Fine structures in Raman spectra of tetrahedral tetrachloride molecules in femtosecond coherent spectroscopy

Herewith, we present fast Fourier transforms of time resolved signals, obtained by use of the femtosecond transient transmission (TT) spectroscopy, for three tetrachlorides, CCl₄, SiCl₄, and GeCl₄, and chloroform, CHCl₃. Due to coherent excitation of molecules, the isotopic splitting of their spectral bands in the range of symmetric stretching vibration can be observed with high resolution not available in spontaneous Raman scattering. The intensity distribution in the isotopic fine structure pattern appears to differ for various studied molecules, which is explained by the role of intermolecular interactions and the local order of molecules in the liquids. In particular, in SiCl₄, the vibrational band exhibits anomalous ratios of the peak amplitudes, which do not agree with the natural abundance of the isotopologues. Using the simple oscillatory model of the liquid and fitting theoretical curves to the experimental results, we have been able to find the intermolecular force constants for all three liquids and to formulate the conclusion that the anomalous spectral pattern in SiCl₄ results from strong interactions between the closest Cl atoms belonging to adjacent molecules. Application of the windowed Fourier transform enables us to study the dynamics of intermolecular interactions. The strength of intermolecular interactions in CCl₄, SiCl₄, and GeCl4, found by the TT technique, is compared with the results obtained by means of the femtosecond optical Kerr effect spectroscopy.