Surface analysis using scattered primary and recoiled secondary neutrals and ions by TOF and ESA techniques

Nuclear Instruments and Methods in Physics Research 218 (1983) 719 726 North-Holland, Amsterdam

719

SURFACE ANALYSIS USING SCATI'ERED PRIMARY AND RECOILED NEUTRALS AND IONS BY TOF AND ESA TECHNIQUES J. W a y n e R A B A L A I S ,

J. A l b e r t S C H U L T Z

SECONDARY

and Ranjit KUMAR

Department of Uhemistry, Unwersi(v of Houston, Houston. Texas 77004. USA

A spectrometer system for simultaneous mass, velocity, and energy analysis of scattered and sputtered ions and velocity analysis of fast neutrals ejected from a surface by bombardment with a pulsed, mass selected ion beam is described. Combination secondary ion and scattered neutral plus ion time-of-flight (TOF) spectra are presented. Electrostatic sector analysis (ESA) coupled with the TOF technique is used to obtain energy distributions of scattered and sputtered ions and TOF/ESA spectra. The neutralization probability of scattered ions, i.e. the fraction of particles surviving a scattering collision as ions, is obtained by collection of spectra of neutrals plus ions and neutrals alone. TOF and ESA spectra are used to illustrate the measurement of particles scattered and sputtered by direct recoils and surface recoils. The feasibility of detecting such fast sputtered neutrals without postionization is demonstrated. TOF spectra of scattered primary and fast recoiled surface neutrals and ions from selected He +, Ne +, and Ar + bombardment of CsBr, Ge, GeO 2, H20 adsorbed on Ge, and clean and adsorbate covered La are presented. Ion fractions are determined for scattered and recoiled particles and the significance of Auger and resonant neutralization channels is demonstrated. Adsorbed hydrogen and oxygen are detected by surface recoiling and direct recoiling processes.

1. Introduction Simultaneous mass, velocity, and energy analysis of scattered a n d sputtered ions and velocity analysis of fast neutrals can be obtained [1 4] from a c o m b i n a t i o n of time-of-flight (TOF), quadrupole mass spectrometry, a n d electrostatic sector analysis (ESA). By using a multiplier that is sensitive to both ions and fast neutrals and by using a pulsed ion b e a m with T O F analysis which collects particles of all energies concurrently in a multichannel mode, surface analysis can be obtained [1] with primary ion doses of only - 1 0 1 2 i o n s / c m 2. At ion energies _< 10 keV, a binary elastic collision model provides a good description [5] of the collision dynamics a n d the laws of conservation of energy and m o m e n t u m can be used to describe scattered and sputtered particle energies. A particle of energy E o and mass M 1 singly scattered (SS) from a surface atom of mass M 2 into a scattering angle 0 will retain an energy E 1, defined as E 1 =E0(1

+A)-2[cosO+(AZ-sin20)l/2]

z

(1)

for M 2 > M 1 and A = M 2 / M 1. Multiple scattering (MS) sequences can be approximated by repeated application of eq. (1). Sputtered atom energy distributions typically [6,7] peak at low energies (about 2 - 2 0 eV) and tail out to several h u n d r e d eV; this is a result of a mechanism [8,9] by which the primary energy is dissipated to target atoms through collision cascades, some of which are eventually directed towards the surface, resulting in the ejection of atoms. Specific trajectory sequences 0 1 6 7 - 5 0 8 7 / 8 3 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

[1,3,10-16], however, can yield high energy sputtered particles with narrow energy distributions, such as for example, direct recoil from either a primary ion or a heavy substrate atom. The energy E r of a target atom of mass M 2 which is directly recoiled (DR) from a primary ion of energy E 0 a n d mass M 1 is E r = 4EoA(1 + A)-2

cos2~,

(2)

