Axis switching in the B 2A′–X 2A′ transition of HCO and fluorescence lifetimes of the B 2A′(0,0,0) rotational states

Fluorescence spectra of the B̃ 2A′–X̃ 2A′(0,0) band of thermalized HCO at 298 K were recorded. The lifetimes of nascent HCO B̃ 2A′(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 Ka=0, 1, and 2, respectively. The lifetimes of these states decrease as the Ka and N quantum numbers increase; they vary in the ranges 24–89 ns for Ka=0 states, 12–52 ns for Ka=1 states and 14–18 ns for Ka=2 states. An a-type Coriolis interaction between the B̃ 2A′ and à 2A″ states is proposed to account for predissociation of the low vibrational levels of B̃ 2A′. The spectral intensity shows anomalous behavior in that the intensity ratio of transitions QR0/QP0 is nearly 2; similar phenomena are observed for other branches. The mechanism of axis switching in the B̃ 2A′–X̃ 2A′ 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 X̃ 2A′ state from fluorescence spectra B̃ 2A′–X̃ 2A′.