ON THE INFRARED-SPECTROSCOPY OF SIF4 AND SF6 IN AR CLUSTERS - LOCATION OF THE SOLUTE

We have measured the infrared photodissociation spectra of argon clusters containing SiF4, as a function of the cluster size n (for n≤ 103) using molecular beam laser spectroscopy. The clusters were produced by both the conventional seeded expansion of a dilute mixture and by a "pickup" method where, upon colliding with it, the chromophore sticks to the surface of a cluster made in a neat solvent expansion. Furthermore, the spectra of small SF6/Arn clusters (n≤50) have been remeasured with the improved resolution resulting from the use of two line and tunable isotopic CO2 lasers. These data, together with previously published data on SF6/Ar, indicate a remarkably different behavior for these two solute-solvent combinations. The preferred "site" for SiF4 is at the surface of Ar clusters of all sizes, regardless of how the molecule is introduced to the cluster, while appreciable amounts of SF 6 reside at the surface only when the cluster is large and the impurity is deposited onto the cluster surface. The behavior of SiF4 and SF6, together with the analogous behavior of other polyatomic chromophores, the IR spectra of which have been measured and reported previously [D. J. Levandier, M. Mengel, R. Pursel, J. McCombie, and G. Scoles, Z. Phys. D 10, 337 ( 1988); D. J. Levandier, S. Goyal, J. McCombie, B. Pate, and G. Scoles, J. Chem. Soc. Faraday Trans. 86, 2361 (1990)], can be rationalized in terms of molecular dynamics simulations of similar systems which are presented in the paper by Perera and Amar [L. Perera and F. G. Amar, J. Chem. Phys. 93, 4884 (1990)]. The combination of the theoretical and experimental results confirm the usefulness of infrared photodissociation spectroscopy for the study of the structure of clusters and suggest that assuming any particular location for an impurity in a cluster in the absence of experimental evidence or, at least, a dynamics calculation, can easily lead to wrong conclusions. © 1990 American Institute of Physics.