446. [3] A. N~rmann, H. Derks, W. Heiland, R. Monreal, E. Goldberg and F. Flores, Surf. Sci. 217 (1989) 255. [4] M.L. Yu and B.N. Eldridge, Phys. Rev. B 42 (1990) 1000. [5] R. Souda, T. Aizawa, W. Hayami, S. Otani and Y. lshizawa, Phys. Rev. B 42 (1990) 7761. [6] O. Grizzi, M. Shi, H. Bu, J.W. Rabalais and R.A. Baragiola, Phys. Rev. B 41 (!$,,0)4789. [7] F,. Faglauer, W. Engiert, W. Heiland and D.P. Jackson, Phvs. Rex,. Lett. 45 (lO~0~ 740.
[14] [151 [16]
[17] [18] [19] [20]
Surf. Sci. 173 (1986) 565; K.J. Snowdon, R. Hentschke, A. N~irmann and W. Heiland, Surf. Sci. 173 (1986) 581. A. Narmann, R. Monreal, P.M. Echenique, F. Flores, W. Heiland and S. Schubert, "vs. Rev. Lett. 64 (1990) 1601. Th. Fauster and M.H. Met,,ner, Surf. Sci. 166 (1986) 29. O. Grizzi, M. Shi, H. Bu and J.W. Rabalais, Rev. Sci. Instrum. 61 (1990) 740. H.D. Hags/rum, in: Inelastic Ion-Surface Collisions, Eds. N.H. T o l l W. Heiland and C.W. White (Plenum, New York, 1977). J.W. Rabalais, Science 250 (1990) 521. J F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids (Pergamon, New York, 1985). O. Grizzi, M. Shi, H. Bu, J.W. Rabalais and P. Hochmann, Phys. Rev. B 40 (1989) 10127. ,i.C. Brenot, D. Dhuicq, J.P. Gauyacq, J. Pommier, V. Sidis, M. Barat and E. Pollack, Phys. Rev. A 11 (1975) 1933.
77
Structure sensitivity of scattered Ne + ion fractions from a Ni{ 100} surface C.C. Hsu and J.W. Rabalais Department of Chemistry, University of 51ouston. Houston, TX 77204-5641, USA Received 23 February 1991; accepted for publication 9 April 1991
The incident, a, and azimuthal, 8, angle dependence of the sampling depth and scattered ion fractions for 4 and 8 keV Ne + scattering from a clean Ni{100} surface have been investigated. Time-of-flight techniques are used for velocity analysi~ of both neutrals (N) and ions (I). Spectra of N + 1 and N (obtained by electrostatic deflection of I) allow determination of scattered N e " ion fractions as Y = [(N + !) - N ] / ( N + I). The results show that (i) the ion frac;' ns exhibit a strong d e p e n d e n c e on a and 8, (ii) the critical incident angle % used in strut.ture determinationa is dependent on the type of scattering spectra collected, i.e., N + I or 1 only, and (iii) layer-specific information can be obtained by a judicious choice of ot and 8.
1. Introduction The use of low-energy ( < 10 keV) ions in surface analysis is becoming increasingly important due to the extreme surface sensitivity and selectivity obtainable. These properties are duc to two factors: (i) The energies of penetrating ions are rapidly attenuated due to the large collision cross sections. (ii) Ions penetrating into subsurface layers are more efficiently neutralized than those scattering from the first atomic layer. The kinematics of (i) are accurately described by classical mechanics [1]. Although much progress has been made in understanding the dynamic charge transfer process in ion-surface collisions [2-9]. a quantitative model of the electronic transitions in (ii) is not available. Theoretical models [10-13] of ion-surface neutralization using time-dependent perturbanon methods have improved, however. they are usually complex and applied to highly simplified s'vstems. As a rc'_;ult, it is important to obtain ion- surface neutralization da~a on wel~-defined systems in order to enhance our understanding of the charge exchange mechanism. The consequences of this charge exchange are particularly important in low-energy ion scattering ,';pectrometry where two different methods are used
for analysis of the scattered particles. In one case an electrostatic analyzer (ESA) [14] is used for energy analysis of the charged particles and in the other case time-of-flight (TOF) techniques [15] are used for vch.)cit\' analysis of bo',h '.he charged and neutral particles. Ion scattering spectrometry i:, used mainly for s~rfao elemental and structural analy.',;s. When only the chargeC particles are analyzed, it is difficult t~> dis~il~guish between intensity changes caused by the elemental composition of the target, structural effects such as shadowing arid blocking, and neutralization effects. Charge exchange is also important in secondary ion mass spectrometry (SIMS) where the abundances of sputtered ions are modulated by the probability of the escaping particle being in a charged state. Most of the experimental data on ion-surface charge exchange has been imerpre~ted in :crms ~)f a model which separa,es the ion ~rajec~orv into three segments, i.e.. ~he incoming ~raiectory. the dose encoumcr, and ti~c ou~goip.g ,rajector'~.~ . . .,~.i~,a,,, ,~ the incoming and outgoing trajectori~. charge exchange is believed to involve the valence cr conduction band electrons in resonant and Auger transitions at distances of "angstr6ms between the projectile and surface atoms, t h e modds [ 6] usu-
0039-6028/ 91/$03.50 ' 1991 - Elsevier Science Publishers B.V. AI tights reserved
78
C.C. Hsu, J. W. Rabalais / Structure sensitivit7 of scattered Ne + ion fractions from Ni{lO0}
ally involve an ion survival probability, P, of the form P = exp( -,'~./,, j_ ).
