Vibrational Dynamics in Hydrogen-Bonded (Pyridine + Water) Complexes Studied by Spectrally Resolved Femtosecond CARS

The technique of femtosecond time-resolved coherent anti-Stokes Raman spectroscopy (fs-CARS) was used to study the vibrational dephasing dynamics in hydrogen-bonded (pyridine + water) complexes as a function of pyridine mole fraction x(Py). By detecting the spectrally resolved CARS signal, a mapping of the vibrational coherence dynamics of ring modes at ∼990 and ∼1030 cm^-1 was achieved. The quantum beatings among different modes of the hydrogen-bonded network are clearly exhibited in the transient signal. Its spectral analysis yields the involved frequencies by employing FT methods to the time domain signal. The oscillatory pattern in the CARS transients are adequately explained when contributions from several modes are accounted for. The individual vibrational dephasing times (T₂) in the range ∼1.7-5.3 ps were obtained with high precision. The assumption of homogeneous broadening of the ring modes under investigation is made for those mole fractions, where the pyridine molecules have a surrounding consisting mainly of only one species. According to the concentration profile this is valid for x(Py)=1.00 (neat pyridine) and x(Py)=0.20, for which the hydrogen-bonded network is dominated by PyW_n species i.e. water-water hydrogen bonding). For both mole fractions the condition (Δν˜_1/2)_hom=1/πcT₂, which relates line width in wavenumber from Raman data and dephasing time from time-resolved data, is fulfilled. Inhomogeneous broadening is assumed for the intermediate concentration x(Py)=0.60 since several distinct species, such as: Py₂W, PyW, and PyW₂ are known to co-exist leading to complex chemical equilibria.