Mechanism of short-pulse-induced solute migration in comparison to continuous-light-driven thermal diffusion

Abstract

The aim of this work is to clarify the mechanism of photo-absorption-caused solute migration at the microscopic level. Experimentally, we respectively measured the short-pulse-induced and continuous-light-driven migrations of chloroaluminum phthalocyanine molecules dissolved in ethanol at two concentrations, 4.2 × 10¹⁷ cm⁻³ (7.0 × 10⁻⁴ M) and 1.2 × 10¹⁷ cm⁻³ (2.0 × 10⁻⁴ M). Theoretically, by verifying that individual solute molecules in the concentrated solution, compared to those in the dilute solution, absorb more photo energy collectively but less photo energy individually, we consider solute migration as net movement of individual solute molecules and then sequentially analyse how individual solute molecules absorb photo energy, convert the absorbed photo energy into translational excess energy intra-molecularly and carry out movement. Subsequently, by summing up movement of individual solute molecules in a unit of volume, we deduce the solute migration behaviours which coincide with the experimental results: for short pulse excitation, solute migration is more/less in the concentrated solution depending on the pulse energy; for continuous light irradiation, solute migration is always more in the concentrated solution regardless of the light power. Note that, in our theoretical deduction, the short-pulse-induced and continuous-light-driven solute migrations differ in that the former proceeds before inter-molecular relaxation becomes apparent and the latter carries on with inter-molecular relaxation practiced sufficiently. Accordingly, the former is non-quasistatic and the latter is quasistatic and thus referred to as thermal diffusion.