where qa is the angle between the direction of incidence of the primary ion and the recoiling target atom. The energy E~ of light surface recoiled (SR) atoms can be calculated by application of eqs. (1) and (2) to specific trajectory sequences, e.g. zero degree recoil from the primary ion followed by scattering from a heavier target a t o m into a detection angle 0. Since secondary particles ejected by (DR) and (SR) mechanisms can have high kinetic energies, b o t h neutrals a n d ions can be measured a n d detected by T O F techniques without the need for postionization. The large scattering cross sections in this energy range and the specific collision trajectories involved result in extremely high surface (outermost atomic layer) sensitivity a n d the ability to extract surface structural i n f o r m a t i o n [11]. This c o n t r i b u t i o n describes the spectra of scattered a n d sputtered particles from selected He +, Ne +, and A r + b o m b a r d m e n t of CsBr, Ge, G e O 2, H 2 0 adsorbed on Ge, and clean a n d adsorbate covered La. The ion fractions of scattered and recoiled particles are determined and the direct detection of surface hydrogen is exhibited.

J. Wayne Rabalals et al. / Surface analysis using scattered neutrals and ions

720 2. Experimental methods

The T O F systems used in these studies have been described elsewhere [17,18] and are shown schematically in fig. 1. System A has two flight tubes for time-of-flight analysis (one contains a quadrupole mass analyzer) at 90 ° scattering angles. System B has detectors at two scattering angles (45 ° and 105 °) and an ESA analyzer. Typical pulsed beam widths were 0.2-0.5 /*s and pulse rates of 10 40 kHz were used for the various primary

TOF/QUAD

SPECTRA

I}

A=1

X

c[!EJ

ions. Spectral collection times and total primary ion doses were 4 90 min and 1 10 × 1012 i o n s / c m 2. Spectra of scattered and sputtered neutrals plus ions (N + 1) and neutrals only (N) were obtained, the latter by using a deflector voltage to remove the ions. Spectra of ions alone (I) were obtained by subtracting these two spectra and ion fractions were calculated as ( I ) / ( N + l). For Ar 2+ b o m b a r d m e n t the surviving Ar 2+ and Ar t fractions were obtained by using a retarding grid analyzer in front of the multiplier to transmit either (Ar + Ar +) or (Ar + A r + + Ar 2+) and a deflector to transmit only Ar. The sample of CsBr was prepared in the form of a film - 5000 A thick by vacuum evaporation onto stainless steel plates. The sample of GeO 2 was mounted by pressing the powder into a layer of epoxy on metal plate and the Ge and La samples were cleaned by 3 keV Ar + bombardment.

3. Results and discussion

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T O F spectra of neutrals plus ions, neutrals, and ions for 3 keV Ar + b o m b a r d m e n t of CsBr are shown in fig. 2. The (SS) peak for Ar scattering from Cs and Br is at 12.7/*s and 16.1 /*s, respectively, and a (MS) shoulder is observed in the region 11-12.5 #s. The broad hump in the region 5 . 5 - 1 1 . 0 / , s is not due to scattered Ar + since the equivalent energy of the atom would be greater than the beam energy. This broad structure is due to (SR) H, C, O, and F impurities on the CsBr surface. It should be noted that (SR)H has a fast T O F even though it has a low energy. From the three spectra of fig. 2 it is observed that Ar + ions survive not only the (SS) collisions with Cs and Br, but also (MS) collisions. For the Ar + scattering

TOF ION & NEUTRAL

^A

-~

SCATTERED PARTICLE ENERGY (keV) 3.0 2.0 1.5 1-0 0.6

>__ "r TOF/ESA

TOF/ESA SPECTA Fig. 1. (A) Schematic representation of the TOF quadrupole mass spectrometer for ion and fast natural scattering and secondary ion emission from surfaces. A, gas inlet; B, ion source, C, Wien filter; D, neutral stop/energy filter; E, pumping line; F, pulse plates; G, sample; H, Faraday cup, I, drift tube; J, quadrupoles; K, channeltron; L, deflector; and M, retarding energy analyzer. (B) Schematic diagram of the T O F / ESA analysis system showing the pulsed ion beam and the electrostatic sector analyzer with a grid window. TOF analysis of neutrals and ions is obtained by means of a line-of-sight detector through the grid window. Energy analysis of the ions is obtained through the ESA by sweeping the voltages on the sector halves.

