Translational and internal state distributions of NO produced in the 193 nm explosive vaporization of cryogenic NO films: Rotationally cold, translationally fast NO molecules

Abstract

We report the translational, rotational, and spin-orbit state distributions of fast NO molecules which are generated by the 193 nm laser vaporization of 30 K multilayer NO films. Rotational distributions in v=0 are obtained by laser multiphoton ionization for five different velocities ranging from 900 to 2200 m s−1, corresponding to translational energies ET=0.14 to 0.71 eV. In every case, the average molecular rotational energy is more than 10 times smaller than the component of translational energy normal to the surface. Average rotational energies 〈ER〉 range from 0.009±0.002 to 0.024±0.006 eV (with corresponding best fit temperatures, TRot =105 to 220 K). For the molecules with ET=0.14 and 0.22 eV, the spin–orbit population ratios are typically comparable with TRot. For higher translational energies, the typical spin–orbit ratios are larger than expected from TRot and increase to a value F2/F1 of 1.1±0.50.4 (statistical ratio) for NO molecules with ET =0.71 eV. In some cases, the rotational distributions have a non-Boltzmann, high J component. Preliminary investigations for molecules with ET=0.56 eV indicate that the vibrational distribution v=1/v=0 is 3±1% (T≊785 K). The rotations in v=1 are also cold (〈ER〉≊0.01 eV, TRot ≊130 K). The mechanism that causes the ejection of translationally fast, rotationally cold NO molecules is considered in terms of either a collisional cooling process following desorption or rotationally constrained desorption dynamics.