AXIS SWITCHING IN THE $\bar{B}^{2}A'-\tilde{X} ^{2}A'$ TRANSITION OF HCO AND FLUORESCENCE LIFETIMES OF THE $\bar{B}^{2}A'(0,0,0)$ ROTATIONAL STATES
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Date
1996
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Ohio State University
Abstract
Fluorescence spectra of the $\bar{B}^{2}A^{\prime}-\tilde{X} ^{2}A^{\prime}(0,0)$ band of thermalized HCO at 298 K were recorded. The lifetimes of nascent HCO $\bar{B}^{2}A^{\prime}(0,0,0)$ 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. These states have lifetimes decrease as the $K$ and $N$ quantum numbers increase. The measured lifetimes vary in the ranges 24-89 ns for $K_{a} = 0^{\circ}$ 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 $\bar{B}^{2}A^{\prime}$ and $\bar{A}^{2}A^{\prime}$ states is proposed to be the major mechanism to account for predissociation of the low vibrational levels of $\tilde{B}^{2}A^{\prime}$. The spectral intensity shows anomalous behavior in that the intensity ratio of transitions $^{Q}R_{0}/^{Q}R_{0}$ is nearly 2; similar phenomena are observed for other branches. The mechanism of axis switching in the $\tilde{B}^{2}A^{\prime}-\tilde{X}^{2}A^{\prime}$ transition, i.e. the geometry change during the excitation, accounts for the intensity anomaly. Corrections for the lifetime variation and intensity anomaly are necessary to obtain accurate relative populations of the $\tilde{X}^{2}A^{\prime}$ state from fluorescence spectra $\tilde{B}^{2}A^{\prime} -\tilde{X}^{2}A^{\prime}$.
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Author Institution: Department of Chemistry, National Tsing Hua University