We present pump-probe and Raman measurements on individual plasmonic nano-junctions. The time-resolved measurements reveal differences between capacitive and conductive junctions, and paint a detailed picture of the ultrafast electrodynamics of the nano-junction. The insights gleaned from these measurements help interpret the ultrafast response of single molecules placed in the junction.
We present combined surface-enhanced stimulated Raman scattering (SE-SRS) and surface-enhanced coherent anti-Stokes Raman scattering (SE-CARS) measurements on individual plasmonic antennas dressed with bipyridyl-ethylene molecules. By carefully optimizing the conditions for performing SE-SRS experiments, we have obtained stable and reproducible molecular surface-enhanced SRS spectra from single nano-antennas. Using surface-enhanced Raman scattering (SERS) and transmission electron microscopy of the same antennas, we confirm that the observed SE-SRS signals originate from only one or a few molecules. We highlight the physics of surface enhancement in the context of coherent Raman scattering and derive sensitivity parameters under the relevant conditions. The implications of single molecule SRS measurements are discussed.
Surface enhanced Raman scattering (SERS) is a popular technique for detecting and analyzing molecules at very low concentrations. The sensitivity of SERS is high enough to detect single molecules. It has proven difficult, however, to perform similar measurements in the so-called nonlinear optical regime, a regime in which the molecule is responding to multiple light pulses. Nonetheless, recent experiments indicate that after careful optimization, it is possible to generate signals derived from nonlinear analogs of SERS. Such measurements make it possible to view molecular vibrations in real time, which amounts to the femto- to pico-second range. In this contribution, we discuss in detail under which conditions detectable surface-enhanced coherent Raman signals can be expected, provide experimental evidence of coherent Raman scattering of single molecules, and highlight the unique information that can be attained from such measurements.
Femtosecond laser pulses focused inside liquid helium initiate an excitation sequence that leads to formation of molecular Rydberg states He<SUB>2</SUB> detected by fluorescence spectroscopy. Unlike in the case of longer laser pulses, the excitations may be created in a controllable way, at light intensities below dielectric breakdown. The initial step is ionization of He atoms, as demonstrate by charge separation in external electric field. A sequence of the subsequent processes is proposed, which accounts for rapid production of He<SUB>2</SUB>, in less than 10 ns, observed by nanosecond time- resolved laser induced fluorescence following the excitation pulse. the lowest triplet state excimers He<SUB>2</SUB>(<SUP>3</SUP>a), probed in the latter experiment, are long-lived and survive in concentrations of the order of 10<SUP>11</SUP>-10<SUP>12</SUP> cm<SUP>-3</SUP> on a millisecond time scale. Femtosecond time- resolved spectroscopy was performed on He<SUB>2</SUB>* molecules in liquid He, using the pump-probe sequence He<SUB>2</SUB>*(<SUP>3</SUP>a) + 790 nm yields He<SUB>2</SUB>*(<SUP>3</SUP>c), He<SUB>2</SUB>*(<SUP>3</SUP>c) + 790 nm yields He<SUB>2</SUB>*(<SUP>3</SUP>f). The observed decay of the transient signals with characteristic time 3.5 ps is thought to be due to solvent motion corresponding to the relaxation of the liquid helium 'bubble' around the intermediate He<SUB>2</SUB>*(<SUP>3</SUP>c) state.
The mixed-order semiclassical molecular dynamics method is used for the calculation of quantum time correlation functions in extended systems. The method allows the consistent treatment of a selected number of degrees of freedom to second-order in the stationary phase approximation through the Herman and Kluk propagator, while the rest of the system is treated to zeroth-order, using frozen Gaussians. The formulation is applied to calculate the absorption spectrum, of the B $IMP X transition of Cl<SUB>2</SUB> isolated in solid Ar, a spectrum that shows zero-phonon lines and phonon sidebands with relative intensities that depend on the excited state vibrational level. The explicit simulation of quantum time correlation functions of the system consisting of 321 degrees of freedom, reproduces the spectrum and allows its interpretation in terms of the underlying molecular motions.IN order to extend the semiclassical methods to longer timescale a new extension of Herman-Kluk propagator is developed, which combines classical propagation of trajectories for length where the initial value propagator remains accurate, followed by Monte-Carlo regeneration of the ensemble of trajectories and continuation of propagation. This new method is tested for the calculation of long time dynamics in a 1D Morse oscillator.
Understanding many-body dynamics on a molecular level is a major aim in condensed phase photodynamical research. Much can be learned about this general field through studies of molecular photodissociation in model systems, namely crystalline rare gas solids. The aim of this presentation is to illustrate this proposition by highlights drawn from a variety of related investigations. Under the title of photodissociation in solids, several related processes can be categorized: charge transfer induced 1 radiative dis sociation ,2 atomic photomobility,3 are examples.