Vacuum/volume 31/numbers 10-12/pages 507 to 509/1981 0042-207X/81/100507-03502 00/0 Printed ,n Great Br,tam @ 1981 Pergamon Press Ltd

A n g l e - i n t e g r a t e d st ick ing probabi l i t ies: N 2, and D 2 on W{O01} Heather Bickley, James S Ar low, M A Morr is and David A King, The Donnan Laboratorms, The Untverstty, Ltverpool L69 3BX, UK

H2

A new techmque has been used to measure/nctdence-angle-averaged sttcktng probab/httes, (s>, for N 2, H u and D u on W{O01} A comparison with normal-mctdence values, s ±, obtained by molecular beam techntques provtdes a measure of the angular dependence of s For N 2 on W{O01}, the initial (zero-coverage hm/t) sttckmg probabthty (S>o ts 0 63+0 03, comparing closely wtth So ± = 0 5 8 ± 0 01 reported by King and Wells, indicating a relattvely small vartatton m s with incidence angle After saturatton and anneahng at 1000 K to form an underlayer, mtrogen Is readsorbed at 340 K with (S>o=O 1 For both H u and D 2 at 340 K we hnd (S>o to be 0 72±0 02, whtch ts slgmhcantly htgher than the value reported for So ~, 0 60±0 02 The stgnthcance of the results for adsorption mechamsms ts dtscussed

1. Introduction

Relatively few accurate, absolute methods are available for measur ing sticking probabfllUes at single crystal surfaces I For high sticking probabi l i t ies (0 1 ~<s~< 1) the reflection-detector techniques are the most accurate, where the reflection coefficient is measured by sampling the back-scat tered flux for a given molecular beam incidence angle Studies of the dependence ors on incidence angle by these methods are restricted experimentally to the range from normal incidence to ~ 6 0 ° f rom normal , and generally very small var ia t ions have been noted 2 4, with the exception of H2 and D 2 on W ~ l l 0 ) 4, where So is reported to decrease by ~60°~, over this range, and H z / D 2 on stepped Pt{ll l~j planes s, where the reactivity was highest when the incident beam was directed at the steps A compar i son of angle- averaged st icking probabi l i t ies , ( s ) , with n o r m a l incidence values, s~, f rom molecular beam experiments should provide a sensitive measure of the incidence-angle dependence of s and to this end we have applied a recently developed 6 absolute method for measuring ~ to the adsorp t ion of N2, H 2 and D2 on W~0011 Accurate absolute values of s ~ are available 3 v 8 for each of these systems, allowing a direct compar i son For H2 and D 2 on W I001 I, there is some controversy in the l i terature concerning the initial sticking probabi l i ty v'8, the coverage dependence of s 8 l o, and the existence of an isotope effect 7 8 10 tx ~4 We have

a t tempted to resolve these disagreements in the present study

2. Experimental

The principles and opera t ion of the u l t rahigh vacuum chamber used for measur ing absolute sticking probabi l i t ies have been described elsewhere 6 Gas is in t roduced to the adsorp t ion cell via a capillary, and a s teady state pressure is main ta ined In the cell by cont inuous pumping The crystal is suspended in front of a tube

connected to an ionizat ion gauge (IG-1) and possessing an orifice directed at the centre of the crystal The orifice diameter is 1 mm, and the (accurately measured) orifice-to-crystal distance IS 2 26 m m A second ' dummy ' tube in the adopt ion cell is directed at a glass disc, I e an inert surface, but is otherwise identical to the first, connected to a second ionizat ion gauge, IG-2 If the sticking probabi l i ty on the crystal is non-zero, the reflected flux from the crystal Into IG-1 is reduced compared to tha t into IG-2 The sticking probabi l i ty is ob ta ined by compar ing pressures P1 and P2 measured on IG-1 and IG-2, respectively If the fraction of r andomly dis tr ibuted molecules in the adsorp t ion cell which would have entered IG-I but are blocked from doing so by the crystal IS f, then ( s ) is given by

( s ~ = (1 /~ ( P 2 - P O / P 2

whence ( s ) is independent of the gauge cahbra t lon factor An accurate evaluat ion of f is necessary Steckelmacher 12 has

derived accurate formulae fo r f fo r var ious experimental configur- at ions In the present work we have used an elhptlcally shaped crystal, with major axis 9 m m and minor axis 5 5 mm, for this geome t ry , j is accurately given by our original expression 6, and is evaluated as 0 69

