THE ELECTRONIC SPECTRA OF THE ALKALINE EARTH MONOCYANIDES: COMPARISON WITH THE MONOHALIDES

Abstract

We have observed the electronic spectra of the alkaline earth monocyanides by pulsed dye laser fluorescence. These molecules are produced by the reactions $M + BrCN \rightarrow MCN + Br$, where M $=$ Ca, Sr, and Ba. For the Sr and Ba reactions, we also observed weak bands due to MBr and CN. However, the dominant features of the fluorescence excitation spectra are broad quasicontinua several hundred Angstroms wide which occur in the wavelength regions of the analogous alkaline earth monohalides (MX). We assign these features to the MCN molecules since the chemical reactions are run under single-collision conditions. Moreover, this reaction pathway is the same as that inferred previously far the K and Cs + BrCN reactions from a crossed-beam study with a hot-wire $detector.^{1}$ We have also determined the radiative lifetimes of the MCN electronic states by direct observation of fluorescence decay. The lifetimes are as follows: [FIGURE]. The quoted errors are 3 standard deviations, and the labels B and C Indicate the analogous MX wavelength regions $(B = B^{2}\Sigma^{+}$ and $C = C^{2}\Pi)$. The CaCN B state spectrum also spanned the wavelength region of the CaX $A^{2}\Pi$ states, but we did not observe any significant variation in lifetime with wavelength. The MCN and MX B state lifetimes are similar in magnitude, but the MCN C state lifetimes are considerably longer than the analogous MX $C^{2}\Pi$ lifetimes. A possible interpretation is that unlike the $halides,^{2}$ the monocyanide excited states are not all simply described by one-electron excitations into various metal atom orbitals but also perhaps possess some excited state $CN^{-}$ character.