John Christopher Deak

Ultrafast infrared–Raman studies of vibrational energy redistribution in polyatomic liquids

Vibrational energy relaxation and vibrational cooling of polyatomic liquids were studied with the ultrafast infrared–Raman (IR–Raman) technique. In the IR–Raman technique, a type of two-dimensional vibrational spectroscopy, a vibrational transition is pumped with a mid-infrared pulse and the instantaneous populations of all Raman-active transitions are simultaneously probed via incoherent anti-Stokes Raman scattering of a time-delayed visible pulse. The theoretical framework for these measurements, including force–force correlation function methods and perturbative techniques, is reviewed. Experimental aspects of the IR–Raman technique are discussed, including laser instrumentation, experimental set-up, the nature of the pumping and probing processes, detection sensitivity and optical background, and the interpretation of results including spectroscopic artifacts. Then examples are provided from recent research by our group, focusing on timely problems such as the pseudo-vibrational cascade, the dynamics of doorway vibrations, dynamics of overtones with Fermi resonance, multiple vibrational excitations via combination band pumping and spectral evolution in associated liquids. Copyright

Vibrational energy redistribution in liquid benzene

Abstract Ultrafast anti-Stokes Raman spectroscopy after mid-IR excitation of a C–H stretching transition is used to study vibrational energy relaxation (VER) and vibrational cooling (VC) of liquid benzene. By adding CCl4 to benzene and monitoring the CCl4 Raman transitions, the time-dependence of energy build-up in the bath was measured, which showed that VC in benzene occurs about 10 times slower than the 8 ps decay of the C–H stretch. A surprising result is that most VER lifetimes are practically the same as in a low-temperature crystal. This result is explained by the VC model of Hill and Dlott (J. Chem. Phys. 89 (1988) 830).

High-power picosecond mid-infrared optical parametric amplifier for infrared Raman spectroscopy

A high-power (50-MW), kilohertz, picosecond, mid-IR optical parametric amplifier that is pumped by an amplified Ti:sapphire laser and also produces a fixed-frequency visible pulse is described. Mid-IR pulse energies of 40-55 microJ with 0.6-0.8-ps durations and 35-cm (-1) bandwidths are reported in the 3650- 2800-cm (-1) range. The combination of picosecond mid-IR and visible pulses is useful for two-color spectroscopies, which require simultaneous time and frequency resolution. To illustrate the above, we present vibrational relaxation data for the polyatomic molecule nitromethane, using time-resolved infrared Raman spectroscopy.

When vibrations interact: ultrafast energy relaxation of vibrational pairs in polyatomic liquids

Abstract The ultrafast IR–Raman technique is used to compare picosecond timescale vibrational energy relaxation (VR) of a polyatomic liquid, acetonitrile (CH 3 CN), following excitation of either a C–H stretching fundamental (∼3000 cm −1 ) or a pair of vibrations at about the same energy, the CN stretch and C–C stretch. When the vibrational pair is excited via combination band pumping, the CN stretch lifetime decreases by more than an order of magnitude, and an oscillation is observed in the C–C stretch population. These studies of a pair of interacting vibrations on the same molecule provide insight into the complex behavior expected from vibrationally excited nascent products of chemical reactions of condensed-phase polyatomic molecules.

Vibrational energy in molecules probed with high time and space resolution

This article reviews experimental measurements of vibrational energy in condensed-phase molecules that simultaneously provide time resolution of picoseconds and spatial resolution of ångströms. In these measurements, ultrashort light pulses are used to input vibrational energy and probe dynamical processes. High spatial resolution is obtained using vibrational reporter groups in known locations on the molecules. Three examples are discussed in detail: (1) vibrational energy flow across molecules in a liquid from an OH–group to a CH3–group; (2) vibrational energy flow across a molecular surfactant monolayer that separates an aqueous and a non-polar phase in a suspension of reverse micelles; and (3) vibrational energy input by laser-driven shock waves to a self-assembled monolayer of long-chain alkane molecules. These experiments provide new insights into the movement of mechanical energy over short length and time scales where ordinary concepts of heat conduction no longer apply, where the concepts of quantum mechanical energy transfer reign supreme.

Measuring properties of interfacial and bulk water regions in a reverse micelle with IR spectroscopy: a volumetric analysis of the inhomogeneously broadened OH band.

The water OH stretching band (3000-3600 cm(-1)) was analyzed for absorption contributions from the respective bulk and interfacial water regions of a reverse micelle. This analysis was performed by correlating volume changes of these regions to changes in the OH band absorption as the micelle radius grows. The volumetric analysis is based on the well established expanding core-shell model for AOT reverse micelles and yields the dimensions of the water regions and their individual spectral responses in the OH band. The interfacial shell thickness was determined to be 0.45 nm for AOT reverse micelles in i-octane. It was found that each water region absorbs at most frequencies in the OH band; however, absorption on the red side of the OH band is dominated by bulk water, while absorption on the blue side is dominated by interfacial water. The bulk spectral response was found to be more similar to pure water, while the interfacial spectrum is strongly blue-shifted reflecting the weaker hydrogen bonding in this region. AOT reverse micelles with radii in the range 2-4 nm conformed well to the volumetric model. However, it was found that determination of the bulk water spectral response is particularly sensitive to uncertainty in the micelle radius.

Vibrational energy transfer in reverse micelle molecular nanostructures

Vibrational energy transfer across the single molecular layer of surfactant in several reverse micelles (Aerosol OT, lecithin and polyalkyleneoxide modified polydimethylsiloxane) has been studied using ultrafast IR-Raman spectroscopy.

The Primary Processes in Heme Protein Relaxation: The Coupled Reaction Coordinate Problem in Molecular Cooperativity

The phenomena of molecular cooperativity involves an interaction between two or more different protein moieties in which one protein’s function is controlled synergistically by its neighbor. Changes in state of the adjacent protein, such as changes in ligation of a receptor molecule, affect the reaction rate of its neighbor, often several nanometers away. Our current understanding of this process is based on structural changes at one site affecting the adjacent activation barrier. In order to affect a reaction coordinate at a distance, these structural changes must involve highly correlated atomic displacements coupling thousands of degrees of freedom. The key question then is what is the mechanism by which the reaction forces at one site propagate to adjacent sites, i.e., what is the communication pathway?

Energy Transduction and Deterministic Protein Motions

The activity of numerous biological molecules is controlled by the binding of a receptor which produce changes in structure. These structural changes, in turn, modify the barriers to various catalytic and transport functions. The molecular response function to the stimulus for motion involves specific structural changes that require the correlated displacement of thousands of atoms. Given the enormous number of degrees of freedom involved in these functionally relevant motions, it is clear that the energy of interaction with the receptor is being transferred in a highly directed fashion into the key atomic displacements. To understand the biomechanics, it is necessary to determine how energy is exchanged amongst these different degrees of freedom and the length scale of the forces displacing the atoms. In this regard, heme proteins provide ideal model systems. Large amounts of energy can be optically deposited in the center of the protein and the spatial dispersion or redistribution of this energy can be monitored using optical probes sensitive to vibrational or translational energy. It is also possible to optically trigger the functionally important structural changes involved in the al-losteric regulation of oxygen transport in heme proteins. Thus, both energy exchange processes and functionally important motions can be studied in a single system.