Raman study of vibrational relaxation of acetonitrile in solution

Line widths of isotropic Raman spectra of the ν₁ (a₁) CH or CD stretching bands of acetonitrile CH₃CN and CD₃CN were measured in a number of solvents. The vibrational dephasing theory, modified for use in binary mixtures, predicts quantitatively the solvent dependence of the Raman line widths.     

Direct observation of vibrational relaxation of dye molecules in solution

The characteristic time for vibrational relaxation in an optically excited molecular electronic state has been measured with picosecond resolution for the molecules rhodamine B and rhodamine 6G in solution using a new experimental technique.     

Ultrafast electronic and vibrational energy relaxation of Fe(acetylacetonate)3 in solution

Transient mid-infrared spectroscopy is used to probe the dynamics initiated by excitation of ligand-to-metal (400 nm) and metal-to-ligand (345 nm) charge transfer states of FeIII complexed with acetylacetonate (Fe(acac)3, where acac stands for deprotonated anion of acetylacetone) in solution. Transient spectra in the 1500-1600 cm-1 range show two broad absorptions red-shifted from the bleach of the nu(CO) (approximately 1575 cm-1) and nu(C=C) (approximately 1525 cm-1) ground state absorptions. Bleach recovery kinetics has a time constant of 12-19 ps in chloroform and tetrachloroethylene and it decreases by 30-40% in a 10% mixture of methanol in tetrachloroethylene. The transient absorptions experience band narrowing simultaneously with blue-shifting of the absorption maxima. Both phenomena have time constants of 3-9 ps with no evident dependence on the solvent. The experimental observations are ascribed to fast conversion of the initially excited charge transfer states to the ligand field manifold, and subsequent vibrational cooling on the lowest ligand field excited state prior to electronic conversion to the ground state. The analysis of time dependent bandwidths and positions of the transient absorptions provides some evidence of mode specific vibrational cooling.     

Intermolecular vibrational relaxation in liquids

The influence of intermolecular vibrational relaxation on dipole moment correlation functions, as obtained from IR band shapes, is discussed. It is explicitly shown that vibrational relaxation due to intermolecular interactions depends on the reorientational behaviour of the molecules in the liquid.Therefore, an a priori separation of the dipole moment correlation function into independent reorientational and vibrational factors is not generally possible. The implications for various procedures used to “correct” Raman and IR band shapes for vibrational relaxation are discussed.The expression derived for the intermolecular vibrational relaxation is used to calculate theoretically the effect of transition dipole-transition dipole coupling on dipole moment correlation functions.Experimental data obtained from isotopic dilution measurements support the interpretation of the isotopic dilution effect in terms of the transition dipole-transition dipole coupling.     

Reorientational and vibrational relaxation in liquid trichloracetonitrile

The first- and second-order reorientational correlation functions were determined from i.r. and Raman bandshapes of ν_s(CN)in liquid trichloracetonitrile and discussed in terms of the J-diffusion model. A consistent fit for G_1r(t) and G_2r(t) was possible for short times only (0–1.5 ps). The vibrational correlation functions, determined from Raman studies of the ν_s(CN) and ν_s(CC) bandshapes, are discussed in terms of recent theories of vibrational dephasing.     

Electronic to vibrational energy transfer and relaxation in matrices. I. Hg in N2 matrix

Under KrF laser (249 nm) excitation of Hg atoms in a N₂ matrix in the temperature range 12–25 K we observed (i) a strong, broad-band fluorescence in the near-UV region assigned to a (Hg-N₂)^* exciplex (decay time from 800 to 100 μs) and (ii) infrared Δυ = −1 emission from high vibrational levels (υ = 6−12) of N₂ molecules in the electronic ground state, populated by the electronic-to-vibrational energy transfer (decay time 7 s on υ = 10−12 at 14 K). From time-resolved UV and IR spectra, one can conclude that the predominant part of the EV transfer takes place from thermally non-equilibrated levels, on a short time scale. This is unexpected in view of gas phase data and preliminary ab initio potential energy surface calculations. Temperature effects are discussed. Excited N₂ levels decay along a radiative path (predominant for the highest levels) and by VV transfer to N₂(υ=0) molecules with a rate decreasing rapidly with υ but strongly increasing with the sample temperature.     

On vibrational relaxation in liquid trimethylchlorosilane

Vibrational correlation functions and related spectral data were determined in several ways for the ν_s(SiCl) and sym-ν_s(Si(CH₃)₃) vibrations of liquid trimethylchlorosilane, from Raman bandshape analysis. The influence of isotopic composition and Fermi resonances on th accuracy of obtained correlation functions is discussed.     

Nonperturbative vibrational energy relaxation effects on vibrational line shapes

A general formulation of nonperturbative quantum dynamics of solutes in a condensed phase is proposed to calculate linear and nonlinear vibrational line shapes. In the weak solute-solvent interaction limit, the temporal absorption profile can be approximately factorized into the population relaxation profile from the off-diagonal coupling and the pure-dephasing profile from the diagonal coupling. The strength of dissipation and the anharmonicity-induced dephasing rate are derived in Appendix A. The vibrational energy relaxation (VER) rate is negligible for slow solvent fluctuations, yet it does not justify the Markovian treatment of off-diagonal contributions to vibrational line shapes. Non-Markovian VER effects are manifested as asymmetric envelops in the temporal absorption profile, or equivalently as side bands in the frequency domain absorption spectrum. The side bands are solvent-induced multiple-photon effects which are absent in the Markovian VER treatment. Exact path integral calculations yield non-Lorentzian central peaks in absorption spectrum resulting from couplings between population relaxations of different vibrational states. These predictions cannot be reproduced by the perturbative or the Markovian approximations. For anharmonic potentials, the absorption spectrum shows asymmetric central peaks and the asymmetry increases with anharmonicity. At large anharmonicities, all the approximation schemes break down and a full nonperturbative path integral calculation that explicitly accounts for the exact VER effects is needed. A numerical analysis of the O-H stretch of HOD in D(2)O solvent reveals that the non-Markovian VER effects generate a small recurrence of the echo peak shift around 200 fs, which cannot be reproduced with a Markovian VER rate. In general, the nonperturbative and non-Markovian VER contributions have a stronger effect on nonlinear vibrational line shapes than on linear absorption.     

