Verifying thermodynamic equilibrium of molecular manifolds: Kennard-Stepanov spectroscopy of a molecular gas

Abstract

The degree of thermalization of electronically excited state manifolds of an absorber can be tested via optical spectroscopy. In the thermalized manifolds case, the ratio of absorption and emission is expected to follow a universal Boltzmann-type frequency scaling, known as the Kennard-Stepanov relation. Here we investigate absorption and emission spectral profiles of rubidium, caesium and potassium molecular dimers in high pressure argon buffer gas environment and study the effect of collisionally induced redistribution. We find that, despite the use of nonlinear excitation techniques, the ratio of absorption and emission well follows the Kennard-Stepanov scaling for a variety of molecular transitions. We conclude that the upper electronic state rovibrational manifold of the molecular gas is well in thermodynamic equilibrium. Further, we demonstrate an accurate, calibration-free determination of the gas temperature from the measured spectroscopic data.

Rubidium spectroscopy at high-pressure buffer gas conditions: detailed balance in the optical interaction of an absorber coupled to a reservoir

“…The Boltzmann-type universal frequency scaling is long known to be fulfilled for some dye molecules in liquid solution [6], and has also been observed in semiconductor systems [7]. We have demonstrated that the Kennard-Stepanov relation is fulfilled with a good accuracy for rubidium atoms in dense buffer gas environment [8] and in more recent work also for several molecular dimers, K 2 , Rb 2 and Cs 2 under similar conditions [9]. These measurements have demonstrated the validity of this thermodynamic scaling in the gaseous phase.…”

“…8, where the clear quadratic dependence is understood from the two-step excitation dynamics. The molecular Rb 2 (3(D) 1 Π u ) state can then be populated through the collision of rubidium Rb(6p) or Rb(5d) atoms with either ground state atoms or ground state rubidium molecules, see our earlier work [9] for details. Due to both the lower densities and the lower oscillator strength of the molecular transitions, the corresponding optical densities are significantly lower than those of the earlier described atomic transition measurements, thus suppressing reabsorption effects, even at higher cell temperatures.…”

“…8, where the clear quadratic dependence is understood from the two-step excitation dynamics. The molecular Rb 2 (3(D) 1 Π u ) state can then be populated through the collision of rubidium Rb(6p) or Rb(5d) atoms with either ground state atoms or ground state rubidium molecules, see our earlier work [9] for details. The data are taken at a temperature of 673 K for 150 bar argon buffer gas pressure.…”

Spectroscopy of high pressure rubidium-noble gas mixtures

“…5c. As the observed crossover point shifts to lower frequencies for smaller beam diameters, it seems that loss processes depend nonlinearly on intensity, as could well be explained by excitation to higher lying electronic rubidium states by energy pooling in the dense gas mixture [38,39].…”

Light Emission by a Thermalized Ensemble of Emitters Coupled to a Resonant Structure

“…This configuration may lead to collective effects such as superradiance . For a system of dye molecules in a solution, intramolecular relaxation takes place . For Nile Blue and Oxazine, this thermalization takes place in a time on the order of 0.4 ps .…”