Femtosecond infrared pump-stimulated Raman probe spectroscopy: the first application of the method to studies of vibrational relaxation pathways in the liquid HDO/D <subscript>2</subscript> O 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 a sub-100 fs time resolution, a spectral resolution better than 10 cm -1 and a spectral window covering an extremely broad range of molecular vibrations (at least: 200-4000 cm -1 ) with a "single laser shot". The new method was applied to study vibrational relaxation pathways in the liquid HDO/D 2 O system. We determined the lifetimes of OH stretching vibrations to be in the range 310-500 fs depending on the isotopic dilution, which is in 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.