Rotational Excitation of OH(A²Σ⁺,υ'=0) Resulting from Dissociative Collision of H₂O with Metastable Argon Atoms

Spectroscopic investigations were made of the rotational population distribution of OH( A 2 Σ + ,υ'=0) split from H 2 O by steady-state flowing argon afterglow at the pressures from 0.022 up to 12 Torr. In order to observe the distribution close to the initial rotational population distribution free from collisional deactivation, an improved apparatus was constructed and a flowing afterglow was obtained at the pressures of one order of magnitude lower than in the other works. The observed distribution can be expressed by a combination of two thermal population distributions. Effects of pressure on the distribution were examined and the rate constant for collisional rotational relaxation in A 2 Σ + ,υ'=0 was obtained as (7.9±3.5)×10 -12 cm 3 mol sec -1 for Ar. Mechanisms responsible for rotational excitation are discussed.