Diffraction and vibrational dynamics of large clusters from He atom scattering

Abstract

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...

Structure and vibrational spectra of methanol clusters from a new potential model

“…͑The steepness of its repulsive part may be attributed to the simple R Ϫ12 representation, the exaggerated well depth stems from the overestimated dipole moment.͒ It is noted that SCF calculations for methanol clusters from dimer to hexamer were also performed 21,22 which are discussed elsewhere. 7 In order to determine the repulsion parameter K of Eq. ͑4͒ we have calculated the second virial coefficient B(T) and fitted it to experimental data from measurements of the excess molar enthalpy of methanol-nitrogen.…”

“…In addition, the potential consists of repulsion, dispersion, and non-additive induction terms. The cluster approach was chosen to calculate the spectral line shifts using the new intermolecular potential and a simplified intramolecular force field by Schlegel et al 7,23 The outline of the paper is as follows: In Sec. II we give a description of the interaction potentials and results for the second virial coefficient.…”

“…18 It is noted that the new experimental results up to the nonamer will be presented in a forthcoming paper. 19 All calculations for methanol clusters based on separated intra-and intermolecular potentials [7][8][9] were carried out using the Optimized Potential for Liquid Simulation ͑OPLS͒ by Jorgensen 20 for the intermolecular part. While a qualitative agreement between measured and calculated data was found a͒ Present address: Department of Chemistry, University of Nottingham, Notthingham NG7 2RD, England.…”

“…Recently we have described an approach for the evaluation of frequency shifts of molecular clusters in various approximations. 7 The basic idea is to construct a ͑total͒ cluster potential using separated intra-and intermolecular potential models. Then the normal mode analysis based on the harmonic oscillator is applied.…”

Determination of Ag submonolayer structures on Pt(111) from simulation and data of He scattering experiments

“…As with most other diffraction techniques, the He diffraction data are related to a wide extent of the surface and are not directly invertible [10][11][12][13][14]. The TEAS data are bound to the correlation of the positions of the adlayer atoms in a wide range of distances [15,16] and are presented without a quantitative interpretation in most of the literature.…”