Counterpropagating pulsed molecular beam scattering of NH3–Ar. I. State resolved integral cross sections

Abstract

A new approach to molecular beam scattering is described. The method uses counterpropagating molecular beam pulses to define a scattering geometry of cylindrical symmetry while resonance enhanced multiphoton ionization is applied for the state specific product detection. The simple correlation of laboratory and center-of-mass quantities allows a straightforward determination of differential cross sections from measured ion time-of-flight distributions. In addition, the short duration of the pulses causes a delay dependence of the scattering signal which is used as an additional control parameter to define the size of the scattering volume. The method is applied to the rotational excitation of NH3 in collisions with Ar at a collision energy of 158 meV. Delay and depletion studies yield an effective mean free path of 60 cm, confirming single collision condition. While parity averaged integral cross sections are determined for the para modification of NH3, fully state resolved integral cross sections are determined for o-NH3. The general behavior of the integral cross sections for both modifications is well described by an exponential energy gap law. Deviations of individual cross sections from the scaling law confirm the propensity for inelastic collisions with Δk=3. Transitions to parity levels, which are forbidden in the centrifugal sudden approximation, show significantly less intensity.