On the El-E2 Labeling of Energy Levels and the Anomalous Excitation of Interstellar Methanol

Abstract

The notation for torsion-rotation energy levels of methanol is reviewed, and it is emphasized that the levels previously labeled as Ex and E 2 do not form separate noninterconverting stacks when both positive and negative values of the rotational quantum number k are taken into account. Dropping of the Ex-Ez notation in favor of the use of a signed quantum number k is therefore recommended. Experiments are described demonstrating explicitly the transfer of molecules from E x to E 2 levels by one or more allowed electric-dipole transitions. The implications of Ex-Ez conversion for anomalous excitation of interstellar methanol are discussed, and it is pointed out that if A/c = 0 collision-induced transitions are preferred strongly in CH3OH-H2 collisions, as was proposed from microwave double-resonance studies, then it is likely that the 5_i-> 4 0 and 4_i 3 0 E transitions detected in Sgr B2 are inverted.