A novel polarization-modulation transient method has been developed for studying fast anisotropic relaxation in electronic excited states of polyatomic and biologically relevant molecules under excitation with femtosecond laser pulses. The method is based on the modulation of pump beam polarization with a photo-elastic modulator and detection of an anisotropic contribution to the transient signal by a highly-sensitive demodulation balanced scheme. The method was tested on aqueous solution of coenzyme NADH (nicotinamide-adenine-dinucleotide) pumped at 360 nm and probed at 720 nm. Anisotropic vibrational relaxation and rotational diffusion have been observed in the sub-picosecond time domain. The method significantly enhances the accuracy of transient measurements and allows for recording of high-quality signals at low energy (a nJ) pump pulses.
The luminescence spectrum of aqueous solution of Radachlorin photosensitizer in the near IR spectral range (950-1350 nm) has been determined at the excitation in both the Soret and Q absorption bands. Major sources of the recorded luminescence were analyzed. Kinetics of photosensitizer and singlet oxygen phosphorescence signals were studied by means of time-resolved spectroscopy. The corresponding characteristic lifetimes were determined.
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