Microwave Studies of Collision‐Induced Transitions between Rotational Levels. VII Collisions between NH3 and Nonpolar Molecules

State-to-State Rate Coefficients for NH3-NH3 Collisions from Pump-Probe Chirped Pulse Experiments

The kinetics of rotational inelastic NH₃-NH₃ collisions are recorded using pump-probe experiments, carried out with a K-band waveguide chirped pulse Fourier transform microwave spectrometer, in which the population of one inversion doublet is altered by the pump pulse. Due to self-collisions, the resulting deviation from equilibrium propagates to other states and, thus, can be interrogated by probe pulses as a function of the pump-probe delay time. A clear hierarchy of the state-to-state collision processes is found and subsequently translated into propensity rules. State-to-state rate coefficients are estimated, first via an analysis of the kinetics, and then more robustly and accurately derived from the pressure-dependent measurements using a global fitting procedure.

Direct observation of product-pair correlations in rotationally inelastic collisions of ND3 with D2

We present a combined experimental and theoretical study of state-to-state inelastic scattering of ND₃(j = 1₁^-) with D₂ (j = 0, 1, 2, 3) molecules at collision energies around 800 cm^-1. Using a crossed molecular beam apparatus which employs the combination of Stark deceleration and velocity map imaging, we observe the correlated rotational excitations of both collision partners. For D₂, both elastic (Δj_D2 = 0), inelastic excitation (j = 0 →j = 2) and inelastic de-excitation (j = 2 →j = 0) processes are observed. For a number of final ND₃ states, inelastic channels in which D₂ is rotationally excited or de-excited appear surprisingly strong. The experimental results are in excellent agreement with the predictions from quantum scattering calculations which are based on an ab initio ND₃-D₂ potential energy surface.