Picosecond dynamics of hydrogen bond rearrangements during phase separation of a triethylamine and water mixture.

The earliest stages of phase separation in a liquid triethylamine (TEA)-water mixture were observed using a picosecond IR laser pulse to produce a temperature jump and ultrafast Raman spectroscopy. Raman spectral changes in the water OH stretching region showed that the temperature rise induced by IR pulses equilibrated within a few tens of picoseconds. Amplitude changes in the TEA CH-stretching region of difference Raman spectra consisted of an initial faster and a subsequent slower process. The faster process within 100 ps is attributed to hydrogen bond weakening caused by the temperature rise. The slower process attributed to phase separation was observed for several nanoseconds, showing the number of hydrogen bond between TEA and water gradually decreased with time. The kinetics of hydrogen bond scission during phase separation indicated a linear growth of the phase-separated component, as observed previously on the nanosecond time scale, rather than the more usual exponential growth. A peak blueshift was observed in the difference Raman spectra during phase separation. This shift implies that hydrogen bond scission of TEA-water aggregates involving very few water molecules took place in the initial stage of phase separation (up to 2 ns), and then was followed by the breaking of TEA-water pairs surrounded by water molecules. This effect may be a result from spatial inhomogeneities associated with the phase separation process: aggregates or clusters existing naturally in solution even below the lower critical soluble temperature.

Vibrational energy relaxation of liquid aryl-halides X-C6H5 (X = F, Cl, Br, I).

Anti-Stokes Raman spectroscopy was used to probe vibrational energy dynamics in liquid ambient-temperature aryl-halides, X-Ph (X = F, Cl, Br, I; -Ph = C(6)H(5)), following IR excitation of a 3068 cm(-1) CH-stretching transition. Five ring vibrations and two substituent-dependent vibrations were monitored in each aryl-halide. Overall, the vibrational relaxation (VR) lifetimes in aryl-halides were shorter than those in normal benzene (H-Ph). The aryl-halide CH-stretch lifetimes increased in the order F, Cl, Br, I, ranging from 2.5 to 3.4 ps, compared with 6.2 ps in H-Ph. The aryl-halide energy transfer processes were similar overall with four exceptions. Three of the four exceptions could be explained as a result of faster VR of midrange vibrations (1000-1600 cm(-1)) in the heavier aryl-halides. The fourth appeared to result from a coincidental resonance in chlorobenzene that does not occur in the other aryl-halides. Among the aryl-halides, the decay of CH-stretching excitations (∼3070 cm(-1)) was slower in the heavier species, but the decay of midrange vibrations was faster in the heavier species. This seeming contradiction could be explained if VR depended primarily on the density of states (DOS) of the lower tiers of vibrational excitations. The DOS for the first few (1-4) tiers is similar for all aryl-halides in the CH-stretch region, but DOS increases with increasing halide mass in the midrange region.

Vibrational energy dynamics of normal and deuterated liquid benzene.

Ultrafast Raman spectroscopy with infrared (IR) excitation is used to study vibrational energy dynamics of ambient temperature liquids benzene and benzene-d(6). After IR pumping of a CH-stretch or CD-stretch parent excitation, the redistribution of vibrational energy is probed with anti-Stokes Raman. Ten benzene or 12 benzene-d(6) vibrations out of 30 total have large enough cross sections to be observed. The pathways, quantum yields, and lifetimes for energy transfer among these vibrations are quantified. Using a CCl(4) molecular thermometer, we demonstrate an ultrafast Raman calorimetry method which allows measurement of the rate that benzene vibrational energy is dissipated into the bath. On the basis of energy conservation, we then determine the time-dependent dissipation of aggregate vibrational energy from the unobserved, "invisible" vibrations. During the approximately 1 ps IR excitation process, vibrational energy is coherently redistributed to several vibrational modes ("coherently" means the rate is faster than (T(2))(-1) of the pumped transition). This energy is then further redistributed in an incoherent intramolecular vibrational relaxation process with a 6 ps T(1) time constant. The subsequent dynamics involve energy transfer processes accompanied by vibrational energy dissipation to the bath. This vibrational cooling process has a half-life of 30 ps in benzene and 20 ps in benzene-d(6), and thermalization is complete in approximately 100 ps. The observed strongly Raman-active vibrations have about the same amount of energy per mode as the invisible vibrations. The invisible vibrational energy in benzene decays somewhat faster than the observed energy. These two decay rates are about the same in benzene-d(6).

Vibrational energy dynamics of glycine, N-methylacetamide, and benzoate anion in aqueous (D2O) solution.

Ultrafast infrared-Raman spectroscopy is used to study vibrational energy dynamics of three molecules in aqueous solution (D(2)O) that serve as models for the building blocks of peptides. These are glycine-d(3) zwitterion (GLY), N-methylacetamide-d (NMA), and benzoate anion (BZ). GLY is the simplest amino acid, NMA a model compound with a peptide bond, and BZ a model for aromatic side chains. An ultrashort IR pulse pumps a parent CH-stretch on each solute. Anti-Stokes Raman monitors energy flow through the solutes' strongly Raman-active transitions. Stokes Raman of D(2)O stretching functions as a molecular thermometer to monitor energy dissipation from solute to solvent. A three-stage model is used to summarize the vibrational energy redistribution process and to provide a framework for discussing energy dynamics of different molecules. The initial CH-stretch excitation is found to be delocalized over some or all of the solute molecule in NMA and BZ but not in GLY. The overall time constants for energy dissipation are 7.2 ps for GLY, 4.9 ps for NMA, and 8.0 ps for BZ. CH-stretch energy in GLY is redistributed in a nearly statistical manner among observed GLY vibrations. In NMA the energy is distributed among about one-half of the observed vibrations, and in BZ much of the observed energy is channeled along a CH-stretch to the ring stretch pathway. The strongly Raman-active vibrations accurately represent the flow of vibrational energy through NMA but not through GLY or BZ.

Measurement of the distribution of site enhancements in surface-enhanced Raman scattering.

On nanotextured noble-metal surfaces, surface-enhanced Raman scattering (SERS) is observed, where Raman scattering is enhanced by a factor, G, that is frequently about one million, but underlying the factor G is a broad distribution of local enhancement factors, eta. We have measured this distribution for benzenethiolate molecules on a 330-nanometer silver-coated nanosphere lattice using incident light of wavelength 532 nanometers. A series of laser pulses with increasing electric fields burned away molecules at sites with progressively decreasing electromagnetic enhancement factors. The enhancement distribution P(eta)deta was found to be a power law proportional to (eta)(-1.75), with minimum and maximum values of 2.8 x 10(4) and 4.1 x 10(10), respectively. The hottest sites (eta >10(9)) account for just 63 in 1,000,000 of the total but contribute 24% to the overall SERS intensity.

Ultrafast flash thermal conductance of molecular chains.

At the level of individual molecules, familiar concepts of heat transport no longer apply. When large amounts of heat are transported through a molecule, a crucial process in molecular electronic devices, energy is carried by discrete molecular vibrational excitations. We studied heat transport through self-assembled monolayers of long-chain hydrocarbon molecules anchored to a gold substrate by ultrafast heating of the gold with a femtosecond laser pulse. When the heat reached the methyl groups at the chain ends, a nonlinear coherent vibrational spectroscopy technique detected the resulting thermally induced disorder. The flow of heat into the chains was limited by the interface conductance. The leading edge of the heat burst traveled ballistically along the chains at a velocity of 1 kilometer per second. The molecular conductance per chain was 50 picowatts per kelvin.