D. J. Myers

VIBRATIONAL LIFETIMES AND VIBRATIONAL LINE POSITIONS IN POLYATOMIC SUPERCRITICAL FLUIDS NEAR THE CRITICAL POINT

Picosecond infrared pump–probe experiments are used to measure the vibrational lifetime of the asymmetric (T1u) CO stretching mode of W(CO)6 in supercritical CO2, C2H6, and CHF3 as a function of solvent density and temperature. As the density is increased at constant temperature from low, gaslike densities, the lifetimes become shorter. However, in all three solvents, it is found that within a few degrees of the critical temperature (Tr≡T/Tc≈1.01), the lifetimes are essentially constant over a wide range of densities around the critical value (ρc). When the density is increased well past ρc, the lifetimes shorten further. At higher temperature (Tr=1.06) this region of constant vibrational lifetime is absent. Infrared absorption spectra of W(CO)6 and Rh(CO)2acac in supercritical CO2, C2H6, and CHF3 acquired for the same isotherms show that the vibrational spectral peak shifts follow similar trends with density. The peak positions shift to lower energy as the density is increased. Near the critical point, t...

Temperature dependence of vibrational lifetimes at the critical density in supercritical mixtures

Experimental measurements are reported for the temperature dependence of the vibrational lifetime, T1, of the asymmetric CO stretching mode of tungsten hexacarbonyl in supercritical ethane at constant density from just above the critical temperature to substantially higher temperatures. T1 is found initially to increase with temperature along an isochore (reaching a maximum at about 70° above the critical point of ethane), and then subsequently to decrease. Using a recent classical theory of vibrational relaxation, we attempt to rationalize the T1 data. This behavior can be semiquantitatively reproduced by the theory if quantum corrections to the classical rate expressions are assumed to be temperature independent in the limit when the transition energy is much greater than thermal energy. In this case, the theory indicates that the initial increase in T1 with temperature arises because of a competition between properties of the solvent which are changing rapidly as the temperature is raised above the cri...

Vibrational relaxation of a polyatomic solute in a polyatomic supercritical fluid near the critical point

Vibrational lifetimes and absorption spectra of the asymmetric CO stretching mode (∼1990 cm−1) of W(CO)6 in supercritical CO2 are reported as functions of solvent density and temperature. Close to the critical temperature, the observables are density independent over a twofold range of density. Possible explanations are discussed for this unique behavior.

Temperature dependent vibrational lifetimes in supercritical fluids near the critical point

Vibrational relaxation measurements on the CO asymmetric stretching mode (similar to 1980 cm(-1)) of tungsten hexacarbonyl (W(CO)(6)) as a function of temperature at constant density in several supercritical solvents in the vicinity of the critical point are presented. In supercritical ethane, at the critical density, there is a region above the critical temperature (Tc) in which the lifetime increases with increasing temperature. When the temperature is raised sufficiently (similar to T-c + 70 degrees C), the lifetime decreases with further increase in temperature. A recent hydrodynamic/thermodynamic theory of vibrational relaxation in supercritical fluids reproduces this behavior semiquantitatively. The temperature dependent data for fixed densities somewhat above and below the critical density is in better agreement with the theory. In fluoroform solvent at the critical density, the vibrational lifetime also initially increases with increasing temperature. However, in supercritical CO2 at the critical density, the temperature dependent vibrational lifetime decreases approximately linearly with temperature beginning almost immediately above T-c. The theory does not reproduce this behavior. A comparison between the absolute lifetimes in the three solvents and the temperature trends is made.

Vibrational dynamics of large hot molecules in the collisionless gas phase

Infrared ps pump-probe experiments are presented for the P, Q, and R rotational branches of the asymmetric CO stretching mode of tungsten hexacarbonyl (1997 cm−1) in the collisionless gas phase. The pump-probe decays are tri-exponentials (140 ps, 1.3 ns, and >100 ns) in contrast to single exponential decays observed in supercritical fluids and liquid solvents. The 1.3 ns decay component is the vibrational energy relaxation (VER) time. The long component occurs following intramolecular VER into a distribution of low-frequency modes. After VER is complete, the R signal is 48%, the Q signal is 29%, and the P signal is −10% (absorption increase) compared to the t=0 signal. These long-lived signals result from an increase in the occupation numbers of low-frequency modes (internal heating) that causes a shift of the vibrational spectrum. The fastest decay is produced by spectral diffusion. The spectrally narrow pump pulse burns a hole in the inhomogeneous ground state spectrum and generates a narrow spectral po...

