| Summary: | 博士 === 國立中正大學 === 物理學系暨研究所 === 101 === Using chloroaluminum phthalocyanine molecules (C32H16AlClN8) dissolved in ethanol, dubbed as ClAlPc-ethanol, as an example, we investigate the mechanism of solute migration induced by 632.8 nm CW laser light and 532 nm 19 picosecond (ps) laser pulses.
In the study with CW laser light, we measure the transmittance and deflection of the beam normally incident on the sample, prepared to have two concentrations (4.21017 cm3 and 1.21017 cm3), with the entrance surface parallel and normal to the optical table respectively. In the former geometry, we find a monotonous increase of transmittance with time and no transmitted beam deflection. When the higher-concentrated solution shows a temperature gradient larger than the lower-concentrated one, we find that it shows a stronger solute migration. Furthermore, we quantitatively simulate the observed transmittance by involving thermal diffusion alone. Here by thermal diffusion, we mean temperature gradient driven mass transport of a component in a binary system. It is a quasistatic process. In the latter geometry, we find a transition of transmittance from ascent to descent with time and a downward transmitted beam deflection. We additionally invoke convection to explain the results.
In the study with 19 ps laser pulses, we conduct Z-scan measurements on the samples at two concentrations (4.21017 cm3 and 1.21017 cm3). With stronger absorption and larger temperature rise, the higher-concentrated solution shows less migration behavior. Thermal diffusion is thus disregarded. On the other hand, simulation shows that before the excited solution restores local thermal equilibrium, individual solute molecules in the lower concentrated solution gain, due to nonlinear excitation, more translational excess energy t which drives the solute migration. We, therefore, conclude that this migration is non-quasistatic.
To explain how the CW laser light induced-solute migration is quasistatic and the 19 ps laser pulse-induced migration is non-quasistatic, we compare the rate of photo energy deposition into the solute molecules and that of excess energy dissipation throughout the neighboring solvent molecules. Since a CW laser converts its photo energy into the (solute and solvent) molecules translational excess energy, by photo absorption and the subsequent intra- and inter-molecular relaxation, at a rate lower than that the excited samples restore local thermal equilibrium, the samples deviate from local thermal equilibrium infinitesimally in the middle of solute migration. Therefore, this migration tends to be quasistatic. Here, by local thermal equilibrium, we mean the translational energy t (= t0+t with t0 denoting the translational energy of individual solute molecules given the sample in full thermodynamic equilibrium) of each individual molecule contained within the same macroscopic volume element dV is nearly equal and, hence, the solution temperature pertaining to this dV becomes definable but appears minutely different from that pertaining to a neighboring dV. Here a dV has dimensions in the order of a wavelength and thus light intensity can be considered uniform therein. Since, according to the equipartition theorem, is proportional to t given the sample in (full or local) thermal equilibrium, ( t) is taken as the driving force of the solute migration, namely thermal diffusion which is a quasistatic process. Note that when the samples are nearly in local thermal equilibrium all the time, t pertains to both species of molecules composing the binary systems.
On the other hand, since a 19 ps pulse converts its photo energy into the solute molecules translational excess energy at a rate higher than that the excited solution restores local thermal equilibrium, we first simulate translational excess energy (t) gained by individual solute molecules and then calculate t retained in individual solute molecules after the solution restores local thermal equilibrium by intra- and inter-molecular relaxation. As a result, we discover that a larger temperature gradient is formed in the higher concentrated solution after the excited solution restores local thermal equilibrium; however, individual solute molecules in the lower concentrated solution possess more translational excess energy t before the solution restores local thermal equilibrium.Combing the experimental results and theoretical simulation, we determine that 19 ps pulse-induced solute migration is non-quasistatic, initiated before the excited solution restores local thermal equilibrium.
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