Calculations of vibrationally resonant sum- and difference-frequency-generation spectra of chiral molecules in solutions: Three-wave-mixing vibrational optical activity

摘要

Determining absolute configuration of chiral molecule can be achieved by using vibrational opticalactivity (VOA) measurement methods, such as vibrational circular dichroism and Raman opticalactivity (ROA). Recently, vibrationally resonant sum frequency generation and difference frequencygeneration utilizing circularly polarized beams were theoretically studied and shown that they canbe alternative and complementary VOA measurement techniques [S. Cheon and M. Cho, Phys. Rev.A 71, 013808 (2005); J. Phys. Chem. A 113, 2438 (2009)]. Even for randomly oriented chiralmolecules in solutions, the sum- and difference-frequency-generation signals induced by linearlypolarized incident beams with mutually perpendicular polarization directions can be nonzero and aredetermined by the optical activity hyperpolarizability given by a product of antisymmetric Ramantensor and vibrational transition dipole. If one of the beams involved in the three-wave-mixingprocesses is circularly polarized and if the difference signal is measured, not only the same opticalactivity hyperpolarizability but also that including electric quadrupole-ROA tensor are required todetermine the signals. Here, we carried out quantum chemistry calculations to obtain thesequantities for a representative chiral molecule, (S)-methyl lactate, and numerically simulated thecorresponding spectra. It is shown that the circular polarization three-wave-mixing signal intensitiesare quantitatively similar to those of the linear polarization three-wave-mixing signals, respectively,and that they are sensitive to the absolute configuration of chiral molecule. The calculation resultsthus suggest that these two novel techniques will be of use in studying molecular chirality even intime domain, once polarization-modulated ultrashort pulses are used to carry out circularpolarization three-wave-mixing experiments.