( 1)
where v is a characteristic velocity and vl is the component of velocity normal to the surface (in versely proportional to the time spent near the surface). The constant o~ has been found to be of the order of 106-107 c m / s [2-9]. In the close encounter, the projectile approaches to within tenths of an 'angstrOm of a surface atom and inner shell electron orbitals overlap. Electrons can be promoted [6] to excited and autoionizing states as a result of this overlap. The dynamical evolution of electronic states along this time-dependent trajectory is a complicated phenomenon. In this paper we present the incident and azimuthal angle dependence of the sampling depth and scattered ion fractions for Ne + scattering trom a Ni{100} surface. The purpose is to examine (i) the possibility of obtaining layer-specific information and (ii) the effect of collecting spectra of neutrals plus ions or ions only on the critical incident angles % used in structural analysis. Surface interatomic spacings are often measured by monitoring backscattering intensity as the incident angle ~ between the beam and surface is varied [17]. As ~ is increased, atoms move om of the shadow cones of their neighbors resulting in sharp rises in intensit\' which define ~L. Interatomic spacings are calculated by using % and the shape of the shadow cone. The results obtained herein show that it is possible to obtain layerspecific data and that the scattered ion fractions have significant incident and azimuthal angular dependm!ces (even for a clean metal surface) which do effect the % values.
2. Experimental A derailed description of ti,..-of-flight scattering and recoi~itlg spectnm,etry (T()F-SARS) has been Desented elsewhere [15]. The experimental parameters used herein at.: 4 and 8 keV Ne + primary ions from a Colutron source, ion pulse width ~ 30 us, pulse rate = 30 kHz, and average currc~t 3c~sit\ = 1 hA/ram:. Spectra of neutrals
plus ions (N + I) and neutrals only (N) were collected by electrostatic deflection of the ions and ion fractions (Y) were calculated as Y = [(N + I) - N ] / ( N + I). Typical base pressures of 3 × 10-~o Torr were obtained after baking the chamber. The Ni(100} crystal was prepared by polishing with consecutively finer diamond pastes, with the final polishing using 0.05/.tm alumina. The!crystal was adjusted by laser reflection such that the uncertainty in a over an azimuthal rotation of 135 ° was < 0.3 °. The surface was cleaned in the UHV chamber by several cycles of 2 keV Ar + sputtering at ---500°C followed by annealing at - - 6 0 0 ° C . No contaminants were detectable by either T O F direct recoiling or Auger electron spectroscopy (using LEED optics). A sharp (1 x 1)Ni(100} LEED pattern was obtained after annealing the clean surface. This pattern was used for an approximate azimuthal alignment of the crystal. An azimuthal alignment to < +_1 ° was ootained from the azimuthal pat,-'rn of 4 keV Ne + scattering.
