Femtosecond infrared pump – stimulated Raman probe spectroscopy: the first application of the method to studies of vibrational relaxation pathways in liquid HDO/D2O system.

We have proposed and constructed a setup for a novel method of ultrafast vibrational spectroscopy: femtosecond infrared pump – stimulated Raman probe spectroscopy. This is the first time-resolved spectroscopy providing simultaneously sub-100 fs time resolution, spectral resolution better than 10 cm-1 and spectral window covering extremely broad range of molecular vibrations (at least: 200 cm-1 – 4000 cm-1) with a “single laser shot”. The new method was applied to study vibrational relaxation pathways in liquid HDO/D2O system. We determined lifetimes of OH stretching vibrations to be in the range 310 fs – 500 fs depending on the isotopic dilution, which is in a good agreement with the results from pump-probe femtosecond infrared spectroscopy. Moreover, we observed a strong coupling of OH stretch to OD stretch vibrations and possibly also to the librational modes of water.