On the coherent vibrational phase in polarization sensitive resonance CARS spectroscopy of copper tetraphenylporphyrin

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Abstract
Polarization sensitive multiplex spectroscopy of resonance coherent anti-Stokes Raman scattering (CARS) of copper(II)-tetraphenylporphyrin in solution (CH2Cl2) is reported. The measurements were performed in the Q band range of the porphyrin electronic absorption. Four polarized CARS spectra were resolved in 1300–1650 cm−1 Raman range and were simultaneously fitted with a single set of vibrational parameters (band positions, bandwidths, amplitudes, depolarization ratios, and phases). The obtained coherent vibrational phases of A1g, A2g, and B1g skeletal modes of the porphyrin macrocycle appeared to correlate strongly with the mode vibrational symmetry. The origin of such correlation is analyzed within a model of the third-order nonlinear electric susceptibility χ(3). The proposed model is based on multidimensional displaced harmonic oscillator in the Herzberg–Teller expansion of Raman polarizability. The coherent vibrational phases of modes of different symmetry classes are directly affected by the symmetry dependent vibronic couplings between the pairs of Qx, Qy and Bx, By electronic transitions of the porphyrin. The phase contrast between the modes of various symmetries is most pronounced near the Q00 resonance.