The W crystal was cut and polished to within 1 ° of the 1100~ plane, and was cleaned m sztu by tempera tu re cycling between 400 and 1700 K in an oxygen pressure of 10-6 torr for several days (until no CO desorpt lon peak was observed) and subsequently annealed m tacuo at 2500 K Tempera tures were measured using a W/25°o Re-W thermocouple spotwelded to the side of the crystal, independent ly cal ibrated using an optical pyrometer

3. Results and discussion

3.1. N 2 on W[001}. Six Independent runs were performed to evaluate the var ia t ion of ( s ) with surface coverage, with the

507

Heather Btckley et al Angle-integrated st,ckmg probabd,t,es N 2, H= and D u on W{001}

a • • i i

0 6 ?&- --o~ o~ . . •

<S>, S J" 0 5 ~\ 04 %

03

02 o

01 ,

0 ~ ~ 0 2 4 6 8

Coveroge / otoms cn52 x 1014

Figure l. Variation of <s) with coverage for N 2 on W ~0011 Each symbol represents a separate run Crystal temperature 340 K The full line is taken from the data for s ± against coverage of King and Wells 3. at 340 K

0 8 '

O6

O2

01 , J , ~ ,mw~x

0 5 10 15 20

Coveroge / otoms cr52 x 1014

Figure 2. Varmtlon of (6) with coverage of H 2 and D z on W{001] Each symbol represents a separate run Filled circles D z Crystal temperature 340 K Also shown are the data for ~ against coverage from King and Thomas s (KT) and MadeyV(M)

crystal at ~ 340 K The results are presented in Figure 1, together with the results for s ± obta ined by King and Wells 3 using a col l imated molecular beam technique Since coverages in the present work are dependent on the gauge cahbra t lon factor, which was not evaluated, the coverages have been normal ized to the value obta ined by King and Wells 3 at s ± = 0 03, viz 5 8 X 10 I4

a tom cm -2, thus assuming that ( s ) ~ s ~ near ' sa tura t ion ' Extrapola t ing to zero coverage gives an initial sticking probabi l i ty (6)o in the range 0 63 4- 0 03, which is only marginal ly higher than the value at no rmal inodence , So ± = 0 59_+ 0 013 The differences in the observed dependence of ( s ) and s ± on coverage are also relatively minor , and are p robab ly bet ter a t t r ibuted to experi- mental inaccuracies ra ther than to any real effect

Recently it has been repor ted ~3 that after adsorb ing 5 to 6 x 1 0 1 4 N a toms cm -2 on W[0011 at room temperature , anneal ing to ~ 1 2 0 0 K caused most of the adsorba te to be absorbed into the bu lk For example, the half-order L E E D beams whlch are character is t ic of the overlayer s t ructure are removed by this procedure However, a subsequent brief exposure to ni t rogen at r oom tempera ture was sufficient to restore the half-order beams to their original intensity This effect was studied in the course of the present work Ni t rogen adsorp t ion at 340 K was cont inued until ( s ) had fallen to ~ 0 02, the surface was annealed at 1200 K for I0 s and the crystal allowed to cool to 340 K before commencing r eadso rpuon The sticking p robab lh ty during the initial stages of readsorp t ion was found to be 0 1 4- 0 03 Thus, we est imate tha t the res tora t ion of half-order beam intensity during readsorp t ion repor ted in the L E E D study of Grlffiths and King a3 occurred with a total up take of ~ 6 5 × 1014 a toms cm 2 This confirms the observa t ion I3 tha t the desorpt ton spect rum after readsorp t lon showed an increase m peak area corresponding to a coverage of 6 5 × 1014 atoms cm 2, and rules out the possibility tha t the relatively small desorptxon increment resulted from permanen t absorp t ion into the bulk during t empera tu re pro- g r amnung Res tora t ion of half-order intensities thus seems to imply the (seemingly unhkely) process of the re turn of N a toms from the bulk to the overlayer dur ing r eadso rpuon at room tempera tu re

3.2. H2 and D2on W{001}. Results obta ined from the var ia t ion of ( s ) with coverage for bo th H 2 and De are shown in Figure 2, and are compared with the s ± da ta repor ted by Madey 7 and King and T h o m a s s for this system In each case, coverages have been

normal ized to the now generally accepted 7,s 14 sa tu ra t ion coverage for this system of 20 x l0 ~4 a toms cm -2

