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.