Ultrafast polarization-modulation transient spectroscopy to study electronic excited state dynamics in biological molecules

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.