State-to-state collisional excitation of NH 3 by He and H 2 studied in a laser crossed molecular beam experiment

Abstract

The astrophysically important inelastic collisions of ammonia with He and H 2, are studied in a crossed beam experiment. State-to-state cross sections for the excitation of both ortho and para NH 3 starting from their respective ground states 0 0 and 1 1 are presented. In the experiment a pulsed nozzle beam of NH 3, seeded in Ar, is crossed with a secondary beam of He or H 2. Both the initial cold beam and the collisionally excited NH 3 are detected with complete state selectivity by REMPI (resonance-enhanced multiphoton ionisation) via the B̃ state of ammonia. Initial state preparation is achieved by rotational cooling that yields a primary NH 3 beam that consists almost exclusively of the lowest quantum states of ortho and para NH 3. There are considerable differences between the experimental results and recent theoretical calculations on the collisional excitation of NH 3 by He and H 2, which demonstrates that direct state-to-state experiments are essential for the description of such complicated collision systems.