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.

We find that the reduction in dielectric response (depolarization) of water caused by solvated ions is different for H_{2}O and D_{2}O. This isotope dependence allows us to reliably determine the kinetic contribution to the depolarization, which is found to be significantly smaller than predicted by existing theory. The discrepancy can be explained from a reduced hydrogen-bond cooperativity in the solvation shell: we obtain quantitative agreement between theory and experiment by reducing the Kirkwood correlation factor of the solvating water from 2.7 (the bulk value) to ∼1.6 for NaCl and ∼1 (corresponding to completely uncorrelated motion of water molecules) for CsCl.

In this work we study the effect of caffeine and taurine on the mobility of water molecules at 298 K using femtosecond mid-infrared and dielectric relaxation spectroscopy. We observe both molecules to have a slowing down effect on the mobility of surrounding water molecules: a single caffeine molecule slows down ∼9 water molecules, a single taurine molecule slows down ∼4 water molecules. The reorientation time constant of these slow water molecules is 4-5 times longer than the reorientation time constant of 2.5 ps of water molecules in bulk liquid water.