Microwave Studies of Collision-Induced Transitions between Rotational Levels. II. Observations in H2CO, HCN, and H2CCO

Abstract

The technique of high‐power microwave double resonance is applied to study collision‐induced transitions between rotational levels in H₂CO, HDCO, HCN, DCN, and H₂CCO. The K‐type doublets (or l‐type doublets) of these molecules for selected J levels have been ``pumped'' by high‐power microwave radiation and the decreases in the intensities of the absorption lines of K‐type doublets (or l‐type doublets) of other J levels have been observed. The ``preferred'' collision‐induced transitions previously observed for ethylene oxide have been more strikingly confirmed. Also multiple transitions with ΔJ≥2 have been observed. An approach by the use of steady‐state equations is developed and used for the analysis of the observed results. Relative values of the rate constants for various collision processes (such as the ΔJ=1 collision‐induced transitions versus the ΔJ=0 collision‐induced transitions or collision‐induced transitions with dipole selection rules vs collision‐induced transitions with quadrupole‐type selection rules) have been determined. The analysis has led to the following conclusions. (1) The collision‐induced transitions follow selection rules. For the molecules studied in this paper, the dipole selection rules (ΔJ=0, ±1, parity +↔—) are dominant although not always by a very large factor [see Conclusion (3) below]. (2) The rate constants for the collision‐induced transitions with ΔJ=1 are of the same order of magnitude as those with ΔJ=0, although, molecules make transitions between more widely spaced energy levels in the former case. This result is evidence which supports the idea of ``rotational resonance'' introduced by Anderson for explaining the pressure broadening of ammonia. (3) For HCN, DCN, and H₂CCO, rather large rate constants have been obtained for the ΔJ=2 transitions. This suggests that the collision‐induced transitions with quadrupole‐type selection rules or possibly those with even higher multipole‐type selection rules also play a role in these molecules.