FORBIDDEN TRANSITIONS INVOLVING HIGHLY EXCITED TORSIONAL STATES IN METHANOL-D1 AND CONFIRMATION OF OPTICALLY PUMPED FAR-INFRARED LASER LINES

Abstract `Forbidden' transitions involving highly excited torsional levels in the ground vibrational state of methanol-D 1 (CH 2 DOH) have been identified in the high resolution Fourier transform spectrum, which become allowed through `Fermi' interaction with the first excited OCD-bend state. This is the first time such transitions have been identified for any asymmetric–asymmetric molecule and represents a breakthrough in the assignments for torsional–rotational transitions of singly deuterated asymmetric methanol. This provides data for the modeling of the torsion-mediated interaction Hamiltonian. The results also confirm the assignment of far-infrared (FIR) emissions involving an upshifted pump transition to the OCD-bend state, the mechanism of the anomalous shift of which remained a mystery until now. In addition, comments have been made for the anomalous intensity of the OCD-bend transitions for different symmetry species.