Microwave Studies of Collisionhyphen;Induced Transitions between Rotational Levels. V. ldquo;Selection Rulesrdquo; in NH3ndash;Rarehyphen;Gas Collisions

摘要

The technique of highhyphen;power microwave double resonance has been applied to the study of transitions between rotational levels of ammonia induced by NH3sngbnd;He, NH3sngbnd;Ar, and NH3sngbnd;Xe collisions. Altogether 127 fourhyphen;level systems have been examined for each of the three collisions. From the analysis of the observed results it is confirmed that theAthinsp;harr;thinsp;AandEthinsp;harr;thinsp;Etransitions are ldquo;allowedrdquo; while theAthinsp;harr;thinsp;Etransitions are ldquo;forbiddenrdquo; in accordance with the spin considerations. In addition, the ruleDgr;kthinsp;equals;thinsp;3n(n; integer) is found wherekis a signed value ofK. No obviousJor 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 theDgr;kthinsp;equals;thinsp;3transitions can be caused by the octopole momentOHgr;xxxthinsp;minus;thinsp;3OHgr;xyyof NH3. These considerations lead to the conclusion that NH3ndash;rarehyphen;gas collisions are fairly shorthyphen;range (strong) collisions which cannot be interpreted by a single term in the multipole expansion as has often been done.