Microwave Studies of Collision-Induced Transitions between Rotational Levels. V. “Selection Rules” in NH3–Rare-Gas Collisions

The technique of high‐power microwave double resonance has been applied to the study of transitions between rotational levels of ammonia induced by NH₃–He, NH₃–Ar, and NH₃–Xe collisions. Altogether 127 four‐level systems have been examined for each of the three collisions. From the analysis of the observed results it is confirmed that the $\mathrm{A\hspace{0.17em}\leftrightarrow \hspace{0.17em}A}$ and $\mathrm{E\hspace{0.17em}\leftrightarrow \hspace{0.17em}E}$ transitions are “allowed” while the $\mathrm{A\hspace{0.17em}\leftrightarrow \hspace{0.17em}E}$ transitions are “forbidden” in accordance with the spin considerations. In addition, the rule $\text{Δk\hspace{0.17em}=\hspace{0.17em}3n}$ ($n$ ; integer) is found where $k$ is a signed value of $K$ . No obvious $J$ or parity selection rules are found. A study of the symmetry of the collisional Hamiltonian leads to an understanding of these rules. It is shown that the $\text{Δk\hspace{0.17em}=\hspace{0.17em}3}$ transitions can be caused by the octopole moment ${\Omega }_{\mathrm{xxx}}{\text{\hspace{0.17em}−\hspace{0.17em}3Ω}}_{\mathrm{xyy}}$ of NH₃. These considerations lead to the conclusion that NH₃–rare‐gas collisions are fairly short‐range (strong) collisions which cannot be interpreted by a single term in the multipole expansion as has often been done.