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Pressure broadening and pressure shift coefficients for 127I2 (diatomiciodine) in the presence of various buffer gases were determined respectivelyfrom the line-widths and line-center shifts observed in Doppler-limited,steady-state, linear absorption spectra near 675 nm as a function of buffer gaspressure through a nonlinear regression analysis of observed line shapesagainst a Gaussian-Lorentzian convolution line shape model.  A line shape model, obtained from a first-order perturbation solution of thetime-dependent Schrodinger equation for randomly occurring interactions betweena two-level system and a buffer gas treated as step-function potentials,reveals a relationship between the ratio of pressure broadening to pressureshift coefficients and a change in the wave function phase-factor, interpretedas reflecting the "cause and effect" of state-changing events in themicroscopic domain. Collision cross-sections determined from this model areinterpreted as reflecting the inelastic nature of collision-inducedstate-changing events.  A steady-state kinetic model for the two-level system compatible with theBeer-Lambert law reveals thermodynamic constraints on the ensemble-averagestate-changing rates and collision cross-sections, and leads to the proposal ofa relationship between observed asymmetric line shapes and irreversibility inthe microscopic domain, and suggests that such features may be more fullyconsistent with the use of non-Hermitian Hamiltonians for describing thestate-changing process; in the context of the [non-degenerate] two-level model,the Hamiltonian that describes photon absorption is not the Hermitian conjugateof the Hamiltonian that describes photon emission.

Pressure Broadening and Pressure Shift of Diatomic Iodine at 675 nm