3. Ezperimental results
3.1. Time-of-flight spectra An example of T O F spectra obtained from 8 keV Ne + scattering from Ni{100} along tile (100> azimuth is shown in fig. 1. Spectra of neutrals plus ions (N + I), neutrals only (N), and ions only (I) are presented. The peaks correspond to Ne ° and Ne + quasi single scattering (SS) from Ni, i.e., the projectile experiences one large angle deflection which may be preceded or followed by minor deflections. The T O F peaks are identified by application of the classical binary elastic collision model. The positions cf the peaks are independent of the incident o~ and azimuthal 8 orientations of the sample and very close to the T O F predicted by the binary elastic collision model, indicating that the major contributicm to these peaks is SS. The broad features at lower and higher T O F on both sides of the peak correspond to multiple scattering (MS) from two or more Ni atoms. The I spectrum contains much less MS contributions and, as a result, is significantly sharper than either the N + 1
C.C. Hsu, J. I44 Rabalais / Structure sensitivity of scattered Ne + ion fractions from Ni { 1O0 } ,
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Time of F'ight (fzs) Fig. 1. Example of TOF spectra for 8 keV Ne + scattering from a clean Ni{100} surface along the (100) azimuth with scattering angle O = 1 6 6 ° and incident angle t~=18 °. ( N + I ) = neutrals pros ions, ( N ) = neutrals only and ( I ) = ions only (obtained by subtraction of (N + I ) - ( N ) ) . A schematic of the Ni{100} surface is shown in the inset, where the solid circle.,, represent first-layer atoms and the open circles represent second-layer atoms.
or N spectra. This decreased MS contribution to the I spectrum is due to the higher neutralization probability of ions in MS sequences. The bcattcring intensities f(S) wore determined as the integrated counts in a 0.5 ~s window centered at the SS peak maxim'.lm folMwing background subtraction.
79
through the neighboring atoms; since ion trajectories are focused at the edges of the cones, large enhancements in I(S) are observed at the critic,d angle % where the cone edge and atomic core coincide. The peaks of figs. 2 ar~ 3 can be identified by the interpretative illustrations in the insets of fig. 3. These insets contain views of the atomic arrangements in a plane perpendicular to the surface plane and containing the azimuth indicated. The shadowing and scattering atoms are connected by lines and labeled by letters corresponding to the letters associated with the peaks in the a scans. The a~s indicated in the insets are calculated from the angle of inclination of the shadowing-scattering atom pair with the surface and the radius of the shadow cone at the appropriate distance behind the shadowing atom. Some of the outgoing trajectories at {9 = 166 ° from subsurface atoms are blocked by atoms in
Ne÷--Ni{lO0}'
~,
1
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2.~ I.M I-Z ILl .__1 I.xJ PC
3.2. Incident an;de, ~, scans 3.2.1. Total newral plus ion intensities versus
Scattering i~ltensities I(S) at 0 = 166 ° are plotted as a function of incident angle o~ along diH'erent crystal azimuths 8 for 4 and 8 keV N e ' ~n figs. 2 and 3, respectively. Fo,- ,,uch large @, I(S) is determined b\,' the ability of incident ions to make neariy head-on collisions with the Ni atoms. At small o~, I(S) is low because atoms lie within the shadow cones of their nearest neighbors and direct colli,';ions are not possible. As ~ increases, the edge of the shadow cone moves
O
2O 4 0 60 80 INCIDENT ANGLE a
Fig. 2. IN + ]) scattering intensity, I(S). versus inodent angle. a,, for Ni{lO0} using 4 keV Ne* at O = 1 6 6 ° along three different azimuths 8. The ~ positions are ipdicaled by circles. The letters identify peaks corresponding to the inset of fig. 3. The dashed curves represent data from ref. [14] for the same system using an electrostatic analyzer to collect only ions.
80
/ Structure sensitivity of scattered N e + ion fractions from N i { l O 0 }
C.C. hrsu, J. W. Rabalais
layers above them; such cases are not included in the insets of fig. 3. The peak intensities vary considerably depending, on the degree of shadowing, blocking, or focusing for specific trajectories. Two important observations can be made from figs. 2 and 3. First, the a scans at 4 keV contain fewer peaks and are simpler than those at 8 keV. This is a result of the -- 15% larger shadow cone radius at 4 keV compared to 8 keV as will be shown below. The smaller shadowing and block-
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ing cones at 8 keV allow the projectiles to penetrate a n d escape f r o m d e e p e r layers, hence the larger number of scattering peaks in the higher energy scan. The 4 keV beam samples the first through fifth layers while the 8 keY beam samples from the first down through the ninth layers. Second, since scattering from surface and specific subsurface layers is resolved, collection of scattered ions as a function o f ~t can provide layer-specific i- formation.
