AXIS-SWITCHING TRANSITIONS AND THE STIMULATED-EMISSION PUMPING SPECTRUM OF HCN

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.