The only notable difference between the results for ( s ) and s ~ is in the initial sticking p robabd l ty Thus, all three sets of da ta show a virtually l inear fall in sticking probabi l i ty with coverage, a l though the curve of King and T h o m a s s does show a sharper fall at low coverages Taking the mean of the two da ta sets for So I as 0 55 4- 0 05, the value for ( s )0 , 0 72 4- 0 02, is significantly higher, suggesting an appreciable incidence angle dependence ofs for this system, i e (S)o/So ~ = 1 3 4-0 1 The virtually linear dependence of s on coverage confirms the results of Madey ~ and King and Thomas s, and contras ts with the da ta of T a m m and Schmidt l°, who reported tha t s is initially almost independent of coverage (Jaeger and Menzel 9 have also recently inferred that s is initially independent of coverage from ESD data) In all three sets of data, including the present, where a l inear dependence is found the crystal was cooled to the adsorpt ion tempera ture before dosing with hydrogen whereas T a m m and Schmldt ~° cooled the crystal in a hydrogen ambient , suggesting that their result may be an artefact produced by crystal cooling in the early stages of adsorpt ion

Results for D 2 adsorpt ion are also shown in Figure 2 Within experimental error, there is no difference between the H 2 and D 2

data, and we conclude that s(D2)/s(H2)= 1 0 0 4 - 0 0 4 , in agree- ment with King and Thomas s Madey ~ reported a value of 1 12 4- 0 12, while Schmidt and coworkers 4' lo, ~x report values in the range 14_+005 There is no obvious reason for this experimental discrepancy

3.3. General discussion. In considering the results, it is useful to derive a relat ionship between the incidence-angle-averaged stick- ing probabil i ty, ( s ) , and the differential value s o for a given incidence angle 0 (measured from the surface normal) The parameter ( s ) refers to the average value for a fully randomized Maxwel l -Bol tzman gas, the incident flux of gas molecules I ° can therefore be represented by a cosine law expression

I ° = I cos 0

where I is the flux at no rmal incidence Hence the angle integrated sticking probabi l i ty IS given by

(~/2 (*n/2 Fx/2 ( s ) = | s°l c o s 0 d 0 / | I c o s 0 d 0 = s°cosOdO (1)

do do Jo

5 0 8

Heather Btckley et al Angle-)ntegrated stsckmg probabdltles N2, H=

In order to relate ( s ) to s °, a funct ional dependence ofs ° on 0 must be assumed For example, a reasonable funct ion (based on energy accommoda t ion considerat ions) may be

s o = a{ 1 - e x p ( - - b/cosO)} (2)

where a~< 1 Exper imenta l values of ( s ) and s ± thus allow an evaluat ion of a and b f rom equat ions (1) and (2)

For N 2 o n W{001}, we observe a small dependence o f s on 0 (So±/(S)o = 1 09_+ 0 04) F r o m equat ions (1) and (2), the da ta can be fitted with a = 0 8 and b = 1 3, which applies a l imiting value for s as O~n/2 of 0 8 The funct ional form of equa t ion (2) with these values of a and b is shown in Figure 3, together with the da ta of

s 1 0 ~H2" 0 8 ~ ~ N z

/iNz

0 6

o % 3'o do 9o Incidence Angle / °

Figure 3. Full lines calculated variation of s with incidence angle from equation (2) for H2 and N 2 on W{ 001 ), consistent with the (s) and s ~ data for these systems Dashed lines experimental data of Stembruchel and Schmldt 4

Stelnbruchel and Schmldt 4, showing good agreement As dis- cussed in detail elsewhere 1' 3.1 s, 16 adsorp t ion into the dissoclat- lvely adsorbed fl state occurs t h r ough initial t rapping of the incident molecule into a weakly b o u n d precursor state, and subsequent transfer to the chemlsorbed state The t rapping probabi l i ty ~t at no rmal incidence was est imated as 0 60 from the low tempera tu re l imiting value ofs 3, ~ was found to decrease with increasing gas t empera tu re 3 The present results indicate an Increase in ct wi th incidence angle, the increase is smaller than would be ant ic ipated if only the normal componen t of incident molecule t rans la t iona l energy must be accommoda ted We note tha t a cosine law desorptlon dis t r ibut ion implies tha t s is independent of 0, as discussed recently by Comsa 1 ~, the angular d is t r ibut ion of N 2 backscat tered from steel, a l u m m l u m and glass is close to a cosine law dis t r ibut ion 1 s The present results would imply a slight f lat tening of the desorpt lon d is t r ibut ion compared with cos 0, and are discussed in relat ion to the desorpt lon flux dis t r ibut ion observed for N 2 f rom W{310} In Ref 19