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I

I

I

'o20 -Ar +(3keV)~ CsBr w15 Z Z U

m 5

g So r---i 4

6

I 8

i 10

i

I

I

12 14 16 TIME-OF-FLIGHT (psec)

I

18

I

20

]

Fig. 2. TOF spectra of 3 keV Ar + scattered from a CsBr surface at 0 = 90 ° through a path length of 112.3 cm. (A) A r + A r +, (B) Ar only, (C)= (A)-(B) Ar + only.

J. Wayne Rabalais et al. / Surface analysis using scattered neutrals and ions

721

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-4

A

>= 0

-8

-12 ~ ~Ar/Br

u.l

z -16

z

7////

~

-

-

Cs (Sp)

-20

o

k-

u -24

Cs(5=)

.,.,,1

Ar 2+

-28

Br[4s)

Fig. 3. Electron energy level diagram representing Ar + and Ar 2+ ions approaching a CsBr surface. The valence bands of CsBr are shown along with the discrete energy levels of the ions in their potential well. The abscissa represents the distance from the surface to the ion and the ordinate represents the energy of an electron in the solid or atom. The most probable charge transfer transitions that can occur between the ion and surface while the ion is within angstroms of the surface are indicated. Transition 1 represents a resonant transition from the Cs 5p valence band to the Ar + 3p hole. Transitions 2 and 2' represent a two-electron Auger process in which one Br 4p valence band electron tunnels into the Ar + 3p hole and another is ejected into the vacuum continuum of states.

the peak value of the ion fraction is 0.22 for (SS) from Cs and drops to 0.13 for (SS) from Br. For Ar 2+ scattering both Ar + and Ar 2+ ions are found to survive. The surviving Ar + fraction is 0.18 and 0.08 and the Ar 2+ fraction is 0.10 and 0.09 for (SS) from Cs and Br, respectively. These ion fractions are consistent with a model (see refs. 17, 19, 20, the electronic band structure of CsBr was obtained from ref. 21) in which the ion survival probability is governed by Auger and resonant neutralization (AN and RN) processes during approach a n d / o r departure from a specific atomic site as illustrated in fig. 3. Shifts in the discrete atomic energy levels due to the image potential in the surface are typically < 2 eV and hence will not alter the basic predictions from fig. 3. For Ar + scattering from Cs, the d o m i n a n t energy level with which it interacts is the Cs 5p level, this interaction being through R N transitions. A N transitions are not possible because the condition 1P > 2 Eg, where I P is the projectile ionization potential and Eg is the band gap of the solid, is not fulfilled. For Ar + scattering from Br, only AN transitions with the Br 4p level are possible. Since the only neutralization process for scattering from Cs is RN and from Br is AN, the higher ion fraction surviving a Cs collision (69% more) suggests that the A N process on Br is more efficient than the R N process on Cs [22]. The Ar 2+

L

0

!

5 TIME OF FLIGFrT (~lsec)

!

I0

Fig. 4. TOF spectra of scattered and sputtered neutrals and ions for 3 keV Ar + bombardment of CsBr at a forward (42 °, 170 cm path) and backscattering (105 °, 45 cm path) angle. P represents a bombardment induced photon pulse, Ar/Cs and Ar/Br are scattering peaks for binary elastic collisions of Ar with Cs and Br, H(SR) and O(SR) are surface recoiled hydrogen and oxygen, and H(DR) and O(DR) are direct recoiled hydrogen and oxygen.

