Linear Dichroism and Birefringence of Probe Radiation in Pump-Probe Spectroscopy of Polyatomic Molecules

Abstract

The dichroism and birefringence effects in excited states of polyatomic molecules upon excitation by two successive femtosecond pulses have been studied theoretically as a function of the delay time between the pulses. General expressions have been derived describing the change of the intensity and polarization of the probe pulse after transmission through a solution of arbitrary polyatomic molecules for any initial polarization of each of the laser pulses. The expressions were written in terms of spherical tensor operators and took into account the coherence of vibrational molecular excited states and their population due to vibrational relaxation, rotational diffusion, and radiative transitions. The expressions contain contributions from linear dichroism and birefringence in the molecular excited states. It was shown that, under certain conditions, both effects can be observed simultaneously. The geometry of almost collinear propagation of the pump and probe pulses through the molecular sample was considered, and it was shown that the contributions from linear dichroism and birefringence to the signal can be completely separated in the experiment by means of an appropriate choice of a probe beam polarization analyzer placed in front of the photodetector. The expressions obtained were used to describe the signals obtained using the polarization-modulation technique developed recently by the authors (Gorbunova et al, Phys. Chem. Chem. Phys. 2020, Vol. 22, 18155–18168). It was shown that the modulated dichroism and birefringence signals could be observed in quadrature to the second harmonic of the modulated reference signal.