  Hydrogen-bonded mixtures with varying concentration are a complicated
networked system that demands a detection technique with both time and
frequency resolutions. Hydrogen-bonded pyridine-water mixtures are studied by a
time-frequency resolved coherent Raman spectroscopic technique. Femtosecond
broadband dual-pulse excitation and delayed picosecond probing provide
sub-picosecond time resolution in the mixtures temporal evolution. For
different pyridine concentrations in water, asymmetric blue versus red shifts
(relative to pure pyridine spectral peaks) were observed by simultaneously
recording both the coherent anti-Stokes and Stokes Raman spectra. Macroscopic
coherence dephasing times for the perturbed pyridine ring modes were observed
in ranges of 0.9 - 2.6 picoseconds for both 18 and 10 cm-1 broad probe pulses.
For high pyridine concentrations in water, an additional spectral broadening
(or escalated dephasing) for a triangular ring vibrational mode was observed.
This can be understood as a result of ultrafast collective emissions from
coherently excited ensemble of pairs of pyridine molecules bound to water
molecules.
