Axis switching in the (B)over-tilde(2)A'-(X)over-tilde(2)A' transition of HCO and fluorescence lifetimes of the (B)over-tilde(2)A'(0,0,0) rotational states

Fluorescence spectra of the (B) over tilde(2)A'-(X) over tilde(2)A'(0,0) band of thermalized HCO at 298 K were recorded. The lifetimes of nascent HCO (B) over tilde(2)A'(0,0,0) states produced from acetaldehyde photolyzed at a wavelength of 310.9 nm in a supersonic jet were obtained for rotational states to N=26, 24, and 18 for K-a=0, 1, and 2, respectively. The Lifetimes of these states decrease as the K-a and N quantum numbers increase; they vary in the ranges 24-89 ns for K-a=0 states, 12-52 ns for K-a=1 states and 14-18 ns for K-a=2 states. An a-type Coriolis interaction between the (B) over tilde 2A' and (A) over tilde 2A '' states is proposed to account for predissociation of the low vibrational levels of (B) over tilde(2)A'. The spectral intensity shows anomalous behavior in that the intensity ratio of transitions (Q)R(0)/P-Q(0), is nearly 2; similar phenomena are observed for other branches. The mechanism of axis switching in the (B) over tilde(2)A'-(X) over tilde(2)A' transition is because. the geometry is altered during the excitation; consequently the principal inertial axes are rotated to account for the intensity anomaly. Corrections for the lifetime variation and intensity anomaly are necessary to obtain accurate relative populations of the (X) over tilde(2)A' state from fluorescence spectra (B) over tilde(2)A'-(X) over tilde 2A'. (C) 1996 American Institute of Physics.