CN photodissociation and predissociation chemical lasers: Molecular electronic and vibrational laser emissions

Abstract

Molecular electronic [CN*(A²Π_3/2, v′ = 0) → CN(X²Σ⁺, v″ = 0,1,2)] and vibrational [CN^† (X²Σ⁺, v′) → CN(X²Σ⁺, v′ − 1), v′ = 5,4,3,2] cyanide radical laser emissions have been observed following photodissociative and predissociative fragmentation of cyanide parents [HCN, ClCN, BrCN ICN, (CN)₂, CH₃NC, CF₃CN, C₂F₅CN] at λ ≥ 1550 Å. The observed transitions and their relative gain coefficients and quenching behaviors are used in connection with spectroscopy and fluorescence experiments to formulate a state‐to‐state photochemical reaction mechanism, viz., $$\begin{array}{c}\mathrm{RCN}{\displaystyle {lim}_{\to }^{\mathrm{\hslash \hspace{0.17em}\omega }}}\mathrm{R+CN*}{\mathrm{(A}}^2{\Pi }_i\text{,v′=0}\mathrm{primarily}\mathrm{),}\\ \mathrm{CN*}{\mathrm{(A}}^2{\Pi }_i\text{,v′=0)+}\mathrm{M}\to {\mathrm{CN}}^{†}{\mathrm{(X}}^2{\Sigma }^{+}{\mathrm{,v}}^{″}\text{=4}\mathrm{primarily}\mathrm{)+M,}\end{array}$$ in which nascent photochemical products are formed with high electronic population inversion [N_CN*(A)/N_CN(X) ≫ 1] and no vibronic inversion within the A²Π_i manifolds (i.e., v = 0 is the principal product vibronic state). Near‐resonant collision‐induced intersystem crossing is shown to preferentially populate v = 4 within the X²Σ⁺ electronic manifold. Electronic and vibronic population inversions are treated via state‐to‐state adiabatic correlation diagrams and a simple dynamical model for reaction product vibronic state distributions. Anomalous laser behavior in the BrCN photodissociation system is rationalized via an intramolecular near‐resonant E → E′ energy transfer scheme.