In a crossed molecular beam arrangement helium atoms are scattered from argon clusters which are produced in an averaged size range of n̄=6 to n̄=90 by adiabatic expansion through sonic and conical nozzles. The diffraction oscillations in the total differential cross section are used to derive information on the size distribution of the clusters by comparison with quantum mechanical calculations based on a model potential. In the size range covered by the measurements, the average cluster size is given by n̄=38.4(Γ*/1000)1.64, where Γ* is the scaling parameter of the source conditions introduced by Hagena [Z. Phys. D 4, 291 (1987)]. The results are in agreement with recent measurements of corrected mass spectra but disagree with the results obtained from electron diffraction. General relations are recommended which connect the scaling parameter with the averaged size.

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In a crossed molecular beam arrangement helium atoms are scattered from ammonia clusters of the averaged sizes n̄=18, 745, and 1040 which are generated by isentropic expansions with conical nozzles. The inelastic energy transfer is detected by time-of-flight analysis of the scattered helium atoms with a resolution of less than 5 meV at a collision energy of 95 meV. The energy transfer increases with increasing deflection angle and extends to 65 meV. Intensity maxima are observed between 11 and 16 meV, around 27 meV, and at 33 meV. The one in the middle is attributed to the vibration of a specific cluster network, while the other two occur in the energy regime of the translational or librational modes of the solid with a preference for small or large clusters, respectively.

A helium atom scattering study of the structure and phonon dynamics of the ice surface

Abstract. The vibrational dynamics of large Arn clusters from n=30 to n=4500 is investigated by measuring the energy loss of He atoms in a high resolution scattering experiment. The clusters are generated by adiabatic expansion through conical nozzles and contain a distribution of cluster sizes. The He supersonic nozzle beam provides a resolution of better than 1 meV. The results are compared with calculated spectral density functions for single cluster sizes and bulk phonon spectra. PACS: 36.40+dvery interesting surface properties [2]. In the present paper we report first measurements of He atom scattering from large Arn clusters in the range from n = 30 to 4500. There are mainly two different types of observables: (1) The angular distributions, which contain among other things information about the geometry and the size of the investigated objects through the diffraction oscillations. (2) The inelastic energy transfer, which is related to the excitation of the vibrational modes and which is measured by time-of-flight analysis of the scattered He atom. In this contribution we will mainly concentrate on the latter one. A preliminary account of the results appeared in [3]. I n t r o d u c t i o nThe knowledge of the frequency spectrum of a cluster plays an important role both for the interpretation of static and structural properties as well as for the dynamical behavior. This includes such quantities as the thermal free energy and the specific heat, the melting and condensation processes, and the excitation or deexcitation of the vibrational modes [1]. The latter process is of special interest, since it is very sensitive to the transition of the cluster from the discrete spectrum of vibrational modes of the molecular system to the lattice vibrations and the continuous phonon dispersion curves of the solid. The theoretical methods to study these topics reach from the classical normal mode analysis over molecular dynamics simulations to complete quantum calculations of the lattice dynamics. The experimental methods include IR-and Raman-spectroscopy for the molecular systems and neutron, electron or He atom scattering for probing bulk and surface phonons, respectively. From all these methods the scattering of He atoms appears to be the most general process to study the vibrational spectra of clusters, since this method is mainly sensitive to surface properties and obeys nearly no selection rules. In the last 10 years it has been developed into a very successful and reliable method for measuring surface phonons of the solid and for deriving a series of ExperimentalThe experiments have been carried out in a crossed molecular beam machine which is described elsewhere [4]. Essentially it consists of a He supersonic nozzle beam and a cluster beam for the target which intersect at an angle of 90 °, and a detector with an electronbombardment ionizer and a quadrupole mass filter operating under ultrahigh vacuum conditions. The angular dependence is measured by rotating the source assembly relative to the fixed detector posi...

Vibrational excitation of an Ar55 cluster in collisions with He (Ekin=25 meV) is investigated by using classical trajectories and a highly approximate quantum mechanical method (vibrationally sudden approximation). The energy transfer from the helium atom to the cluster (ΔE) is calculated as a function of the scattering angle θ. It is found (i) that predominantly the modes corresponding to the cluster atoms in the outer shell are excited and (ii) that the probability for multi-phonon (Δn⩾2) excitations steadily increases with the scattering angle. The results of both sets of calculations are generally in good agreement with measured energy loss spectra over the entire range of scattering angles. In the region θ≈15°–30°, which—because single-phonon transitions dominate—is most important for determining the frequency distribution of the cluster, the quantum mechanical calculations are superior to classical mechanics; the latter fails to conserve zero-point energy and therefore leads to unrealistic energy transfer below ΔE≲2 meV.

In crossed molecular-beam experiments, three type of cross sections have been measured: Total differential cross sections with well-resolved diffraction oscillations for oD2–NH3 at E=95.9 and 111.3 meV, differential energy-loss spectra for ND3–oD2 at E=118.3 meV which cover the center-of-mass (c.m.) angular range from 85° to 170° and are obtained by time-of-flight (TOF) analysis, and state-to-state integral cross sections for oNH3–H2 and pNH3–H2 at E=75 meV for many final rotational states which are detected by resonance enhanced multiphoton ionization. These data which are mainly sensitive to the anisotropy of the potential energy surface are well reproduced by quantum calculations in the coupled-states approximation. The potential is constructed by combining large basis-set self-consistent-field (SCF) calculations with damped long-range dispersion coefficients. The two free parameters of the damping function are fitted to a restricted set of configuration interaction (CI) calculations.

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