survival probability from either Cs or Br is similar because the energetic Ar 2+ hole can undergo similar A N transitions with both the Cs 5p and Br 4p levels, however R N transitions with the near-degenerate Cs 5s and Br 4s levels have low probability due to the long residence times [17] of electrons in these bands. The Ar + produced by partial neutralization of Ar 2+ can undergo the same processes stated above for Ar + scattering, and hence the ion fractions are similar. When the CsBr surface is cleaned and then allowed to contaminate for several hours in the U H V system, Cs2OH + secondary ion clusters can be ejected from the surface as low energy SIMS ions suggesting that there is a hydroxide impurity. T O F spectra of such a contaminated surface at 42 ° and 105 ° , as shown in fig. 4, clearly demonstrate the (DR) and (SR) processes. In these spectra the energy (TOF) of the Ar scattering peaks are predicted from eq. (1), the H ( D R ) and O(DR) from eq. (2), and the H(SR) and O(SR) from application of both eqs. (1) and (2). H ( D R ) and O(DR) present in the forward scattering spectrum are of course absent in the backscattering spectrum. H ( D R ) and O(SR) have similar TOFs, making it difficult to determine their individual contributions to the 4.9 /~s peak at 42 °. We feel that most of the 4.9 /~s peak is due to H ( D R )

722

J. W+(vne Rabalais et al. / Surface 61nalysis using scattered neutrals" and ions

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Time of

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Fig. 5. (A) ESA spectrum showing scattered and sputtered ion energy distributions at 47 ° for 3 keV Ar + bombardment of CsBr. (B) Example of a TOF/ESA spectrum, i.e. a TOF mass spectrum of ions focused through the sector at a kinetic energy of 1325 eV.

however, because in other systems [18] where the time resolution is adequate, the (SR) peaks are of minor intensity c o m p a r e d to the (DR) peaks and they tend to be broader than (DR) peaks since they involve a two collision sequence for which many angle combinations are possible. The scattered and sputtered ion energy distributions from clean CsBr at 47 ° , measured with the ESA, are shown in fig. 5(A). The ion composition (mass distribution) of the energy distribution of fig. 5(A) was determined by measuring the ion T O F spectra through the sector [fig. 5(B)] which was preset to pass specific kinetic energies. Ar + surviving scattering from Cs, Br, and Fe (in the sample holder) are observed. The ratio of Ar + ions surviving a Br collision to those surviving a Cs collision is 0.32. Peaks due to (DR) Cs +, Br +, and H + are observed and their energies are predictable from eq. (2). The shape of the Cs + (DR) is similar to that observed [23] for Cs (DR) from a thin Cs film by Ne + b o m b a r d m e n t . The high energy shoulder on the Cs + peak tails out to nearly 2000 eV and must result from

N

j j-.>,

II.O

22.0

3t.0

TIME OF FLIGHT(psec) Fig. 6. TOE spectra of neutrals plus ions (N+I) and neutrals only (N) at a 47 ° scattering angle (170 cm path) for 3 keV Ar + and 6 keV Ar 2+ bombardment of CsBr. The (N) spectra were obtained by using an electrostatic deflector to remove the ions in the flight tube.

(MS) sequences which produce fast Cs +. The low energy shoulder of the Cs + peak extends to zero kinetic energy; the rise in the region < 100 eV corresponds to cascade sputtered Cs + which has such a low energy distribution [24,25]. T O F spectra for 3 keV Ar + and 6 keV Ar 2+ b o m b a r d m e n t of clean CsBr at 47 ° in fig. 6 show Ar scattering peaks from both Cs and Br as well as Br(DR) and Cs(DR). The cesium is mostly produced as an ion while the bromine is mainly neutral. The C s + / C s and B r + / B r ion fractions ( + 20%) are, respectively, 0.90 and 0.08 for 3 keV Ar + and 0.83 and 0.12 for 6 keV Ar 2+. The Ar + ion fractions surviving Cs and Br collisions at this angle are 0.14 and 0.03, respectively, indicating preferential neutralization on Br as described earlier. The Ar 2+ ion fractions (both Ar m and Ar 2+) surviving Cs and Br collisions are 0.20 and 0.09, respectively.