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Fig. 3. (N + ]~) scattering intensity, I(S), versus incident angle, ~, for Ni{100} using 8 key Ne + at 6)= 166 ° a|ong four different azimuths ?,. The ~L: positions are indicated by circles. The insets illustrate the s h a d o w i n g - s c a u e r i n g atom pairs in the surface and subsurface hyers of the plane containing the azimuth. Within the insets are shown, the layer numbering, the critical angles, a~, and the labels corresponding to the peaks in the ,~ scans. Solid circles: atoms in odd-numbered layers; open circles: atoms in even-numbered layers.
C C Hsu, J. W. Rabalais / Structure sens#ivity of scattered Ne + ion fractions from Ni {100}
The three a scans at 4 keV in fig. 2 can be compared directly to those of Fauster and Metzner ( F & M ) [14] for the same system in which only the scattered ions were collected using an electrostatic analyzer (ESA). Due to preferential neutralization of ions scattering from subsurface layers, the F & M scans exhibit only low intensity, broad peaks above a = 40 ° where the subsurface peaks of fig. 2 are clearly observed. The s!~.dow cones were computed from the Ziegler-Biersack-Littmark potential [18]. The sealing factors, C, for the potential screening length were adjusted [19] to provide the best fit between the calculated cone radii, R, and experimentally measured radii at distances L behind the target atoms. Experimental measurements of a~s from first-layer shadowing and scattering pairs along different azimuths were used for this calibration. The optimum fit for 4 and 8 keV was obtained with C = 0.81 and 0.94, respectively. The optimized cones along with the experimental points are shown in fig. 4. For L > 2 A, the difference in R for the two cones is nearly constant at - 0.1 ,~.
--
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L (A) Fig. 4. Calculated shadow cones for 4 and 8 keV Ne ~ + Ni. Experimental points obtained from first-layer scattering are shown on the plot. The screening constants in the potential were adjusted for optimum agreement between experimental points and computed curves.
81
3.2.2. Total (N + I) and component N and I intensities versus a
Scattering intensities for N + I, N, and I were collected as shown in fig. 1 for a values corresponding to the first peak of the a scans of fig. 3. The results are plotted in fig. 5 and the acs are listed in table 1. As the interatomic spacings increase, the a~s decrease as expected. The a~ values for the N + I and I scans for each ~ i m u t h are within experimental error, i.e., < 0.3 °, with the exception of the (100) azimuth where a~I is 1 . 8 ° larger than a~tN +i) . These results were reproduced several times and are well outside of the experimental uncertainty of the measurements. 3.2. 3. Scattered ion fractions versus a
The scattered ion fractions Y determined from the T O F spectra of the a scans of fig. 5 are plotted in fig. 6. The Y values and their behavior differ along the various azimuths, yore> is lowest due to the short interatomic spacings and the simultaneous scattering from both first and secend layers, y(m0> is high because scattering results from only the first layer in the low c~ range: the value increases with increasing a up to a maximum of Y = 32% at o~ = 21 °. This increase of Y with o~ is expected from the decreased ionsurface neutralization probability as the velocity of the particle perpendicular to the surface ~,.... increases. ) ' ~ ~ I decreases above ~ ~ 21 ° due to the onset of second-layer scattering (fig. 3). Intermediate ion fractions are obtained along (310) and (210). y ( 3 1 0 ) iS enhanced by the long interatomic spacings, but is reduced by the combination of both first- and second-layer scattering that is accessible at all o~. y<2m> is also enhanced by the long interatomic spacings; it increases in the range a = 7 o 12 ° due to the increase in ~'a, however it is reduced by the accessibility of secondlayer scattering beginning at a = t3 ° 3.3. Az~mudmt angle 6 scans 3.3.1. Scattering inlensiaes versus 8
Total N + 1I scattering intensities l(S) as a function of azimuthal angle 8 are plotted in t~g. 7 for different incident angles a. Beginning at low o, the o~ = 20.5 ° scan is dominated by iptense max-
C.C. ttsu, J. 14/'. Rabalais / Structure sensitwity o/scattered Ne + ion fractions front Ni (100}
82
ima at (110) with minor maxima near (310). Three factors contribute to the high intensity at (110): (i) The a ,alue corresponds to the maximum in the a scan of fig. 3. (ii) This azimuth presents the highest atomic density, e.g., there are twice as many atoms exposed to the beam compared to the (100) direction. (iii) The incident and scattered ions are focused by the (110) surface semichannels, i.e., the first and second atomic layers serve as "walls" and "base", respectively,
Table 1 Critical incident angle, a,., values for 8 keV Ne + scattering from Ni{ 100} along different azimuths Azinmth
(N + 1)
(1)
(110) (100)
18.2 ° 13.5 °
18.3 ° 15.3 °
(310)
10.3 °
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(210)
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8.8 °
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C.C. Hsu, J. IV. Rabalais ,I Structure sensith~ity of scattered Ne + ion fractions from Ni {100)
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Fig. 6. Scattered ion fractions, Y = [IN + | ) - N ] / ( N + II, versus incident angle, 0~, from the data of fig. 5. An example error bar is shown for one of the points. (100> = closed circles; {210} = open circles: (310> = open triangles; (110> = closed triangles.
