Pressure Broadening and Pressure Shift of Diatomic Iodine at 675 nm

TL;DR

This study measures pressure broadening and shift coefficients for diatomic iodine at 675 nm, using a detailed line shape model and kinetic analysis to understand microscopic collision processes and their thermodynamic implications.

Contribution

It introduces a first-order perturbation line shape model linking microscopic interactions to macroscopic spectral features, and proposes a relationship between line shape asymmetry and microscopic irreversibility.

Findings

Pressure broadening and shift coefficients quantified for I2 near 675 nm.

A novel line shape model relates microscopic collision effects to spectral line features.

Thermodynamic constraints and non-Hermitian Hamiltonian implications discussed.

Abstract

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