Absorbance change, pump intensity

An ultraviolet-laser based thermo-optical absorbance detector for micrometer capillaries was used by Qi et al. [76] to monitor the separation of a mixture of 13 phenylthiohydantoin-amino acids. A modulated pump laser beam periodically illuminated the capillary at a point near its end. Complex deflection and diffraction effects occur at the capillary-solution interface. Perturbation of the refractive index at this interface changes the intensity of the probe beam that is measured using a small photodiode. 

The slope of the logarithmic plot of absorbance change versus the logarithm of the pump intensity is 1.84, indicating the occurrence of a nonlinear absorption process. The TPA cross section of isomer A is about 0.76 GM and an increased 8 was observed for isomer B (8 = 6 GM) due to the closed ring form and higher planarity. One-photon and two-photon induced isomerization of 143 shown in Eq. (62) results in similar spectra and in the same isosbestic points observed at 329 nm, 377 nm, and 429 nm. These results clearly indicate participation of the same excited state applying either OP or TP excitation. Quantum yields are close to unity. 

However, for the dendrimer nanocomposite metallic systems this change in shape was not observed. Again, due to the high stability to intense laser pulses, the anisotropy value of the gold dendrimer nanocomposite, which can be viewed as a measure of the symmetry of the particle, did not change after several repeated cycles of measurements. It is possible that the initial optical pumping of the electron-phonon modes of the metal particles is partially absorbed by the encapsulating PAMAM dendrimer. 

In the basic experiment, a first optical pulse (pump) is absorbed by the sample. A second, time-delayed pulse of weaker intensity is used to probe the change in optical response. Commonly, the test pulse probes the change in transmission, given in the small signal limit as... 

In the backward-wave geometry, the fundamental laser beam is usually split into forward, backward and pump beam. It is necessary to use delay lines in the case where the pulses are very short in order to assure a good temporal overlap between the three input beams. The output phase-conjugated intensity is maximized by changing the path length of one of the pump beams. The maximum value for the DFWM signal is measured for both sample (fample) and a reference (fe/erence)-In the case of non-absorbing medium, they are simply related [24] ... 

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