Optical dichroism pumping

Figure 19.1. Schematic diagram of a general pump-probe-detect laser spectrometer suitable for picosecond electronic absorption, infrared (IR) absorption, Raman, optical calorimetry, and dichroism measurements. For picosecond fluorescence—a pump-detect method, no probe pulse needs to be generated.

The isotropic signal delivers (rotation-free) information on the temporal evolution of the population numbers of the investigated vibrational transition(s). The induced dichroism is governed by the time constant ror (second-order reorientational correlation time, 1 = 2) and possibly population redistribution that may contribute to the loss of induced optical anisotropy. The zero-setting of the delay time scale (maximum overlap between pump and probing pulses) is determined by a two-photon absorption technique in independent measurements with an accuracy of better than 0.2 ps (67). 

Another mechanism responsible for the optically induced anisotropy is angular redistribution (AR) of molecules. This mechanism has been widely developed to explain photoinduced birefringence and dichroism. In most experimental cases, there is evidence of some rotation of molecules during the photoisomerization cycle (see Reference 2, for example). This rotation results in AR, because the molecules remain longer in states with lower excitation probability, and so more molecules are accumulated perpendicular to the pump polarization. The AR process is initiated by the AHB, and these two processes should be studied simultaneously in the framework of general... 

This effect of a change in the SH output polarization depending on the enantiomer or mixture of enantiomer is somewhat analogous to the linear optical phenomenon of optical rotary dispersion (ORD) in bulk chiral liquids. As such, the process for SH radiation is termed SHG-ORD [70]. In general, chiral surfaces will also exhibit distinct radiation characteristics for left- and right-polarized pump beams. Again, by analogy with the linear optical process of circular dichroism (CD), this effect has been termed SHG-CD [69]. 

Mollenauer and co-workers used the MCD effect in the absorption band of an F center in order to detect, for the first time, changes in the ground state spin polarization induced by ENDOR in the relaxed excited state. A He-Ne laser served for optical pumping as well as for monitoring the magnetic circular dichroism, while several preceding experiments still had used conventional lamps. The laser beam, which passed a quarter wave plate with a stress modulation frequency of SO kHz, was irradiated onto a KI crystal that contained the F centers under study. The transmitted laser light was monitored with a photomultiplier and processed with lock-in technique. 

In contrast to fluorescence, which probes molecules in the excited state only, TRPS dichroism may provide us with information about the dye molecular dynamics in the ground singlet state Sq. In the pump-probe scheme, a linearly polarized pumping optical pulse induces an iititial anisotropy in the ground-state population labeled with the suffix i =g. Then the decay of this anisotropy is followed by measuring at variable temporal delays the absorption of a probe beam of frequency z/pr. 

---