Radiative lifetimes of the alkaline earth monohalides

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Abstract
The radiative lifetimes for a number of electronic states of the alkaline earth monohalide molecules MX, where M denotes Ca, Sr, or Ba and where X denotes F, Cl, Br, or I, have been determined directly from the rate of fluorescence decay with time using a pulsed dye laser as an excitation source. The measured lifetimes in nanoseconds are CaF: A1 21.9(4.0), A2 18.4(4.1), B 25.1(4.0); CaCl: A1 29.4(1.8), A2 28.4(2.6), B 38.2(3.9), C 25.0(2.1); CaBr: A1 34.2(2.9), A2 33.7(1.9), B 42.9(3.1), C1 33.2(4.0), C2 31.8(3.7); CaI: A1 41.7(2.3), A2 41.6(3.4), B 50.9(1.7); SrF: A1 24.1(2.0), A2 22.6(4.7); SrCl, A1 31.3(2.7), A2 30.4(3.6), B 38.8(2.1), C1 26.1(1.9), C2 26.0(1.6); SrBr: A1 34.3(2.3), A2 33.2(1.6), B 42.2(1.6), C1 30.3(2.6), C2 28.1(1.3); SrI: A1 43.3(1.6), A2 41.9(1.3), B 46.0(2.0), C1 36.0(3.7); BaF: C1 23.8(2.3), C2 23.5(0.7); BaCl: C1 17.5(0.8), C2 16.6(0.6); BaBr: C1 17.9(1.0), C2 16.5(1.4); BaI: C1 17.9(1.9), C2 16.0(1.5), where A1, A2, B, C1, and C2 denote, respectively, the A 2Π1/2, A 2Π3/2, B 2Σ+, C 2Π1/2, and C 2Π3/2 states, and where the uncertainties, representing one standard deviation, are given in parentheses. The lifetimes of the CaX and SrX states are seen to be quantitatively similar, while those of BaX are quite different. No vibrational dependence of the lifetime for the BaI C2 state is observed for 0 ≤ ν′ ≤ 39. For other MX molecules the vibrational dependence of the lifetime could not be ascertained because the MX molecule was not produced in a sufficiently wide range of ground state vibrational levels. Various one‐electron models for the MX transitions are discussed. It is found that a consistent explanation of the transition moments can be obtained by assuming that the transition is between two nonbonding orbitals centered on the metal atom whose mixing coefficients in terms of a truncated metal atom basis set are adjusted to reproduce a subset of the experimentally determined transiton moments. For the X, A, and B states of CaX and SrX, the nonbonding orbital is primarily n s σ, n p π, and n p σ, respectively, in character; for BaX it is suggested that (n − 1)d π and (n − 1)d σ play an important role in the description of the nonbonding orbital in the A, B, and C states.