From femtoseconds to minutes: spectroscopic study of optically-induced thermal diffusion in aqueous solution of rhodamine B

Given that nonequilibrium molecular motion in thermal gradients is influenced by both solute and solvent, the application of spectroscopic methods that probe each component in a binary mixture can provide insights into the molecular mechanisms of thermal diffusion for a large class of systems. In the present work, we use an all-optical setup whereby near-infrared excitation leads to a steady-state thermal gradient in solution, followed by characterization of the nonequilibrium system with electronic spectroscopy, imaging and intensity. Using rhodamine B in water as a case study, we perform measurements as a function of solute concentration, temperature, wavelength, time, near-infrared laser power, visible excitation wavelength, and isotope effect. The results are complemented with non-equilibrium molecular dynamics simulations and analysis of infrared spectra and heat diffusion and stochastic dynamics simulations of the coupled Brownian/spectroscopic non-equilibrium dynamics. Overall, the results presented here exemplifies how spectroscopic probing of solute and solvent can be useful for understanding molecular mechanisms of optically-induced thermal diffusion in aqueous solution of rhodamine B.