Showing papers by "Ranjan K. Singh published in 2002"
TL;DR: In this article, the 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.
Abstract: 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 (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 e. water-water hydrogen bonding). For both mole fractions the condition (Δν 1 / 2 ) h o m = 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.
33 citations
TL;DR: In this article, a logarithmic plot as a function of 1/T gives two straight lines with an abrupt discontinuity, which clearly exhibits a phase transition at about 210 K, and the temperature dependence of the linewidth of the ∼1165 cm−1 band was used to characterize this transition.
Abstract: Raman spectra of terephthalidine-bis-heptylaniline (TB7A) were recorded in the region 1125–1225 cm−1 from room temperature to 10 K at 18 different temperatures The splitting of the ∼1165 cm−1 band, and the variation of the ratio of integrated intensities of the original band at ∼1165 cm−1 to the new component at 1170/1171 cm−1 reveals a solid modification The analysis of the temperature dependence of the linewidth was made using Rakov's relation A logarithmic plot as a function of 1/T gives two straight lines with an abrupt discontinuity, which clearly exhibits a phase transition at about 210 K The temperature dependence of the linewidth of the ∼1165 cm−1 band was used to characterize this transition A relatively large variation (∼3 cm−1) of the linewidth on going from 250 to 10 K in TB7A having an odd (n = 7) number of carbon atoms in the alkyl tail compared with those observed (∼1 cm−1) in other homologues having an even (n = 2 or 10) number of carbon atoms in the alkyl tail is a clear demonstration of the even–odd effect Copyright © 2002 John Wiley & Sons, Ltd
15 citations