Here, a is a constant. The slope b = μac0 is then used for the calculation of the linear light absorption coefficient μa. Such fitting has usually a very good correlation (r > 0.999) with the detected signal front, which provides high accuracy of the light absorption coefficient determination (RSD ca. 0.1% [27], [28] and [31]). As described above by Eqs. (2) and (6), a part of the signal profile which is closer than τL to the signal maximum is not exponential and should not be used in fitting.
The speed of sound c0 in the solution is also measured experimentally by a signal propagation time indicated in the position of the signal maximum. For the determination of the ultrasound speed in the solution under study, we measured the time delay between the primary detected OA signal and its double propagation within the OA cell. This measurement is totally independent on the position of the OA source. All the quartz components of the OA cell were polished and plane-parallel. It made it possible to have the medium thickness fixed with the precision better than few μm for all the investigated solutions. Note that the value of c0 could be inconstant in the course of a chemical reaction due to the formation and decomposition of species affecting the thermophysical parameters of the medium. The relative error of the speed of sound measurement was lower than 0.5%.
Thus, time-resolved detection of OA signal profiles provided with the value of light absorption coefficient μ a [cm−1] for the current experiment, i.e. for a current state of the solution. In case of kinetic measurements, values of μ a detected at different moments were treated as a kinetic signal depending on the reaction time t reac after mixing all the reagents (see below), which was fitted as
where View the MathML source is the maximum value of light absorption coefficient in the current reaction and A is an experimental constant. The rate constant kv was then determined from Eq. (8).
Here, a is a constant. The slope b = μac0 is then used for the calculation of the linear light absorption coefficient μa. Such fitting has usually a very good correlation (r > 0.999) with the detected signal front, which provides high accuracy of the light absorption coefficient determination (RSD ca. 0.1% [27], [28] and [31]). As described above by Eqs. (2) and (6), a part of the signal profile which is closer than τL to the signal maximum is not exponential and should not be used in fitting.
The speed of sound c0 in the solution is also measured experimentally by a signal propagation time indicated in the position of the signal maximum. For the determination of the ultrasound speed in the solution under study, we measured the time delay between the primary detected OA signal and its double propagation within the OA cell. This measurement is totally independent on the position of the OA source. All the quartz components of the OA cell were polished and plane-parallel. It made it possible to have the medium thickness fixed with the precision better than few μm for all the investigated solutions. Note that the value of c0 could be inconstant in the course of a chemical reaction due to the formation and decomposition of species affecting the thermophysical parameters of the medium. The relative error of the speed of sound measurement was lower than 0.5%.
Thus, time-resolved detection of OA signal profiles provided with the value of light absorption coefficient μ a [cm−1] for the current experiment, i.e. for a current state of the solution. In case of kinetic measurements, values of μ a detected at different moments were treated as a kinetic signal depending on the reaction time t reac after mixing all the reagents (see below), which was fitted as
where View the MathML source is the maximum value of light absorption coefficient in the current reaction and A is an experimental constant. The rate constant kv was then determined from Eq. (8).
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