Fi+, 8. Scattering intensity, IIS), for ions only tl) versus azimuthal angle, 8, for two different incidem angles, ~.
to guide the trajectories. It has been shown [15] that such semichanneling can produce sharp spatial anisotropy in the I(S) distributions. At (310),
only factor (iii) contributes to enhancement +~f intensity. As a increases, the ma-,ima at (I10) shift towards the <210> and <310> positions. This is due to the accessibility of sub,,urface }aver scattering along these azimuths (fi~:. 3) and the k ~ s s o f t h e e n h a n c e m e n l factors(i} ~;,!!iii) above as ~ increases. The scattering intensities I(S) of the ion component I are plotted in fig. 8 as a function of 8 for two of the a values shown in fig. 7. At low incidence (c~ = 20.5 o ), both the N + ~ and [ intensity patterns are similar with peaks along <110). At high incidence (~ = 4 5 °), the ~ intens, ty is independent of 8 while the N + ~ intensity exhibits a strong 8 dependence. This ]~>,,,, of 8 sensitivity in the ion intensi,'v at high a results from the
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scattering produces mainly neutraH,: :rod their intensities are strongly ~qodulated by >,i~:~do~ving and blocking. On the other hand, most of ,,he scattered ions o r i g n a t e from first-layer scattering and this produces a constant ion intensity at high a. Therefore, the d e p e n d e n c e of I(S) on 8 at high a is due to variations in the neutral yield rather than the ion yield.
C.C. Hsu, J. W. Rabalais / Structure sensitivity of scattered Ne + ion fractions from Ni {100}
84
3.3.2. Scattered ion fractions versus The scattered ion fractions Y are plotted as a function of 8 in fig. 9 for the same o~ values used in fig. 7. At low a, maxima are observed at <100> and (210) due to exclusively first-layer scattering and long interatomic spacings. Minima are observed at (110) and <310> due to a combination of first- and second-layer scattering. As a increases, the maximum at (100> dominates because subsurface-layer scattering becomes accessible for the other azimuths. When a = 50 o this condition no longer persists because subsurface-layer scattering is accessible along (100). For a = 55 °, subsurface scattering is readily accessible in all directions restdting in low Y and weak azimuthal dependence. The large variations in the scattered ion fractions Y (,from -- 2 to 40%) as a function of 8 (fig. 9) are mainly due to the modulation of surface and subsurface scattering as 8 is scanned. The Y values are controlled by charge exchange along the outgoing trajectories since the distances of closest approach at these energies and scattering angle are
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INCIDENT ANGLE Fig. 10. Plots of scattered ion fraction,., Y, and Y / P , where P is the ion survival probability from eq. (1), versus incident angle, a, for the (100> (circles) aria (210> (triangles) azimuths. An example error bar is shown for one of the points.
well within the threshold for reionization of Ne, as will be detailed in section 4.2. This can be demonstrated along the (100) and (210) azimuths where pure first-layer scattering is observed up to a = 23 ° and c ~ 12 °. respectively. Plotting eq. (1) as Y/exp(-vJv±) versus a with v,: = 0.60 x 107 and t.5 x 107 c m / s for the <210) and (100> azimuths, respectively, produces a constant that is independent of a as shown in fig. 10. This simple treatment is not possible along other azimuths due to combined multilayer scattering. The difference in a, for the two azimuths may arise from the dependence of the neutralization rate on interatomic spacings.
q. Discussion
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t:::i,D. o. So:tiered ion fractions, )', versus azimuthal angle 8, for
the four incident angles of fig. 7.
