Knowledge Bank

The technique of high power microwave double $resonance1$ is used to study collision-induced transitions between rotational levels in $H2CO$, HDCO, HCN, DCN and $H2CCO.2$ 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. An approach by the use of steady state equations is developed and used for the analysis of the observed results. The following conclusions have been obtained. (1) The collision-induced transitions follow selection rules. For the molecules studied in this paper, the dipole selection rules $(\Delta J=0,\pm 1$, parity $+⟷-$) are dominant although not always by a very large factor. (2) The rate constants for the collision-induced transitions with $\Delta J=1$ are of the same order of magnitude as those with $\Delta J=0$, although molecules make transitions between more widely spaced energy levels in the former case. (3) For HCN, DCN, and $H2CCO$, rather large rate constants have been obtained for the $\Delta J=2$ transitions. This suggests that the collision-induced transitions with quadrupole-type selection rules or possibily those with even higher multipole-type selection rules also play a role in these molecules. The above technique is extended for the study of the $NH3$ molecules. An apparatus which is capable of measuring intensity variations of the absorption lines to within 0.2% has been constructed. A detailed analysis of the results is being carried out.