J. Wayne Rabalais et al. / Surface analysis using scattered neutrals" and ions

Fig. 7. TOF spectra at 0 = 90 ° (125 cm path) of scattered and sputtered neutrals plus ions (N + I), neutrals only (N), and ions only ( I ) = ( N + I ) - ( N ) for 3 keV He + , Ne +, and Ar + scattering from clean Ge and H 2 0 adsorbed on Ge. For clean Ge the ion fraction is negligible, hence ( N + I ) = (N) and only the (N + 1) spectrum is shown. The curves labelled V represent acceleration of H + ions from the H(SR) structure of H 2 0 / G e using +400 V (He+), +600 V (Ne+), and +300 V (Ar+). Calculated positions for single scattering (SS) at 90 ° and double scattering (DS) at two 45 ° angles are: H e / G e (SS) 3.5 #s, H e / O (SS) 4.2 #s, H e / G e (DS) 3.4 ffs, H e / O (DS) 3.8 ffs, H e / G e e (DS) 3.6/~s, N e / G e (SS) 9.7/is, N e / G e (DS) 8.6 #s, A r / G e (SS) 19.2 ffs, A r / G e (DS) 14.5 ffs. Calculated positions for surface recoiling (SR) hydrogen and oxygen are: H e / H ( S R ) 2.1 ffs, H e / O ( S R ) 10.2 ffs, N e / H ( S R ) 4.0 #s, N e / O ( S R ) 8.2 /,ts, A r / H ( S R ) 5.5 p,s, A r / O ( S R ) 9.1 ,u,s.

Ar +

"3

Ld Z Z<~

5

N+I

z

._.,---if,

(D >I--

IO

~

Z LAJ t-Zm Ld > t-~J

20

50

40

50

60

Ne +

He +

I\o. I

I

3 6 T ME-OF-FLIGHT

5

(psec)

I0

723

whereas -4-6% s u r v i v e for N e + a n d A r + b o m b a r d m e n t o f H 2 0 / G e . C a l c u l a t i o n s s h o w t h a t the l e a d i n g e d g e of the (SR) s t r u c t u r e at 3 . 9 / x s for N e + a n d 5.4 ffs for A r + c a n o n l y be d u e to H ( S R ) with 530 a n d 278 eV, respectively; O ( S R ) is slower [8.24 ffs (Ne) a n d 9.06 # s (Ar)]. O n l y a s m a l l f r a c t i o n ( - 6 - 8 % ) of t h e s e (SR) a t o m s are c h a r g e d . N o (SR) s t r u c t u r e is o b s e r v e d in t h e H e + s p e c t r u m d u e to t h e low c r o s s s e c t i o n s i n v o l v e d [26]. A p p l i c a t i o n of a s a m p l e bias v o l t a g e to t h e N e + a n d A r + s p e c t r a of H 2 0 / G e s h i f t s t h e H ( S R ) i o n s f r o m t h e fast s h o u l d e r to f o r m a s h a r p , s h o r t t i m e p e a k . T h e l e a d i n g edge o f this p e a k c o r r e s p o n d s to h y d r o g e n with a n e n e r g y e q u a l to t h e c a l c u l a t e d H ( S R ) e n e r g y p l u s the b i a s voltage. T h e G e e 2 s p e c t r a (fig. 8) are d o m i n a t e d b y (SR) a n d

15

3.2. Germanium, H 2 0 / Ge, and GeO 2 T h e s p e c t r a of c l e a n G e (fig. 7) are s i m p l y i n t e r p r e t a ble in t e r m s of (SS) a n d ( M S ) elastic collisions; t h e (SS) a n d ( M S ) s e p a r a t i o n is largest for A r + s c a t t e r i n g , w h e r e t h e ( M S ) p e a k d o m i n a t e s over the (SS) peak. T h e spectra of H 2 0 a d s o r b e d o n G e e x h i b i t b r o a d e n e d scatteri n g p e a k s a n d (SR) s t r u c t u r e s at s h o r t e r times. S c a t t e r e d i o n f r a c t i o n s s u r v i v i n g f r o m c l e a n G e are o n l y - 1%,

-t-

+

Ar

Ne >t-Z 1LJ

~

I

H

2

0

/

G

e

Z bJ

_J liJ r~

k

5

. . l _ _ J . _ _ J . _ _

I0

15

20

TIME-OF-FLIGHT

IO

20

3o

40

50

(Is s e c )

Fig. 8. TOF spectra at 0 = 90 ° (125 cm path) of scattered and sputtered neutrals plus ions (N + I), neutrals only (N), and ions only (I) = (N + I ) - ( N ) for 3 keV Ne + and Ar + scattering from G e e 2. The Ge spectrum is shown for comparison. The calculated SS, MS, and SR event times are given in the caption of fig. 7. The structure observed in the I spectra for Ne + from 8-20 #,s and for Ar + from 10-45 #s corresponds to Ge + and the impurities Na + and K + which have energies corresponding to surface recoils (SR) and cascade sputtering (S) plus the surface charging potential.

