Axis-switching transitions and the stimulated emission pumping spectrum of HCN

Abstract

Six of the 14 unidentified bands in the stimulated emission pumping (SEP) spectrum of HCN are shown to be forbidden transitions to l″=1 e parity levels of the ground state. The band origins agree with predictions within the error of the anharmonic expansion; the rotational constants, when corrected for rotational-l doubling, agree within experimental error. Rotational-l resonance between l″=0 and l″=2 is found in highly excited bending levels, confirming the extrapolation of the rotational-l resonance and doubling constant q2 from microwave and infrared measurements to 17 000 cm−1. The rotational intensity of the l″=1 bands due to the axis-switching mechanism of Hougen and Watson [Can. J. Phys. 43, 298 (1965)] is shown to be greater than some of the observed allowed rotational transitions to l″=2 when laser polarization effects are taken into account. A qualitative Franck–Condon analysis of the SEP spectrum provides unusually strong evidence for the axis-switching mechanism. The eight remaining unassigned bands are evidently perturbed and are assigned based on agreement between sums of observed rotational constants and sums of zero-order (unperturbed) rotational constants predicted by the anharmonic expansion, the magnitude of the rotational-l resonance, and the expected Franck–Condon factors.