Collisional relaxation of single rotational states in highly vibrationally excited acetylene

The combination of vibrational overtone excitation with time-resolved laser induced fluorescence detection of the excited molecule permits the study of collisional energy transfer in highly vibrationally excited molecules with single quantum state resolution. We apply this new technique to aceytlene excited in the region of the second CH stretching overtone transition (3νCH) (εvib =9640 cm−1) and probed via the à electronic state. Our results show that self-relaxation proceeds at essentially the gas kinetic collision rate while quenching by the rare gases He, Ar, and Xe is only about a factor of two slower. The insensitivity of the relaxation rate to the structure of the collision partner clearly points to rotational relaxation or intramolecular vibrational energy transfer as the mechanism for collisionally depopulating the initially prepared state.