Variation of dipole–dipole interaction with rotational state: Experiment and theory

Abstract

An experimental investigation of the rotational-state dependence of intermolecular interactions for dipole–dipole systems was performed with the aid of light-induced drift. Data are presented of the relative change in collision rate upon excitation, Δν/ν, of HF with respect to polar and nonpolar collision partners (CH3F, CH3Cl, HCl, OCS, and CH4, H2, CO2, respectively). A continuously tunable color-center laser was used to rovibrationally excite HF in the fundamental vibrational band (v=0→1). Both P- and R-branch excitations were studied, with the rotational quantum number ranging from J=0 to 6. By combining Δν/ν for all pairs of P(J) and R(J−1) transitions, the separate v and J dependencies of the collision rate ν are obtained. It is found that for HF–CH3F ν decreases by more than 40%, and for HF–CH4 by only 4%, as J increases from 0 to 6. These data show that the familiar 1/r3 dipole–dipole interaction is highly J dependent. We attribute this to the increased averaging-out of the dipole–dipole interaction as the rotational quantum number increases. A theoretical treatment based upon the first Born approximation for the total cross section of two rotating dipolar molecules is proposed. Comparison between measurements and theoretical results shows good agreement.