Non-exponential relaxation of a single quantum vibrational excitation of a large molecule in collision free gas phase at elevated temperature

Abstract The intramolecular vibrational dynamics of the asymmetric CO stretch of W(CO) 6 (∼2000 cm −1 ) have been studied in the collision free gas phase at 326 K using ps mid-infrared pump–probe spectroscopy. The pump–probe decay is a tri-exponential with components 140 ps, 1.28 ns, and >100 ns. The middle component is the population relaxation into the collection of low frequency intramolecular modes. This relaxation heats the low frequency modes causing a spectral shift that gives rise to the long component. The fast component, which is described theoretically, is caused by the dynamics of the superposition state composed of thermally populated low frequency modes.

Density dependent vibrational relaxation in supercritical fluids

y1 . . The vibrational lifetimes of the asymmetric CO stretching mode 1990 cm of W CO in the gas phase and as a 6 function of density in supercritical ethane are examined using infrared pump-probe experiments. The gas phase measure- ment provides the zero density rate constant, which permits the effect of the ethane solvent to be separated from internal relaxation dynamics. The non-exponential gas phase vibrational dynamics are briefly discussed. The density dependence of the lifetime is compared to an extension of a hydrodynamic theory of vibrational relaxation in supercritical fluids to the regime of large wavevector. Agreement between experiment and theory is found to be good. q 1999 Elsevier Science B.V. All rights reserved.

Isomerization and intermolecular solute-solvent interactions of ethyl isocyanate: Ultrafast infrared vibrational echoes and linear vibrational spectroscopy

Thermally induced gauche–trans isomerization and direct solute–solvent interactions of the solute, ethyl isocyanate (EIC), in the solvent, 2-methylpentane (2MP), are investigated using ultrafast infrared vibrational echo experiments and linear vibrational absorption spectroscopy of the isocyanate (N=C=O) antisymmetric stretching mode (2278 cm−1). Both the EIC vibrational echo measured pure vibrational dephasing and the absorption spectra show complex behavior as a function of temperature from room temperature to 8 K. The EIC data are compared to absorption experiments on the same mode of isocyanic acid (HNCO), which cannot undergo isomerization. To describe the observations, a model is presented that involves both intramolecular dynamics and intermolecular dynamical interactions. At room temperature, gauche–trans isomerization is very fast, and the isomerization dynamics contribution to the vibrational echo decay and the absorption line shape is small because it is motionally narrowed. The dominant contri...

Temperature and density dependent solute vibrational relaxation in supercritical fluoroform

Temperature- and density-dependent vibrational relaxation data for the v6 asymmetric stretch of W(CO)6 in supercritical fluoroform (trifluoromethane, CHF3) are presented and compared to a recent theory of solute vibrational relaxation. The theory, which uses thermodynamic and hydrodynamic conditions of the solvent as input parameters, shows very good agreement in reproducing the temperature- and density-dependent trends of the experimental data with a minimum of adjustable parameters. Once a small number of parameters are fixed by fitting the functional form of the density dependence, there are no adjustable parameters in the calculations of the temperature dependence.

Temperature dependence of solute vibrational relaxation in supercritical fluids: experiment and theory

Vibrational lifetime (T-1) data for the asymmetric CO stretching mode of W(CO)(6) in supercritical ethane and carbon dioxide as a function of temperature at various fixed densities are compared to a recent extended hydrodynamic theory [1]. In ethane at the critical density, as the temperature is raised, T-1 initially becomes longer, reaching a maximum similar to 70 K above the critical temperature. T-1 decreases with further increase in temperature. The theory is able to reproduce this behavior nearly quantitatively without free parameters. At high density in ethane and in CO2, the inverted temperature dependence is not observed, in agreement with theoretical calculations.