4. !. Effect of neutralization on ~,
!on neutralization during scattering can affect the ~ values in two ways. First, since there is a very high neutralization probability for subsurface scattering, I(S) versus a scans obtained by collec-
C.C. Hsu, J. W. Rabalais / Structure sensitwi O, of scatwre, ,¢e + ion fractions from Ni [ lO0}
tion of only ions exhibit extremely low intensity peaks at high a. It is sometimes difficult to even observe these high o~ peaks and it is usually not possible to extract accurate a~ values for secondand third-layer scattering from such scans. A good example of this is shown in fig. 2. Second, variations in the scattered ion fractions Y as a function of a can cause shifts in the a~ positions if oaly the ions are collected. Considering the second factor above, i.e., the variation of Y with a, tabl~ 1 shows that the % values determined from N + I and I only are within the experimental uncertainty of the measurement for all azimuths except (100). Fig. 6 shows that the highest Y values are obtained along (100) and that Y increases by 100% in the critical low-a region. This large increase in Y results in a shift ~f the a~ value for the I scan to 1.8 ° higher than that of the N + I scan. No such shifts are observed for the other azimuths because the Y values do not change by such large amounts in the critical low-c~ region. These results indicate that the % values obtained from measurements of I are sensitive to Y and that the structures de~ termined from them carry a corresponding uncertainty. For example, the 1.8 ° shift in % along (100) translates to a decrease in the interatomic spacings of - 0 . 4 ,~. This artifact is not encountered when N + 1 measurements arc used.
4.2. Mechanism for production of large Ne + ion fractions The mechanism for production of large Ne + ion fractions in scattering from light metal surfaces has been identified [6]. It involves the evolution of molecular orbitals (MOs) of a transient diatomic molecule formed from the overlap of inner shell atomic orbitals (AOs) of the individual atoms [20]. Due to the availability of electronic sta~e~ a., coastrained by the Pauti principle and symmetr 5 ru~c~, electr3ns can be promoted into excited and autoionizing states. As a result, ions that are neutralized along the incoming trajectory can be efficiently rdonized in the close encounter if the distance of closest approach ( d ) is small enough to allow inner shell electronic orbitals to overlap.
85
For t: e case of Ne + and Ni, the radii of the N e L and N i M shells are both --0.3 A. For 4 and 8 keV Ne + scattering from Ni at O = 166 °, d - 0.27 and 0.19 A, respectively. For such close approaches, two N e 2 p electrons can be promoted into the highly excited 4fo MO, which leads to formation of the doubly excited autoionizing electronic state, N e * * (2p43s2), as the atoms recede from each other [61. Due to the long lifetime (--- 1.5 x 10-~4 s) of this state, autoionization to Ne+(2p s) occurs along the exit trajectory when the ion is away from the surface and the neutralization probability is low. As a result, scattering from the surface layer yields high ion fractions. Scattering from subsurface layers yields low ion fractions because autoionization occurs along the exit trajectory before the N e * * can escaoe the lattice. o
5. Conclusions Collection of scattered neutrals plus ions b\, TOF techniques as a func~_ior~ oi inciden~ ~, and azimuthal, 8, angles allows one to sample the surface as well +~,, specific subsurface layers do'+ r, to at toast nine atomic laye~+~. Thi> i~tu~uatc~ tt;c utility of T O F scattering techniques for obtaining layer-specific information about interfaces and very thin films. The scattered Ne + ion fractions are large (up to - 4 0 % ) and have a strong ~ and 8 dependence, even on a clean metal surface such as Ni. For such cases, scattering intensity versus ~x or 8 plots, which are used heavily in structural analysis, can be distorted by e|ectronic effects when only ions are collected rather than neutrals plus ions. Such an effect has been found in ~he determination of the c~iticai incideat angles ~ along various azimuths. The behavior of the ion fra,:~ion~ i~ consistent with a model in which Ne is excited into autoionizing states during the close atomic encounter which decay along the outgoing trajectory. The surviving ion fraction depends heavily on the neutralization probability of ~hese escaping nascent ions.
86
C.C. Hsu. J. ~1":Rahalais / Structure sen,~itieiO' of scattered Ne + ion fractions from Ni{lO0}
Acknowledgements This material is based on work supported by the National Science Foundation under Grant No. DMR-8914608. The authors thank H. Bu and R. Trehan for assistance with the experiments and interpretations.
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[13]
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