A. Tokmakoff

Theory of vibrational relaxation of polyatomic molecules in liquids

A simple tractable theory of vibrational relaxation of polyatomic molecules in polyatomic solvents, which is also applicable to solid solutions, is presented. The theory takes as its starting point Fermi’s golden rule, avoids additional assumptions such as the rotating wave or random phase approximations, and treats both the internal degrees of freedom of the relaxing molecule and the bath degrees of freedom in a fully quantum mechanical manner. The results yield intuitively understandable expressions for the relaxation rates. The treatment of the annihilation as well as the creation of all participating bosons allows the theory to go beyond earlier analyses which treated only cascade processes. New predicted features include temperature effects and asymmetry effects in the frequency dependence. The theory is constructed in a manner which facilitates the use of recent developments in the analysis of instantaneous normal modes of liquids.

HOMOGENEOUS VIBRATIONAL DYNAMICS AND INHOMOGENEOUS BROADENING IN GLASS-FORMING LIQUIDS : INFRARED PHOTON ECHO EXPERIMENTS FROM ROOM TEMPERATURE TO 10 K

A study of the temperature dependence of the homogeneous linewidth and inhomogeneous broadening of a high‐frequency vibrational transition of a polyatomic molecule in three molecular glass‐forming liquids is presented. Picosecond infrared photon echo and pump–probe experiments were used to examine the dynamics that give rise to the vibrational line shape. The homogeneous vibrational linewidth of the asymmetric CO stretch of tungsten hexacarbonyl (∼1980 cm−1) was measured in 2‐methylpentane, 2‐methyltetrahydrofuran, and dibutylphthalate from 300 K, through the supercooled liquids and glass transitions, to 10 K. The temperature dependences of the homogeneous linewidths in the three glasses are all well described by a T2 power law. The absorption linewidths for all glasses are seen to be massively inhomogeneously broadened at low temperature. In the room temperature liquids, while the vibrational line in 2‐methylpentane is homogeneously broadened, the line in dibutylphthalate is still extensively inhomogeneo...

Temperature-dependent vibrational relaxation in polyatomic liquids: Picosecond infrared pump-probe experiments

Vibrational lifetimes of metal carbonyl solutes in two liquids were studied as a function of temperature from the melting points to the boiling points. Picosecond infrared pump–probe experiments were performed at the absorption maxima of the T1u CO stretching vibration (∼1980 cm−1) of Cr(CO)6 and W(CO)6 dissolved in carbon tetrachloride (CCl4) and chloroform (CHCl3). The temperature dependencies of the lifetimes in CCl4 are dramatically different from those in CHCl3. A decrease in the vibrational lifetime with temperature is seen for both metal carbonyls in CCl4. However, the vibrational lifetime of W(CO)6 in CHCl3 becomes longer as the temperature is increased, while the lifetime of Cr(CO)6 decreases only slightly. To understand the vibrational dynamics in these systems it is necessary to consider the temperature dependencies of the thermal populations of low frequency phonons (instantaneous normal modes) of the liquids, the phonon density of states, and the anharmonic coupling matrix elements.

The low frequency density of states and vibrational population dynamics of polyatomic molecules in liquids

Instantaneous normal mode calculations of the low frequency solvent modes of carbon tetrachloride (CCl4) and chloroform (CHCl3), and experiments on the vibrational population dynamics of the T1u CO stretching mode (∼1980 cm−1) of tungsten hexacarbonyl in CCl4 and CHCl3 are used to understand factors affecting the temperature dependence of the vibrational lifetime. Picosecond infrared pump–probe experiments measuring the vibrational lifetime of the T1u mode from the melting points to the boiling points of the two solvents show a dramatic solvent dependence. In CCl4, the vibrational lifetime decreases as the temperature is increased; however, in CHCl3, the vibrational lifetime actually becomes longer as the temperature is increased. The change in thermal occupation numbers of the modes in the solute/solvent systems cannot account for this difference. Changes in the density of states of the instantaneous normal modes and changes in the magnitude of the anharmonic coupling matrix elements are considered. The ...

