K. D. Rector

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...

Vibrational anharmonicity and multilevel vibrational dephasing from vibrational echo beats

Vibrational echo experiments were performed on the IR active CO stretching modes (∼2000 cm−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 25.4 cm−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 dephasing mechanisms in liquids and glasses: Vibrational echo experiments

Picosecond vibrational echo studies of the asymmetric stretching mode (2010 cm−1) of (acetylacetonato)dicarbonylrhodium(I) [Rh(CO)2acac] in liquid and glassy dibutyl phthalate (DBP) (3.5 K to 250 K) are reported and compared to previous measurements of a similar mode of tungsten hexacarbonyl [W(CO)6]. The Rh(CO)2acac pure dephasing shows a T1 dependence on temperature at very low temperature with a change to an exponentially activated process (ΔE≅400 cm−1) above ∼20 K. There is no change in the functional form of the temperature dependence in passing from the glass to the liquid. It is proposed that the T1 dependence arises from coupling of the vibration to the glass’s tunneling two level systems. The activated process arises from coupling of the high-frequency CO stretch to the 405 cm−1 Rh–C stretch. Excitation of the Rh–C stretch produces changes in the back donation of electron density from the rhodium dπ orbital to the CO π* antibonding orbital, shifting the CO stretching transition frequency and caus...

Vibrational echoes: a new approach to condensed-matter vibrational spectroscopy

This review describes the first ultrafast infrared vibrational echo experiments, which are used to examine liquids, glasses and proteins. Like the nuclear magnetic resonance (NMR) spin echo and other NMR pulse sequences, the vibrational echo can extract dynamical and spectroscopic information that cannot be obtained from a vibrational absorption spectrum. The vibrational echo measures the homogeneous vibrationallinewidtheven if the absorption line is massively inhomogeneously broadened. When combined with pump-probe (transient absorption) experiments, the homogeneous pure dephasing (energy level fluctuations) is obtained. Conducting these experiments as a function of temperature provides information on dynamics and intermolecular interactions. The nature of the method and the experimental procedures are outlined. Experimental results are presented for the metal carbonyl solutes, W(CO) and Rh(CO) acac, in several glassy and liquid solvents. The dynamics of the CO ligand bound at the active site of the prot...

Vibrational echo spectroscopy: Spectral selectivity from vibrational coherence

Theory and experimental data are presented which illustrate a new method for performing two-dimensional vibrational spectroscopy using ultrafast pulsed infrared lasers, called vibrational echo spectroscopy (VES). The VES technique can generate a vibrational spectrum with background suppression using the nonlinear vibrational echo pulse sequence. The vibrational echo pulse sequence is used with the delay between the excitation pulses fixed while the excitation wavelength is varied. A detailed theory of VES is presented which calculates the full third order nonlinear polarization including rephasing and nonrephasing diagrams. Finite width laser pulses are used and the calculations are performed for a model spectrum with two or more peaks. Two mechanisms that can result in background and peak suppression are illustrated. The mechanisms are based on differences in homogeneous dephasing times (T2) or transition dipole matrix element magnitudes. Although the VES line shape differs from the absorption line shape...

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.

Two-pulse echo experiments in the spectral diffusion regime

The two-pulse echo sequence is examined for the case in which the frequency modulation time τm of the transition is intermediate between the well known limiting cases of very fast modulation (motional narrowing) and very slow or static modulation (inhomogeneous broadening). Within this spectral diffusion regime, the interpretation of the echo decay differs markedly from standard treatments. If the frequency-frequency correlation function initially decays as 1−tβ, the echo decay time TE is proportional to τmβ/(β+2). These results reduce to those of Yan and Mukamel [J. Chem. Phys. 94, 179 (1991)] for β=1. Drawing on a viscoelastic model, the theoretical results are compared to viscosity and temperature dependent vibrational echo experiments on myoglobin–CO. A τm1/3 dependence is observed, as is predicted for an exponential decay of the frequency–frequency correlation function.

Dynamics in globular proteins: vibrational echo experiments

Abstract The temperature-dependent vibrational pure dephasing of the CO stretching mode of carbonmonoxyhemoglobin (HbCO) in an ethylene glycol:water mixture is reported and compared to previously measured dephasing of carbonmonoxymyoglobin (MbCO). HbCO displays a T 1.3 -dependent pure dephasing rate between 15 and ∼150 K, suggesting glass-like behavior. Above 150 K, the temperature dependence becomes steeper. The functional form of the HbCO and MbCO pure dephasing temperature dependences are identical within error. However, the hemoglobin pure dephasing is 27% smaller at all temperatures studied, suggesting that the fast dynamics or the protein electric field–CO coupling is smaller in hemoglobin than in myoglobin.

Nitro group asymmetric stretching mode lifetimes of molecules used in energetic materials

Picosecond infrared pump-probe measurements of the vibrational relaxation of the nitro NO functional group 2 y1 . asymmetric stretch ; 1580 cm of several molecules used in energetic materials, TNAZ, RDX, HMX, and CL-20 .abbreviations defined in text , and nitromethane, are presented. In addition, a temperature-dependent study was performed between 50 and 298 K on TNAZ. All of the lifetimes fall in the range of 2-6 ps with the exception of nitromethane, which has a lifetime of 16 ps. The temperature dependence of TNAZ is flat and solvent independent, suggesting an intra-molecular relaxation pathway that does not involve low-frequency modes. q 1999 Elsevier Science B.V. All rights reserved.

Ultrafast infrared spectroscopy in biomolecules: Active site dynamics of heme proteins

Rapid advances in the generation of intense tunable ultrashort mid-infrared (IR) laser pulses allow the use of ultrafast IR pump-probe and vibrational echo experiments to investigate the dynamics of the fundamental vibrational transition of CO bound to the active site of heme proteins. The studies were performed using a free-electron laser (FEL) and an experimental set up at the Stanford University FEL Center. These novel techniques are discussed in some detail. Pump-probe experiments on myoglobin-CO (MbCO) measure CO vibrational relaxation (VR). The VR process involves loss of vibrational excitation from CO to the protein and solvent. Infrared vibrational echoes measure CO vibrational dephasing. The quantum mechanical treatment of the force-correlation function description of vibrational dynamics in condensed phases is described briefly. A quantum mechanical treatment is needed to explain the temperature dependence of VR in Mb-CO from 10 to 300 K. A molecular-level description including elements of heme protein structure in the treatment of vibrational dynamics is also discussed. Vibrational relaxation of CO in Mb occurs on the 10−11-s time scale. VR was studied in proteins with single-site mutations, proteins from different species, and model heme compounds. A roughly linear relationship between carbonyl stretching frequency and VR rate has been observed. The dominant VR pathway is shown to involve anharmonic coupling from CO through the π-bonded network of the porphyrin, to porphyrin vibrations with frequencies > 400 cm−1. The heme protein influences VR of bound ligands at the active site primarily via altering the through π-bond coupling between CO and heme. Preliminary vibrational echo studies of the effects of protein conformational relaxation dynamics on ligand dephasing are also reported.