VARIATION OF DIPOLE-DIPOLE INTERACTION WITH ROTATIONAL STATE - EXPERIMENT AND THEORY

An experimental investigation of the rotational-state dependence of in termolecular interactions for dipole-dipole systems was performed with the aid of light-induced drift. Data are presented of the relative ch ange in collision rate upon excitation, Delta nu/nu, of HF with respec t to polar and nonpolar collision partners (CH3F, CH3Cl, HCl, OCS, and CH4, H-2, CO2, respectively). A continuously tunable color-center las er was used to rovibrationally excite HF in the fundamental vibrationa l band (upsilon=0-->1). Both P- and R-branch excitations were studied, with the rotational quantum number ranging from J=0 to 6. By combinin g Delta nu/nu for an pairs of P(J) and R(J-1) transitions, the separat e upsilon and J dependencies of the collision rate nu are obtained. It is found that for HF-CH3F nu decreases by more than 40%, and for HF-C H4 by only 4%, as J increases from 0 to 6. These data show that the fa miliar 1/r(3) dipole-dipole interaction is highly J dependent. We attr ibute this to the increased averaging-out of the dipole-dipole interac tion as the rotational quantum number increases. A theoretical treatme nt based upon the first Born approximation for the total cross section of two rotating dipolar molecules is proposed. Comparison between mea surements and theoretical results shows good agreement. (C) 1997 Ameri can Institute of Physics.