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 PressLtd Angle-integrated sticking probabilities: N 2, and D 2 on W{O01} Heather Bickley, James S Arlow, M A Morris 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 58±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 measuring sticking probabfllUes at single crystal surfaces I For high sticking probabilities (0 1 ~<s~< 1) the reflection-detector techniques are the most accurate, where the reflection coefficient is measured by sampling the back-scattered 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 ~60 ° from normal, and generally very small variations have been noted 2 4, with the exception of H2 and D 2 on W~ll0) 4, where So is reported to decrease by ~60°~, over this range, and Hz/D 2 on stepped Pt{lll~j planes s, where the reactivity was highest when the incident beam was directed at the steps A comparison of angle- averaged sticking probabilities, (s), with normal incidence values, s~, from 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 adsorption 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 comparison For H2 and D 2 on W I001 I, there is some controversy in the literature concerning the initial sticking probability v'8, the coverage dependence of s 8 l o, and the existence of an isotope effect 7 8 10 tx ~4 We have attempted to resolve these disagreements in the present study 2. Experimental The principles and operation of the ultrahigh vacuum chamber used for measuring absolute sticking probabilities have been described elsewhere 6 Gas is introduced to the adsorption cell via a capillary, and a steady state pressure is maintained In the cell by continuous pumping The crystal is suspended in front of a tube connected to an ionization 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 mm A second 'dummy' tube in the adoption cell is directed at a glass disc, I e an inert surface, but is otherwise identical to the first, connected to a second ionization gauge, IG-2 If the sticking probability on the crystal is non-zero, the reflected flux from the crystal Into IG-1 is reduced compared to that into IG-2 The sticking probability is obtained by comparing pressures P1 and P2 measured on IG-1 and IG-2, respectively If the fraction of randomly distributed molecules in the adsorption 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/~ (P2-PO/P2 whence (s) is independent of the gauge cahbratlon factor An accurate evaluation of f is necessary Steckelmacher 12 has derived accurate formulae forffor various experimental configur- ations In the present work we have used an elhptlcally shaped crystal, with major axis 9 mm and minor axis 5 5 mm, for this geometry,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 temperature cycling between 400 and 1700 K in an oxygen pressure of 10-6 torr for several days (until no CO desorptlon peak was observed) and subsequently annealed m tacuo at 2500 K Temperatures were measured using a W/25°o Re-W thermocouple spotwelded to the side of the crystal, independently calibrated using an optical pyrometer 3. Results and discussion 3.1. N 2 on W[001}. Six Independent runs were performed to evaluate the variation of (s) with surface coverage, with the 507