Probe beam linear dichroism and birefringency in the pump-and-probe spectroscopy in polyatomic molecules

Probe beam dichroism and birefringency occurring in the excited states of polyatomic molecules under excitation with two femtosecond laser pulses have been theoreticaly studied as a function of delay between the pulses. General expressions describing the change of intensity and polarization of the probe laser pulse passed through the solution of arbitrary polyatomic molecules at any initital polarization of the laser pulses have been derived using the spherical tensor approach. The expressions take into account the coherence in excited molecule vibrational states and decay of these states due to vibrational relaxation, rotational diffusion, and radiative transitions. The expressions describe the effects of probe beam linear dichroism and birefringency occurring in molecular excited states. As shown, under certain conditions both effects can be observed simultaneously. It has been concluded that within the geometry of almost collinear propagation of the pump and probe laser pulses through the molecular sample the contributions from linear dichroism and birefringency can be completely separated from each other by an appropriate choice of the polarization analyzer placed in front of the photodetector. The obtained expressions were applied for description of the signals that can be recorded experimentally by means of the polarization-modulation scheme developed in the recent author's publication (Gorbunova et al,Phys. Chem. Chem. Phys. 2020, Vol. 22, 18155–18168). The theory predicts that the modulated signals of dichroism and birefringency appear in quadrature with respect to the double frequency reference modulation signal.