Comment on collisionless vibrational relaxation

It is shown that collisionless vibrational relaxation is associated with homogeneous spectral broadening. A relaxation time constant exists only if several states are contained within the homogeneous width. Transitions to high vibrational levels are usually associated with inhomogeneous spectra. Under customary conditions of narrow-band optical excitation, only a fraction of the inhomogeneous width is excited. This fraction as well as the time scale of the temporal evolution depend on external parameters like pulse length and intensity. From published measurements of absorption with long and short pulses, evidence is deduced against any importance of collisionless relaxation in infrared multiphoton excitation.     

Electronic and vibrational spectra of diphenylmethane

From the critical analyses of Raman and infrared spectra, different normal modes of vibration of diphenylmethane (DPM) have been identified. The near ultraviolet absorption spectra of the molecule are found to consist of two band systems, one around 220 nm and the other around 270 nm with respective f-values 5.23 x 10(-2) and 6.44 x 10(-3). The first system is broad and shows few diffuse structures, whereas the later one exhibits very well-resolved structure. They are respectively assigned as 1L(a) and 1L(b) bands. The Raman excitation profiles of several normal modes have been analyzed to get structural and other information of different excited electronic states.     

Improved relations for vibrational relaxation in gases

Explicit expressions are presented for calculating vibration-to-translation (VT) energy conversion probabilities, essential in molecular laser isotope separation. VT conversions in molecular collisions occur by two mechanisms: (1) high-energy impact transfers prevailing at higher temperatures, and (2) Van der Waals-bonding encounters followed by (pre-)dissociations at lower temperatures. While mechanism (1) has been studied for over fifty years culminating in the Schwartz–Slawsky–Herzberg relation, a useful analytic expression for (2) has so far been lacking. An improved dimer formation theory developed by the author together with molecular pre-dissociation physics now provides a VT conversion relation for mechanism (2), which correctly predicts observations.     

Temperature dependence of vibrational relaxation in carbon tetrachloride

The temperature dependence of vibrational relaxation in carbon tetrachloride has been investigated from near the melting point to near the boiling point. The method of spontaneous Brillouin scattering has been used to determine the hypersonic frequencies and velocities as functions of temperature. The dispersion in the velocities are then compared with theoretical predictions in order to investigate the character of the relaxation.     

Light-scattering study of vibrational relaxation in liquid xylenes

Brillouin spectra obtained in dynamic light-scattering experiments are reported for the three isomeric xylenes (ortho-, meta-, and paradimethylbenzenes) between 288 and 363 K. Limiting sound velocities and relaxation times, as obtained from the polarized spectra using the theory developed by Mountain [J. Res. Natl. Bur. Stand. 70A, 207 (1966)], reveal the existence of a relaxation process. Our results suggest that the relaxation process in liquid xylenes has a purely vibrational nature. Vibrational-translational energy exchanges in xylenes are analyzed in terms of available molecular models and compared to those previously obtained for toluene and benzene. The results presented here confirm the important role played by the molecular geometry in the vibrational relaxation process, as the relative arrangement of the methyl groups has significant effect in determining the relaxing vibrational modes.     

Tunable raman excitation and vibrational relaxation in diatomic molecules

A generalization of the Raman vibrational excitation technique is presented. It is applied here to the room temperature study of V→T transfer in pure N₂ and O₂ or in mixtures with H₂ and He. The results on N₂ are the first obtained at room temperature, those on O₂ are in good agreement with existing data and provide a check of validity of the method.     

Femtosecond vibrational relaxation of large organic molecules

The intramolecular relaxation time of highly excited states of the vibronic manifold of the first excited singlet state of large organic molecules such as Nile Blue, Rhodamine 640, DODC iodide, Cresyl Violet, and Oxazine 725 in solution is shown to be extremely fast, less than 30 fs. We believe that this is a characteristic time of the relaxation due to anharmonic coupling among the many degrees of freedom of the molecule.     

Electronic structure and vibrational spectrum of acetylthiocarbamide

An approximate quantum chemical optimization of the geometric parameters of the acetylthiocarbamide molecule CH₃CONHCSNH₂ was carried out using the MNDO/H approximation. Bond lengths, bond angles, enthalpy of formation, total energy, ionization potential, and dipole moment were estimated, and the effective charges on the atoms and the bond orders were calculated. An analysis of the normal vibrations of the acetylthiocarbamide molecule and its deuteroanalog CH₃CONDCSND₂ was carried out. The force fields have been estimated. The frequencies, potential energy distribution among the vibrational coordinates, and the frequencies for the partially and completely deuterated acetylthiocarbamide molecules have been calculated.A. A. Baikov Institute of Metallurgy, Russian Academy of Sciences. A. A. Sechenov Moscow Academy of Medicine. N. S. Kurnakov Institute of General and Inorganic Chemistry. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 2, pp. 58–65, March–April, 1993.