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 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).
Temporal bound solitons are observed experimentally in a passively mode-locked figure-eight fiber laser with a dispersion-imbalanced nonlinear optical loop mirror (DI-NOLM). Soliton interactions are suppressed by use of a spectral bandpass filter, and Gordon-Haus timing jitter is eliminated with a DI-NOLM, which removes the cw light component in the laser cavity. The bound solitons are found to be stable for several hours in the laser cavity when no external perturbation is applied.
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.
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