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

Abstract

Vibrational Dynamics / Femtosecond CARS / Hydrogen-Bonded ComplexesThe 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 (T 2 ) 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 (∆ν 1/2 ) hom = 1/πcT 2 , 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: Py 2 W, PyW, and PyW 2 are known to co-exist leading to complex chemical equilibria.

Time- and frequency-dependent model of time-resolved coherent anti-Stokes Raman scattering (CARS) with a picosecond-duration probe pulse

“…(B3) demonstrates that the outgoing signal of fs tr-CARS in principle contains all molecular information (including relative cross-sections, frequencies, and dephasing rates) associated with a spontaneous Raman spectrum of the molecular species, 20, 32, 33 two of the major deficiencies of fs tr-CARS, noted previously, 1 are also emphasized: (1) absolute molecular frequencies are not measured directly; only beat frequencies, ω ca, c a , spectrally centered at the average frequency associated with the two contributing transitions [ 1 2 · (ω ca + ω c a )], are observed in the oscillating intensities of the broadband CARS output spectrum [note that this is not an issue in the specific case of DFWM, where beats between vibrational or rotational states of interest and the vibrationless/rotationless (Rayleigh) transition can be observed directly]; and (2) it is difficult to deduce dephasing rates associated with individual molecular transitions directly from such measurements, since the measured decay timescale of each observed beat frequency results from the average of the dephasing rates of the two contributing transitions. 24 …”

“…[5][6][7][8][9][10][11][12] This probe technique has been developed as a variant of fs time-resolved CARS (tr-CARS), [13][14][15][16][17][18][19][20][21][22][23][24][25] the history of which has been described thoroughly in recent reviews. 26,27 Hybrid fs/ps CARS utilizes the same four-wave-mixing excitation scheme as does fs tr-CARS; however, the use of a time-delayed, narrowband, ps-duration probe pulse-rather than a broadband, fs-duration probe pulse-allows direct frequency-domain detection of Raman-active molecular states.…”

Hydrogen-Bonding between Pyrimidine and Water: A Vibrational Spectroscopic Analysis

“…For a series of pyrimidine/water mixtures, we employ the ring breathing vibration ν 1 of pyrimidine around 990 cm -1 as a marker; this vibration is sensitive to hydrogenbonding between nitrogen-containing heterocycles and H-donor solvents. [11][12][13][14][15][16] The interpretation of vibrational spectroscopic data can be expanded in conjunction with an appropriate theoretical model that correlates the experimentally observed spectral features with explicit molecular structures that are present in the liquid. In this respect, modern quantum chemical approaches can strongly contribute to our understanding of the hydrogen bond.…”

Time-Resolved Surface-Enhanced Coherent Sensing of Nanoscale Molecular Complexes

“…Hydrogen-bonded complexes with water cannot be excluded as water may be present in small amounts on the surface of gold. Pyridine-water complexes have been previously studied using Raman spectroscopy141516, and pyridine was the first molecule for which SERS was demonstrated1. Single molecules and monolayers are usually the targets of research and proof-of-principle demonstrations.…”