A research program into phenomena related to gas-surface interaction was conducted over the last several years with the broad objective of answering the following general question: Given a stream of gas molecules of known properties approaching a specified surface, how does one predict the velocity and flux distributions of the streams which leave the surface. Beginning with very simple configurations, account has been taken of an increasingly detailed extended set of properties of the solid surface, of the gas stream and of the interaction process. The basic problem was formulated with a view to understanding molecular transport phenomena, first on inert metallic surfaces such as the skins of satellite vehicles in orbital flight, and later on ice crystals and nucleating centers in Brownian motion in the upper atmosphere. The experimental work during the present contract period included the development of a new scheme for measuring molecular velocity distribution using velocity spectrometry of electron beam excited metastable species. In the theoretical part of this research, a classical continuum model of the surface was constructed which successfully allows for a number of physical effects that are not predicted by lattice models.

The research conducted under contract F44620-76-C-0036 in the year 1978 is well described by the three papers presented at the XIth International Symposium on Rarefied Gas Dynamics. We submit them in lieu of an interim scientific report. (Author).

An experimental study of homogeneous and heterogeneous nucleation was conducted, using molecular beam techniques: mass spectrometry and time-of-flight velocity spectrometry, with and without mass filtering. Both the clustered beam itself and the scattering of the clusters from surfaces were studied. The beam was found to consist of a broad spectrum of clusters all having approximately the same velocity distribution. Small clusters were found to have a binding energy much less than that of bulk ice. Measurements of a beam scattered from a surface using a time-of-flight velocity spectrometer show four distinct patterns. These are: (1) a diffuse scattering channel (monomers, Maxwellian/cosine at the target temperature), (2) a direct inelastic channel for monomers characterized by a small loss of incident velocity with a broad spatial distribution peaked at the tangent for all incident angles, theta sub i, (3) a well-defined specular directed lobe (theta sub s = theta sub i greater than 50 deg) with particle velocity much slower than (1) or (2) and a velocity distribution narrower than a stationary Maxwellian, and (4) another slow peak with a slightly broader velocity distribution into a sharp intensity peak near the tangent for all theta sub i greater than 50 deg. The amplitude of the specularly directed slow peak (3) appears to give a sensitive indication of surface coverage. (Author).

There is considerable interest, both fundamental and technological, in the way atoms and molecules interact with solid surfaces. Thus the description of heterogeneous catalysis and other surface reactions requires a detailed understand ing of molecule-surface interactions. The primary aim of this volume is to provide fairly broad coverage of atoms and molecules in interaction with a variety of solid surfaces at a level suitable for graduate students and research workers in condensed matter physics, chemical physics, and materials science. The book is intended for experimental workers with interests in basic theory and concepts and had its origins in a Spring College held at the International Centre for Theoretical Physics, Miramare, Trieste. Valuable background reading can be found in the graduate-Ievel introduction to the physics of solid surfaces by ZangwilI(1) and in the earlier works by Garcia Moliner and F1ores(2) and Somorjai.(3) For specifically molecule-surface interac tions, additional background can be found in Rhodin and Ertl(4) and March.(S) V. Bortolani N. H. March M. P. Tosi References 1. A. Zangwill, Physics at Surfaces, Cambridge University Press, Cambridge (1988). 2. F. Garcia-Moliner and F. Flores, Introduction to the Theory of Solid Surfaces, Cambridge University Press, Cambridge (1979). 3. G. A. Somorjai, Chemistry in Two Dimensions: Surfaces, Cornell University Press, Ithaca, New York (1981). 4. T. N. Rhodin and G. Erd, The Nature of the Surface Chemical Bond, North-Holland, Amsterdam (1979). 5. N. H. March, Chemical Bonds outside Metal Surfaces, Plenum Press, New York (1986).