Bromine isotope splitting in vibrational spectra of bromoform by time-resolved transient transmission spectroscopy.

The femtosecond pump-probe technique, i.e. the transient transmission spectroscopy, has been used for the first time, to detect the vibrational spectra of symmetric fundamentals ν and ν in bromoform and chloroform. The spectra were obtained by fast Fourier transforms of the time domain signals. For both, CHCl and CHBr, there are four isotopologues contributing to the spectra, due to the existence of two stable isotopes; chlorine, Cl and Cl, and bromine, Br and Br, respectively. While for chloroform the isotope splitting of the ν spectral band can be observed even in the spontaneous Raman scattering, for bromoform it is not detectable. Herewith we show that using the time domain spectroscopy and the windowed Fourier transform method we can provide the high resolution spectrum of the ν fundamental in bromoform, in which the contributions of all isotopologues are well distinguishable. The data have been collected for few volume concentrations of the studied liquids diluted in the neutral solvent CCl. It is shown that the intensity pattern of the spectra evolves with decreasing concentration and for the ν fundamental it reaches the natural abundance pattern at a very high dilution. The simple theoretical model, which treats the molecules in a liquid as interacting oscillators, allows us to explain the dependence of the shape of the spectrum on the strength of the intermolecular interactions.