Laser-fluorescence study of the reactions of alkaline earth atoms with BrCN: Spectroscopic observation of the alkaline earth monocyanides

Abstract

The reactions of M+BrCN, where M=Ca, Sr, and Ba, have been studied by laser‐induced fluorescence. The dominant reaction pathway is the formation of the alkaline earth monocyanides (MCN); for the Sr and Ba reactions weak features due to products of the other possible pathway, MBr and CN, are also observed. This study represents the first spectroscopic observation of the MCN molecules. The dominant features of their excitation spectra are broad ’’quasicontinua’’ several hundred angstroms wide which occur at the same wavelengths as the spectra of the homologous alkaline earth monohalides (MX). Thus, the electronic structure of the MCN molecules is qualitatively similar to that of MX, which has been adequately explained by an ionic model, as discussed previously by Dagdigian, Cruse, and Zare. Radiative lifetimes of the MCN electronic states have been determined by direct observation of the fluorescence decay after pulsed laser excitation. The lifetimes are: CaCN: B state 40.8±1.5 nsec, C state 165±38 nsec; SrCN: B state 51.2±6.2 nsec, C state 104.4±6.3 nsec; and BaCN: C state 229±13 nsec, where the quoted uncertainties represent three standard deviations. The MCN and MX B state lifetimes are similar in magnitude, but the MCN C state lifetimes are considerably longer than the homologous MX C ²Π lifetimes. One possible interpretation is that the monocyanide excited C states, unlike those of the halides, are not simply described by one electron excitations into nonbonding metal atom orbitals but also include some excited state CN⁻ character.