A1

Surface Science 208 (1989) 1-12 1 North-Holland, Amsterdam

DEBYE-WALLER FACTOR FOR H e / P t ( l I 1) V. B O R T O L A N I , V. C E L L I *, A. F R A N C H I N I , J. I D I O D I , G . S A N T O R O

Dipartimento di Fisica, Uniuersith di Modena, 41100 Modena, Italy

K l a u s K E R N , B e n e P O E L S E M A a n d G e o r g e C O M S A

lnstitut flir Grenzfliichenforschung und Vakuumphysik der Kernforschungsanlage Jiilich, D-5170 Jiilich, Fed. Rep. of Germany

Received 7 June 1988; accepted for publication 24 August 1988

We report accurate measurements of the dependence of the specular reflectivity on surface temperature and on incident angle, for He scattering off Pt( l l l ) . The usual Debye-Waller formula with a Beeby correction fits the data, provided that the effective well depth is adjusted. We show that a full calculation of the Debye-Waller exponent also reproduces the data. The inelastic atom-surface interaction and the surface phonon spectrum used in the calculation are the ones that reproduce the time-of-flight data obtained in separate experiments. No further adjusta- ble parameters are needed.

Surface Science 208 (1989) 13-33 13 North-Holland, Amsterdam

P E R I O D I C P E R T U R B A T I O N S O F T H E O S C I L L A T O R Y C O

O X I D A T I O N O N P t ( l l 0 )

M. E I S W l R T H *, P. M O L L E R ** a n d G. E R T L

Fritz-Haber-lnstitut der Max-Planck-Gesellschaft, Faradayweg 4 - 6, D-IO00 Berlin 33, Germany

Received 4 August 1988; accepted for publication 22 September 1988

The catalytic oxidation of CO on a Pt(110) surface at low pressures operated under conditions (Po~, Pco, T) of isothermal, self-sustained kinetic oscillations was subject to periodic perturba- tions of the oxygen partial pressure with variable frequency and (small) amplitude. The large variety of the effects observed in the response of the system is classified according to the principles of the bifurcation theory for periodically perturbed chemical systems: Besides harmonic resonance, nine bands of subharmonic or superharmonic entrainment could be identified. Quasi- periodic oscillations were obtained between the entrainment bands and showed the effect of frequency pulling. Upon approaching an entrainment edge markedly prolonged relaxation behav- ior (critical slowing down) was observed. The qualitative differences to previous findings with Pt(100) can be traced back to differences in the mechanisms dominating spatial self-organization.