724

J. Wayne Rabalais et al. / Surface analysis using scattered neutrals and ions

s u r f a c e c h a r g i n g effects [26] a n d the scattering peaks are b r o a d e n e d structures. T h e H ( S R ) a n d O ( S R ) s t r u c t u r e s are very intense, i n d i c a t i n g that the o u t e r m o s t layer is a h y d r o x i d e r a t h e r t h a n an oxide. T h e calculated p o s i t i o n o f the H ( S R ) agrees well with the leading edge o f the o b s e r v e d (SR) structure, w h i c h is c o m p o s e d largely of n e u t r a l atoms. T h e r e is an a d d i t i o n a l s h a r p h y d r o g e n peak [2.41 p,s ( N e +) a n d 3.64 /Ls (Ar)] which is c o m p o s e d of only H + ions that are accelerated d u e to s u r f a c e c h a r g i n g d u r i n g b o m b a r d m e n t . T h e kinetic energy d i f f e r e n c e calculated from the leading edge of the T O F p o s i t i o n s for H ( S R ) neutrals a n d ions p r o v i d e s an e s t i m a t e of the p o t e n t i a l t h r o u g h w h i c h the H ( S R ) ions are accelerated from the surface d u e to charging. This d i f f e r e n c e is 355 a n d 906 eV for A r + a n d N e +, respectively. This a c c e l e r a t i n g p o t e n t i a l m a y be specific to the collision site a n d variable d e p e n d i n g on the n a t u r e of the collision pair as well as the kinetic energy (which

will d e t e r m i n e the d i s t a n c e of closest a p p r o a c h ) . Such collisions can cause electron excitations a n d ionizations, p r o d u c i n g highly repulsive local fields specific to the i n t e r a c t i o n site. A d d i t i o n a l ion peaks o b s e r v e d in the G e O 2 s p e c t r a c o r r e s p o n d to G e +, N a +, and K + which have energies equal to (SR) a n d c a s c a d e s p u t t e r i n g plus the surface c h a r g i n g potential. 3.3. L a a n d ads'orbate couered La

Scattering from polycrystalline La foil was s t u d i e d u n d e r two c o n d i t i o n s . T h e first was the cleanest surface o b t a i n a b l e b y 3 keV A r + b o m b a r d m e n t which was c h a r a c t e r i z e d by a ratio o f the (SR) oxygen c o n t a m i n a tion peak to the rare gas scattering peak of < 4% a n d an A E S s p e c t r u m which s h o w e d < 2% o x y g e n in the surface layers. The s e c o n d was a k a surface that was allowed to a d s o r b the residual gases in the v a c u u m for several days

4-

A;

Ne

He Z U.J

iii

m

,-a u.I ev

I

I

3

6

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9

5

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10

20

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20

30

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20

25

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20

30

T I M E - OF- FLIGHT (p sec) Fig. 9. TOF spectra at 0 = 90 ° for scattered and sputtered neutrals plus ions (N + I), neutrals only (N), and ions only (1) = (N + I ) - ( N ) for 3 keV He +, Ne +, and Ar + scattering from a lanthanum and adsorbate covered lanthanum surface. Calculated positions for single scattering (SS) at 90 ° are H e / L a = 3.4, N e / L a = 8.5, and A r / L a = 13.9 p,s and for double scattering (DS) at two 45 ° angles are H e / L a = 3.3, N e / L a = 8.0, and A r / L a = 12.3 `us. Calculated positions for H, C, and O atoms surface recoiled from La (for the trajectory 0 ° recoil from the primary ion followed by 90 ° scattering from ka) are, respectively, 3.9, 6.4, and 7.4 /_ts for Ne + bombardment at 5.4, 7.3, and 8.1 ,us for Ar + bombardment.

