Mechanism of short-pulse-induced solute migration in comparison to continuous-light-driven thermal diffusion
Yi-Ci Li1*, Li-Shu Lee1, Yu-Ting Kuo1, Chi-Chen Wang1, Po-Yuan Huang1, Cheng-I Lee2, Tai-Huei Wei1
1Department of Physics, Chung Cheng University, Chia-Yi, Taiwan
2Department of Life Science, Chung Cheng University, Chia-Yi, Taiwan
* presenting author:Yi-Ci Li, email:yclphy@gmail.com
The subject 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 solute migrations of ClAlPc/ethanol solutions (chloroaluminum phthalocyanine molecules dissolved in ethanol). The samples are prepared at two concentrations, 4.2*1017 cm-3 and 1.2*1017 cm-3. Theoretically, we verify 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. Therefore, we consider solute migration as net movement of individual solute molecules and then 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, the magnitude of the solute migration in the concentrated solution depends on the pulse energy; for continuous light irradiation, the magnitude of the solute migration is always higher in the concentrated than in the dilute solution. Note that, in our theoretical deduction, the short-pulse-induced and continuous-light-driven solute migrations are different in the fashion 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 a non-quasistatic and the latter is a quasistatic process and thus referred to as thermal diffusion.


Keywords: Z-scan, solute migration, thermal diffusion, ClAlPc