  The degree of thermalization of electronically excited state manifolds of an
absorber can be tested via optical spectroscopy. In the thermalized-manifold
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, cesium,
and potassium molecular dimers in a 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.