J. Wayne Rabalais et al. / Surface analysis using scattered neutrals and ions

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725

electrons, resulting in a low density of occupied states for convolution into the A N transition probability. For adsorbate covered La the Ar + level is quasi-degenerate with the adsorbate levels, allowing RN transitions to take place. These adsorbate levels also provide additional A N channels for Ne + and He + neutralization. The data indicate that the high survival probability for A r + ions scattered from clean La is due to the low density of valence b a n d electrons available for convolution into the most energetically feasible neutralization channel, i.e., the A N channel. The effect of adsorbates is to supply electrons at levels where both RN and A N are energetically feasible. These results are in contrast with the low ion survival probabilities from metals with high densities of valence b a n d electrons.

I-LtJ _J

,,, 5 6

~ S s Lo

~

5s

adsorbate covered Lo

Fig. 10. Electron energy level diagram representing He +, Ne +, and Ar ~ ions approaching a clean and adsorbate covered lanthanum surface. The valence bands of lanthanum are shown along with the discrete energy levels of the ions. Typical shifts [20] in these discrete levels due to the image potential in the surface are indicated as dashed levels.

until the oxygen (SR) peak to La scattering peak ratio was - 3 3 % . The d o m i n a n t residual gases consisted of H2, H2 ° , and CO. T O F spectra of the scattered and sputtered particles (fig. 9) exhibit (SS) peaks, a (MS) shoulder (on the short time side of the (SS) peak) for Ar +, and a low intensity b r o a d structure at short times for Ne; + a n d Ar + which consists of (SR) H, C, and O impurities. The positions of these structures are predictable from eqs. (1) and (2). The (SR) structure is greatly e n h a n c e d on adsorbate covered La. The scattered ion fractions are < 1% with the exception of Ar + scattering from La where it is 21.1% for the (SS) peak a n d 13.7% for the (MS) shoulder a n d for Ar + scattering from adsorbate covered La where 10.7% surviving ions are observed at the (SS) position. These data show that the probability of Ar + ions surviving as ions in a scattering collision with La is - 9 7 % higher than it is from adsorbate covered La; the survival probabilities of He + and Ne + are negligible for both surface conditions. Consider the energy level diagram [27] in fig. 10 for rare gas ions a p p r o a c h i n g clean a n d adsorbate covered La surfaces. For clean La both He + and Ne + can undergo RN with the 5p b a n d as well as A N with the 5d, 6s b a n d while A r + can only undergo A N with the 5d, 6s band. This A N transition probability is expected to be small because the 5d, 6s b a n d contains only three

4. Conclusions Pulsed b e a m T O F spectra of scattered ions and neutrals, direct recoiled (DR) and surface recoiled (SR) atoms, and cascade sputtered ions provide sensitive analysis of the outermost atomic layer of a surface using b e a m doses of only - 1012 i o n s / c m 2. The use of (DR) a n d (SR) methods is especially useful for detecting light impurities such as H, C, and O. Detection of (DR) and (SR) adsorbates on single crystal surfaces as a function of angle can yield structural information on the nature of adsorption sites. Studies of the reaction of hydrogen containing molecules with surfaces should benefit from being able to detect surface hydrogen directly. Detection of b o t h scattered and recoiled neutrals and ions allows the determination of ion survival probabilities a n d delineation of s u r f a c e / p r o j e c t i l e charge exchange processes. The low fluence pulsed ion beams allow study of n o n - c o n d u c t i n g surfaces, however surface charging effects occur at high fluence and are observed as shifts in the secondary ion T O F positions. This material is based upon work supported by the R.A. Welch F o u n d a t i o n u n d e r G r a n t No. E-656 a n d the N a t i o n a l Science F o u n d a t i o n under G r a n t No. CHE8209398.

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