  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
(nicotin-amideadenine-dinucleotide) pumped at 360 nm and probed at 720 nm.
Anisotropic vibrational relaxation and rotational diffusion have been observed
in the subpicosecond 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.
