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

Given that non-equilibrium 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 of the solvent leads to a steady-state thermal gradient in solution, followed by characterization of the non-equilibrium 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. We find that spectroscopic excitation of solute and solvent as a function of temperature can be used to understand the thermal diffusion signal. 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. The stochastic/spectroscopic model shows how near-infrared excitation of the solvent influences lifetime dynamics on the picosecond timescale as well as Brownian dynamics in real time. Overall, the results presented here exemplify 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.