Vibrational anharmonicity and multilevel vibrational dephasing from vibrational echo beats

Vibrational echo experiments were performed on the IR active CO stretching modes (∼2000u2009cm−1) of rhodium dicarbonylacetylacetonate [Rh(CO)2acac] and tungsten hexacarbonyl [W(CO)6] in dibutylphthalate and a mutant of myoglobin-CO (H64V-CO) in glycerol–water using ps IR pulses from a free electron laser. The echo decays display pronounced beats and are nonexponential. The beats and nonexponential decays arise because the bandwidths of the laser pulses exceed the vibrational anharmonicities, leading to the excitation and dephasing of a multilevel coherence. From the beat frequencies, the anharmonicities are determined to be 14.7, 13.5, and 25.4u2009cm−1, for W(CO)6, Rh(CO)2acac, and H64V-CO, respectively. From the components of the nonexponential decays, the vibrational dephasing at very low temperature of both the v=0–1 and v=1–2 transitions are determined. At the lowest temperatures, T2≈2T1, so the v=2 lifetimes are obtained for the three molecules. These are found to be significantly shorter than the v=1 life...

Vibrational spectral diffusion and population dynamics in a glass-forming liquid: Variable bandwidth picosecond infrared spectroscopy

The temperature‐dependent vibrational population dynamics and spectral diffusion of the CO stretching mode of tungsten hexacarbonyl in 2‐methylpentane are observed from the room temperature liquid to the low temperature glass using picosecond infrared transient grating and pump–probe experiments. These experiments were performed between 10 and 300 K on the triply degenerate T1u asymmetric CO stretching mode at 1984 cm−1 using pulses with bandwidths narrower and wider than the absorption bandwidth of the transition. The rate of vibrational population relaxation (100≤T1<150 ps) is observed to decrease with increasing temperature. The orientational dynamics for this transition are observed on a faster time scale than the population relaxation. Although the liquid viscosity changes over 14 orders of magnitude, the orientational relaxation rate slows by less than one order of magnitude over the full temperature range. By comparing polarization‐dependent experiments performed with both narrow and broad bandwidt...

Phonon-induced scattering between vibrations and multiphoton vibrational up-pumping in liquid solution

Abstract Fast phonon-induced scattering between an infrared active and a Raman active vibration in solution is investigated using picosecond infrared pump/anti-Stokes Raman probe experiments. Population from the infrared active T 1u CO stretching mode of tungsten hexacarbonyl in carbon tetrachloride at 1980 cm −1 scatters to the Raman active E g mode at 2012 cm −1 but not to the A 1g mode at 2116 cm −1 . Equilibration occurs rapidly compared to the 700 ps population relaxation time. A power dependence of the line shape of the Raman mode indicates that significant population is pumped into high vibrational levels when high infrared pump powers are used.

Multilevel vibrational dephasing and vibrational anharmonicity from infrared photon echo beats

Abstract Vibrational photon echo experiments were performed on the asymmetric CO stretching mode of tungsten hexacarbonyl in glassy dibutylphthalate as a function of temperature using sub-picosecond infrared pulses (1976 cm −1 ) from a free electron laser. The echo decays display pronounced beats and are bi-exponential. The beats and bi-exponential decays arise because the bandwith of the pulses exceed the vibrational anharmonicity, leading to the excitation and dephasing of a multilevel coherence. From the beat frequency, the anharmonicity is determined to be 14.7 cm −1 . From the bi-exponential decay components, the temperature-dependent vibrational dephasing of both the v = 0 → 1 and v = 1 → 2 transitions are determined.

Vibrational photon echoes in a liquid and glass: Room temperature to 10 K

Picosecond infrared vibrational photon echo experiments were performed on the asymmetric CO stretching mode (1983 cm−1) of tungsten hexacarbonyl in 2‐methylpentane from room temperature to 10 K using a free electron laser. This is the first report of a room temperature infrared vibrational photon echo in a liquid.

Nanosecond time scale dynamics of pseudo-nematic domains in the isotropic phase of liquid crystals

Abstract The slow reorientational dynamics of isotropic methoxybenzylidene-butylaniline (MBBA) were measured, using a transient grating optical Kerr effect experiment, over a wide temperature range (49.4°–119.7°C). The range of validity of the Landau—de Gennes theory for hydrodynamic relaxation in ordered fluids is determined. The results are compared to previous experiments on pentylcyanobiphenyl (5CB). Both liquid crystals conform to the Landau—de Gennes theory until the pseudo-nematic domain correlation length falls below three molecular lengths.