Spectrum and Lifetime of NH(a¹Δ → X³Σ'⁻) in Inert Gas Matrices

Metastable NH(a 1 Δ) radicals were produced by the photolysis of matrix isolated HN 3 (HNCO) at 206.2 nm. The (0,0) and (0,1) bands of the forbidden transition to NH(X 3 Σ − ) were observed in emission in the rare gases and in nitrogen, yielding Franck‐Condon factors, Δ G 1/2 values, and matrix shifts. The line structure of the (0,0) band in Xe is due to nearly free rotation of the radical in the electronic ground state. Rotation in the other matrices is strongly inhibited or totally suppressed by the presence of the other photofragment N 2 in or near the cage. Lifetimes of NH(a 1 Δ) in Ne, N 2 , Ar, Kr and Xe are 335 ms, 365 ms, 375 ms, 790 ms and 1020 ms. The strong temperature dependence of the lifetime in Xe is in quantitative agreement with a theoretical model of multiphonon induced inter system crossing to NH(X 3 Σ − , ν = 4), indicating a fluorescence yield near 80% in the low temperature limit. The shorter lifetimes in the other matrices are believed to be due to enhanced molecule‐medium coupling by the nearby N 2 photofragment. Extrapolation of the Xe data yields an estimate of 3.3 s for the radiative lifetime of gaseous NH(a 1 Δ).