Angular variations in the kinetic energy of scattered species are found to provide a useful probe of the transition between gas-surface scattering regimes, complementing angular flux distributions. As incidence energies exceed a few eV, these change from being consistent with scattering from an extended target to being more typical of scattering from individual atoms. Results are presented for the Xe/Pt(l 11) system and are supported by detailed trajectory calculations.PACS numbers: 79.20.Rf An understanding of the dynamics of energy transfer at the gas-surface interface is required for detailed modeling of many different chemical and physical phenomena associated with this interface. These range from the trapping and sticking of atoms and molecules at relatively low en- ergies [1], to sputtering, plasma etching, and implantation at hyperthermal energies [2], Such knowledge is also of value in the design of spacecraft [3] and thermonuclear fusion reactors [4]. Molecular-beam scattering techniques offer a powerful tool for probing such interactionsand have been employed to examine many different systems. However, most studies have concerned angular distributions of scattered species for relatively low incidence energies, providing only a limited picture of the scattering dynamics. Since angular distributions reflect both the static corrugation of the gas-surface potential and differential momentum transfer parallel and perpendicular to the surface, velocity measurements are required for unambiguous interpretation. While high-energy collisions have been recognized as qualitatively distinct from those at low energy for many years [5-12], there is little experimental data that directly relate to the transition between these regimes.In this Letter we report results for the scattering of Xe from Pt(lll) which clearly show that variations in the energies of scattered species with scattering angle can be used to characterize the degree of penetration. At low incidence energies, ZT, < 1 eV, we find that the energies after scattering, £/, decrease with increasing scattering angle Of in a manner approximately consistent with parallel momentum conservation. At high energies, E,-> 5 eV, the opposite trend is observed, with E/ increasing with increasing final angle, in a manner consistent with scattering from one or more individual surface atoms. These qualitative conclusions are supported by detailed trajectory calculations.The molecular-beam surface scattering apparatus and the experimental techniques appropriate to this study have been described elsewhere [12][13][14]. The mounting of the Pt(l 11) crystal is such that thescattering plane intercepts the (111) face close to the [121] azimuth. Contamination levels are below our Auger detection limits (1%), sharp LEED patterns are obtained, and He scattering gives a specular peak width indistinguishable from the in-

Velocities of NO molecules scattered from Ag(lll) have been measured as a function of rotational state for a wide range of incidence energies and angles. We find that increasing rotational excitation is accompanied by decreasing energy transfer to phonons. Results agree quantitatively with an extensive trajectory simulation employing a realistic multidimensional interaction potential, which shows that this correlation is mediated largely by the orientation angle of the colliding molecule. A simple kinematic model suggests that this behavior is a general feature of moleculesurface scattering.PACS numbers: 79.20. Rf, 68.35.Md An understanding of the dynamics of gas-surface collisions is a key requirement for any detailed model of the trapping or sticking of atoms and molecules at surfaces and is of fundamental importance to any comprehensive theory of gas-surface chemistry. Molecular-beam scattering experiments and detailed calculations have addressed many aspects of such interactions, 1 with particular recent interest in the dynamics of rotational excitation. 2 "* 21 However, it seems unlikely that such observations can be inverted to yield an accurate gas-surface potential. For example, experimental rotational state distributions for the NO/Ag(lll) system have been satisfactorily reproduced by use of widely different assumptions. 17 " 21 We believe that this lack of uniqueness can only be remedied by increasing the degree of state selection of experiments towards the ideal case where all velocities and quantum states of the incident and scattered molecules are fully defined. Thus we have begun a program of experiments aimed at characterizing more fully the collision dynamics of the NO/Ag(lll) system, which has already been the focus of considerable experimental 3 " 5 ' 22 and theoretical effort. 16 " 21 We report here the first results of a comprehensive study in which we have made highly state-selected measurements of scattered molecules for a wide range of incidence translational energies, E h and angles, B h from the surface normal and for various surface temperatures.The experimental apparatus and procedures have been described previously by U s 4 ' 5,22 ' 23 and by others io,i3,i4 an( j f u jj details are deferred to a future publication. 24 Briefly, a supersonic molecular beam of NO is directed at a carefully prepared Ag(lll) crystal held in an ultrahigh-vacuum chamber. Scattered NO molecules are detected by use of two-photon ionization in order to ionize selectively specific quantum states via one-photon resonances to the NO A l l+ state at around 225 nm. 25,26 A pulsed molecular-beam source provides 200-/xs-duration beam pulses with NO energies up to about 1.0 eV. The beam pulses are then further chopped by a high-speed chopper. Time-ofarrival distributions are obtained by scanning of the firing time of the laser with respect to the chopper opening. Figure 1 displays some typical time-of-flight distributions. The bottom trace here corresponds to direct detection of a beam of 0.912 eV translational ener...

The potential energy function for interaction of two argon atoms is determined as closely as possible by fitting a multiparameter potential function to experimental data on molecular beam scattering and second virial coefficients, and to the known long-range interaction coefficients. These data are consistent with a range of potential functions between which gas transport properties do not enable a clear choice to be made. In particular, gas viscosities calculated with any of the potential functions show deviations from experiment which are tentatively ascribed to unexpectedly large slip in the viscosity measurements. Third virial coefficients calculated with allowance for the triple-dipole dispersion three-body interaction are close to experimental values whichever function is used, suggesting that other nonadditive interactions must be small. Assuming this, the properties of crystalline argon a t O'K are used to specify a single potential function.Knowledge of intermolecular forces is basic to the understanding of the properties of matter, and the inert gases and argon in particular provide the best test of the adequacy of methods available for determining these forces. Since Guggenheim and McGlashanl demonstrated the arbitrariness of the widely used 12-6 potential there have been a number of studies of this question,2-8 none entirely conclusive. This paper describes an attempt to determine the pair potential for argon as closely as possible using experimental data which depend on interactions of pairs of atoms. Non-additive interactions are then considered in the light of third virial coefficients and crystal properties.Information on two-body interactions is available from second virial coefficients, molecular beam scattering measurements, and low-pressure gas transport properties. In addition the coefficient of R-6 in the long-range part of the interaction is known accurately and the coefficients of R-8 and R-10 have been calculated approximately.

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Brønsted's principle of congruence, according to which the excess thermodynamic functions of a liquid mixture of n-alkanes depend only on the average chain length in the mixture, is shown to apply also to the second virial coefficients of gaseous hydrocarbon mixtures.