Microwave Studies of Collision-Induced Transitions between Rotational Levels. VII Collisions between NH3 and Nonpolar Molecules

Abstract

The technique of high-power microwave double resonance has been used to study rotational transitions induced in NH3 by collisions with various nonpolar gases. The nonpolar gases used were H2, para-H2, HD, D2, O2, N2, CH4, and SF6. Most of the observed collision-induced transitions were found to be those obeying dipole selection rules (ΔJ = 0, ± 1, ΔK = 0, parity + ↔ −) but several weaker transitions with ΔJ = /mp/ 2, ΔK = 0, parity + ↔ +, − ↔ − were also observed. Thus the “selection rules” for the rotational transitions in NH3 induced by collisions with nonpolar gases are similar to those for NH3–NH3 collisions, but very different from those for NH3–rare-gas collisions. Collision-induced transitions with ΔK ≠ 0 or Δ ≥ 3 which are observed clearly in NH3–rare-gas collisions have not been observed here. The “selection rules” give us information concerning the symmetry of the dominant intermolecular interaction. The observed values of relative variations of the signal, ΔI / I, give us relations among rate constants for various transitions. The dependence of ΔI / I on spin species (H2 and para-H2) and isotopic species (H2, HD, and D2) are discussed, and several simple unanswered questions raised by the observations are summarized.