Probe Beam Dichroism and Birefringence in Stumulated Raman Scattering of Biologically Relevant Polyatomic Molecules

The dichroism and birefringence effects in stumulated Raman scattering (SRS) in polyatomic molecules were studied theoretically. 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 that allowed for separation of the both beams polarization matrices and the material part containing three scalar values of nonlinear optical susceptibility where that rank K is limited to the values K=0,1,2. The expressions are valid for arbitrary directions of propagation of both pump and Stokes light beams and arbitrary polarizations of both beams. 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 Stokes pulse beams 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.Comment: 11 pages, 2 figure

Fluorescence Anisotropy in Radachlorin and Chlorin e6 in Water–Methanol Solutions under One- and Two-Photon Excitation

The fluorescence anisotropy of photosensitizers Radachlorin and chlorin e6 was studied using the time-resolved single photon-counting technique under one- and two-photon excitation within the Soret absorption band. A very small negative anisotropy was observed in both photosensitizers under one-photon excitation in the vicinity of the absorption maximum within the wavelength range of 395–405 nm. Meanwhile, two-photon excitation of the photosensitizers in the same spectral range demonstrated high fluorescence anisotropy with the maximum value of about 0.43. The drastic difference of the fluorescence anisotropy parameters at one- and two-photon excitation modes was suggested to be due to the different symmetries of one- and two-photon absorption tensors when two-photon absorption tensor components have comparable values. The variation of excitation wavelengths in the spectral range of 375–425 nm demonstrated nonlinear wavelength dependence of anisotropy of both Radachlorin and chlorin e6, with opposite tendencies at one- and two-photon excitation. The data obtained suggest that one-photon excitation at about 405 nm often utilized in FLIM experiments is not sensitive to fluorescence anisotropy in Radachlorin and chlorin e6 and therefore cannot be used for the determination of anisotropy/rotational diffusion time in these molecules. Meanwhile, two-photon excitation can provide high fluorescence anisotropy and accurate determination of the rotational diffusion time. At the same time, one-photon excitation at about 405 nm can be used for the accurate evaluation of fluorescence lifetimes within the standard FLIM schematic where fluorescence polarization is not taken into account