Thermal-lens study of photochemical reaction kinetics

Abstract

We consider the time dependence of the absorption coefficient due to the photoinduced chemical reaction (PCR) and species diffusion to calculate the temperature rise in the thermal-lens (TL) effect. The TL signal at the detector plane is also calculated. This theoretical approach removes the restriction that the PCR time constant is much greater than the characteristic TL time constant, which was assumed in a previously published model. Hydrocarbon fuel and aqueous Cr(VI) samples are investigated, and quantitative experimental results for the thermal, optical, and PCR properties are obtained. While similar results were obtained for the Cr(VI) solution using the previous and present models, the relative difference between the PCR time constants extracted from the same experimental data for a hydrocarbon fuel sample is found to be more than 220%. This demonstrates the significant difference of the two models.

Investigation into photostability of soybean oils by thermal lens spectroscopy

“…The wavefront of the probe beam is slightly distorted when it propagates through the illuminated volume of the liquid sample. This distortion can be expressed with an additional phase shift to the probe beam by Uðr; tÞ ¼ ð2p=k p ÞL ½nðr; tÞ À nð0; tÞ [22,23], where nðr; tÞ ¼ n 0 þ ðdn=dTÞTðr; tÞ þ ðdn=dCÞCðr; tÞ, n 0 is the refractive index, L is the sample thickness, dn=dT and dn=dC are the refractive index temperature and concentration coefficients, respectively, at the probe beam wavelength k p . The phase shift can be written in terms of Uðr; tÞ ¼ h T ½Tðr; tÞ À Tð0; tÞ þ h c ½Cðr; tÞ À Cð0; tÞ, with h T ¼ ð2p=k p Þ Lðdn=dTÞ and h c ¼ ð2p=k p ÞLðdn=dCÞ.…”

“…The signal is proportional to the optical absorption coefficient of the sample and, therefore, sensitive to changes in the concentration Cðr; tÞ of the absorbing species. The method was applied to examining photoinduced chemical reactions in aqueous solutions [21][22][23][24][25] and hydrocarbon fuels [26]. The theoretical description of the TL effect employed in those studies relied on the time-and spatial-dependence of the optical absorption coefficient with the changes by optical bleaching and mass diffusion within the excited volume of the sample.…”

Correlations among thermophysical properties, ignition quality, volatility, chemical composition, and kinematic viscosity of petroleum distillates

“…Shen Unusual time-resolved thermal lens signals were observed for 163 most of the hydrocarbon distillates. These signals, which resemble 164 those for edible oil samples [14], were probably due to unwanted 165 photochemical reactions induced by the excitation laser [12]. To 166 minimize the effect of these reactions on the measured thermal 167 diffusivity values, a short-time least-squares curve fitting algo-168 rithm was employed using software Mathematica; the curve-169 fitted period for the time-resolved thermal lens signal was much 170 smaller than the photochemical reaction time [12].…”

Determination of photochemical reaction rates using thermal lens spectrometry

“…More details about the experimental setup can be found in Refs. [10] and [14]. The values of ω 0e , m and V = Z 1 /Z c were 72µm, 45.25 and 8.3, respectively.…”