Vibrational lifetimes and vibrational line positions in polyatomic supercritical fluids near the critical point

Abstract

Articles you may be interested inVibrational spectroscopy and relaxation of an anharmonic oscillator coupled to harmonic bath Picosecond infrared pump-probe experiments are used to measure the vibrational lifetime of the asymmetric (T 1u ) CO stretching mode of W͑CO͒ 6 in supercritical CO 2 , C 2 H 6 , and CHF 3 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 (T r ϵT/T c Ϸ1.01), the lifetimes are essentially constant over a wide range of densities around the critical value ( c ). When the density is increased well past c , the lifetimes shorten further. At higher temperature (T r ϭ1.06) this region of constant vibrational lifetime is absent. Infrared absorption spectra of W͑CO͒ 6 and Rh͑CO͒ 2 acac in supercritical CO 2 , C 2 H 6 , and CHF 3 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, the peak positions are density independent, and then redshift further at densities well above c . It is shown that critical fluctuations play a dominant role in the observed effects. Theoretical calculations ascribe the density independence of the observables to the cancellation of various rapidly changing quantities near the critical point. The theory's calculation of density independence implicitly involves averages over all local densities and does not involve any form of solute-solvent clustering.

Vibrational Lifetimes and Spectral Shifts in Supercritical Fluids as a Function of Density: Experiments and Theory

“…Some of these data were reported previously, 8 and like the T 1 data, the improved experimental apparatus produced small changes for data near the critical point. The new and improved results are presented in Figure 3 as the line position shift relative to the peak position at zero density.…”

“…During the previous experiments, the pressure transducer was misreading the pressure by ∼10 psia (∼0.67 atm), a negligible error for the vast majority of reported experimental data. 8,36 However, near the critical density (F c ) and temperature (T c ), small pressure differences can create significant changes in the density. Thus, the densities calculated from the various equations of state were inaccurate for points in the immediate vicinity of (T c , F c ).…”

“…39,8 Briefly, a Q-switched, mode locked, cavity dumped Nd:YAG laser is used to produce ∼500 µJ 532 nm pulses (∼75 ps fwhm) which provide the pump pulses in a LiIO 3 OPA. The doubled output of the YAG laser also synchronously pumps a Rhodamine 610 dye laser.…”

“…7 T 1 is measured using infrared (IR) pump-probe experiments. The vibrational absorption line positions as a function of density are also reported in the three solvents 8 at the two temperatures.…”

“…22,31,34,35 It has been suggested that the formation of clusters can account for anomalous experimental observations of solute properties in SCFs. 22,31 However, in connection with the vibrational relaxation experiments, 8,36 a theoretical approach was used that was based on the changing solvent properties to account for the density dependence of T 1 , and attractive solutesolvent interactions were not part of the description of the solute/ solvent spatial distribution. 1 The earlier version of the theory was based on an analysis limited to small wavevectors (k) and was only able to describe the experimental data qualitatively.…”