The da ta for H 2 on W{001 } show a larger angular dependence of s, compar i son with the s ± da ta of Madey 7 gives So'/(S)o = 1 4, and with the da ta of King and T h o m a s s gives a rat io of 1 2 F rom equat ions (1) and (2) above, a ra t io of 1 2 is ob ta ined with a = 1

and D= on W{001 }

and b = 0 92, which Implies tha t s tends to unity as 0 ~ g / 2 The funct ional dependence is shown in Figure 3, together with the da ta of Stelnbruchel and Schmldt ¢ for H 2 on W{001 } In this case there appears to be an irreconcilable discrepancy with their da ta A higher So±/(S)o rat io implies a different funct ional dependence of s o f rom that suggested by equa t ion (2) The contras t between N 2

and H 2 may reflect different adsorp t ion mechanisms for these two gases Based on an analysis of available experimental data, King 1 has suggested tha t H E adsorp t ion on Wl0011 is domina ted by direct transfer at an incident site to the &ssoclatively chemlsorbed fl state, wi thout t rapping in a precursor state In the absence of the ' scrambling ' influence of the precursor state, it would therefore appear tha t direct ional effects are impor t an t We would again predict, by the principal of microscopic reversibility, a ' squashed ' cosine law desorp t lon dis t r ibut ion for this system

The absence of an isotope effect in the sticking probabi l i ty of H 2 and D E on W{001 } is ano ther impor t an t indicator of adsorp t ion mechanism If sticking were domina ted by p h o n o n transfer to the lattice, a s t rong mass dependence for the incident particle might be expected, since the sticking probabi l i ty would be a sensitive funct ion of the mass rat io ~ On the other hand, if sticking IS domina ted by electron-hole pairing, as has been recently suggested20 22, the ISOtope effect would be small, and domina ted by the different mean velocities of H 2 and D : The observed increase in s with incidence angle for this system is also consistent with electron-hole pmrlng, since the impact t ime is increased at grazing incidence

References

i D A King, Sohd St Mat Sct, 7, 167 (1968) 2 D A King and M G Wells, SurfScz, 29, 454 (1972) a D A King and M G Wells, Proc R Soc, Lond, A339, 245 (1974) 4 C S Stelnbruchel and L D Schmldt, Phys Ret B10, 4209 (1974), Phys Rel Lett, 32, 11 (1974) 5 R G Gale, M Salmeron and G A Somorjal, Phys Ret Lett, 38, 1027 (1977) 6 M A Morns and D A King, Vacuum, 30, 23 (1980) 7 T E Madey, SurfScl, 36, 281 (1973) 8 D A King and G Thomas, SurfScl, 92, 201 (1980) 9 R Jaeger and D Menzel, SurJace Scl, 100, 581 (1980) 10 p W Tamm and L D Schmldt, J Chem Phys, 51, 5352 (1969) 11 p W Tamm and L D Schmldt, J Chem Phys, 55, 4253 (1971) 12 W Steckemacher, Vacuum, 30, 261 (1980) 13 K Grlffiths and D A King, Proc 4th Int ConJ Sohd Surfaces, Suppl Le Vide, 201,237 (1980) 14 L C Feldman, R J Sllverman and ] Stensgaard, SurfScl, 87, 410 0979) is K C Janlda, J E Hurst, C A Becker, P J Cowln, L Wharton and D J Auerbacher, SurfSc4 93, 270 (1980) 16 S P Slngh-Boparal, M Bowker and D A King, SurJSct, 53, 55 (1975) 17 F C Hurlbut, J Appl Phys, 28, 844 (1957) l a g Comsa, Proc 7th Int Vac Congr and 3rd Int Conf Sohd SurJaces, Vienna, 1977, p 1317 19 R C Cosser, S R Bare, S M Franos and D A King, Vacuum, 31,503 (1981) 20 G P BrlVlO and T B Grlmley, SurfSct, 89, 226 (1979) 21 j K Norskov and B I Lundqvlst, SurfSct, 89, 251 (1979) 22 R Brako and D M Newns, Solid State Commun, 33, 713 (1980), D M Newns, Vacuum, 31,685 (1981)

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