Observation of translationally hot, rotationally cold nitric oxide molecules produced by 193-nm laser vaporization of multilayer nitric oxide films

This Letter presents results for the rotational and spin-orbit state excitation of NO molecules which are ejected at hyperthermal velocities by 193-nm laser vaporization of cryogenic multilayer NO films condensed on MgF{sub 2}. For molecules with translational energies of 0.14 and 0.56 eV, the average rotational energies are 0.014 and 0.017 eV, corresponding to temperatures of 160 and 180 K, respectively. The spin-orbit population ratio F{sub 2}/F{sub 1} for the 0.14-eV molecules is 0.35 (T {approx} 170 K); however, the population ratio for the 0.56-eV molecules is higher, 0.7 (T {approx} 500 K). The disequilibrium between translation and rotation may be due in part to a collisional cooling mechanism (adiabatic expansion) which occurs immediately following vaporization. Other dynamical ejection mechanisms are also discussed.