The role of the rotational state in intermolecular interactions of H2O with H2 and CH3Cl

Abstract

Rotational-state resolved measurements of H2O−H2 and H2O−CH3Cl intermolecular interactions were performed. Using light-induced drift as a tool, we measured changes in transport collision ratev upon rovibrational (J, r)-excitation of H2O. We studied P- and R-branch excitation withJ ranging from O through 9 for H2O excited in the fundamental asymmetric stretch mode. Combination of P and R data yields the dependencies ofv upon rotational (J) and vibrational (r) quantum numbers separately. For H2O−CH3Cl it is found thatv decreases by 25% asJ increases from 0 to 9. For H2O−H2 the decrease is only 1.0%. These data seem to exemplify a fundamental aspect of dipole-dipole interaction: the familiar 1r3 interaction term is highlyJ-dependent. This is attributed to the increasing averaging-out of the dipolar potential as the rotational quantum number increases.