Theory of Stark spectroscopy of molecules in 1Π electronic states: Coherence effects and quantum beats

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Abstract
A density operator formalism is used to describe the fluorescence of a molecule in a 1Π electronic state in a static electric field under both pulsed and cw excitation. Coherences can be created both between M levels as well as between the e and f Λ‐doublet levels. Explicit solution of the generalized master equation allows the development of general expressions for the excited state density matrix elements, under conditions where collisions, hyperfine structure, optical pumping, and optical saturation are ignored. Simple expressions are obtained for the fluorescence intensities, valid at high J and whenever the Stark shifts are significantly smaller than zero‐field Λ‐doublet splitting. A simulation study of the expected quantum beat patterns is reported based on the parameters reported by Mandich, Gaebe, and Gottscho [J. Chem. Phys. 8 3, 3349 (1985)] in their experimental study of BCl(A1Π). Attention is focused on the extent to which quantum beat effects will be obscured by the finite widths of the laser excitation pulse and detector response functions.