We present the theoretical framework necessary to describe inelastic collisions between Stark mixed Lgr;hyphen;doublet levels of a molecule in a1Pgr; electronic state, and the subsequent effect on fluorescence intensities. It is convenient to work with the spherical tensor moments of the excited state density matrix. For weak fields and short times after excitation the ratio of the fluorescence intensities for emission on lsquo;lsquo;forbiddenrsquo;rsquo; and allowed1Sgr;+larr;1Pgr; lines is predicted to vary linearly with time and quadratically with field strength. Simulation studies are presented, based on collisional parameters suggested by previous theoretical studies of inelastic collisions of molecules in Pgr; electronic states. The present article provides the foundation for the accurate interpretation of laser diagnostic studies in plasma environments, such as those described by Mandich, Gaebe, and Gottscho in the accompanying article.
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