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We have determined the velocity distributions of individual quantum states of NO scattering from Ag( 111) at specific scattering angles Of using molecular beam techniques to control the incidence energy E j and angle OJ' We find that the mean energies of scattered species E f depend weakly on 0tat low collision energies, but become increasingly independent of this parameter as E[ approaches 1.0 eV. This is true for all final rotation states J. The previously reported insensitivity of the final kinetic energy to J is found to apply at all scattering angles, so that E f vs Of curves for high J fall only slightly below those for low J. This system is highly translationally inelastic at high incidence energies, with up to 55% of EI being lost to phonons at E j = 1.0 eV. Angular distributions are relatively insensitive to J at low E j • but for high Ei the peak flux is found to shift away from the surface normal as Ei increases. The effect of the surface temperature only becomes apparent at low incidence energies. A search fur supernumerary rotational rainbows reveals no discernible oscillations even for the lowest surface temperatures~ We believe that these supernumerary oscillations may be damped by "surface corrugation" effects for this system. Discussion focuses on the observed anticorrelation between kinetic energy transfer to phonons and to rotation, the extent to which parallel momentum is conserved in this system, and energy-angle scaling laws for energy transfer. In this latter case we show that energy transfer in this system scales approximately with the quantity E[ cos OJ, over the full range of conditions covered.

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...