Alan G. Joly

Coherent molecular vibrational motion observed in the time domain through impulsive stimulated Raman scattering

Femtosecond time-resolved observations of coherent molecular vibrations are carried out through impulse-stimulated Raman scattering (ISRS). Individual cycles of vibrational oscillation, as well as vibrational dephasing, are time resolved. ISRS therefore provides a means through which time-resolved spectroscopy of vibrationally distorted molecules can be carried out. Several experimental configurations involving either crossed excitation pulses or a single excitation pulse are discussed theoretically and demonstrated experimentally. >

Metal carbonyl photochemistry in organic solvents: Femtosecond transient absorption and preliminary resonance Raman spectroscopy

Abstract Solvent effects in elementary chemical reactions are studied using femtosecond transient absorption spectroscopy. Photodissociation of M(CO)6 (M = Cr, Mo, or w) is examined in a variety of linear chain alcohols to ascertain the role of the solvent on the initial CO dissociation event. Resonance Raman experiments are also used to obtain more information about the reaction coordinate for CO dissociation. No significant solvent effects are seen on the short time (∼350 fs) transient absorption signal assigned to CO dissociation; however, effects are seen on an intermediate timescale attributed to differences in solvent complexation dynamics. Subsequent vibrational cooling of the nascent Cr (CO) 5 (SOLVENT) complex is observed consistent with earlier results.

Communication: Spectroscopic phase and lineshapes in high-resolution broadband sum frequency vibrational spectroscopy: Resolving interfacial inhomogeneities of “identical” molecular groups

The ability to achieve sub-wavenumber resolution (0.6 cm(-1)) and a large signal-to-noise ratio in high-resolution broadband sum-frequency generation vibrational spectroscopy (HR-BB-SFG-VS) allows for the detailed SFG spectral lineshapes to be used in the unambiguous determination of fine spectral features. Changes in the structural spectroscopic phase in SFG-VS as a function of beam polarization and experimental geometry proved to be instrumental in the identification of an unexpected 2.78 ± 0.07 cm(-1) spectral splitting for the two methyl groups at the vapor/dimethyl sulfoxide (DMSO, (CH(3))(2)SO) liquid interface as well as in the determination of their orientational angles.

Intermolecular vibrational motion in CS2 liquid at 165 ⩽ T⩾ 300 K observed by femtosecond time-resolved impulsive stimulated scattering

Abstract Temperature-dependent molecular dynamics of CS 2 liquid are examined in femtosecond time-resolved impulsive stimulated scattering experiments. At temperatures T ⩽ 240 K, weakly oscillatory time-dependent responses are observed. These are interpreted in terms of librations of molecules about their local potential minima. The librations undergo rapid inhomogeneous dephasing. Temperature-dependent values of the configuration-averaged librational frequency and the extent of inhomogeneity in that frequency are determined.

Molecular dynamics in liquids from femtosecond time-resolved impulsive stimulated scattering

Molecular motions in simple liquids are observed on the femtosecond time scale by means of impulsive simulated scattering. In carbon disulphide and benzene liquids, weakly oscillatory time-dependent responses are observed. These are interpreted in terms of molecular orientational motion which at short times is vibrational (i.e. librational) in character due to intermolecular interactions. In CS/sub 2/, the dephasing of the oscillations is primarily inhomogeneous. Temperature-dependent values of the configuration-averaged librational frequency and the extent of its inhomogeneity are determined. >

Plasmon-induced optical field enhancement studied by correlated scanning and photoemission electron microscopy

We use multi-photon photoemission electron microscopy (PEEM) to image the enhanced electric fields of silver nanoparticles supported on a silver thin film substrate. Electromagnetic field enhancement is measured by comparing the photoelectron yield of the nanoparticles with respect to the photoelectron yield of the surrounding silver thin film. We investigate the dependence of the photoelectron yield of the nanoparticle as a function of size and shape. Multi-photon PEEM results are presented for three average nanoparticle diameters: 34, 75, and 122 nm. The enhancement in photoelectron yield of single nanoparticles illuminated with femtosecond laser pulses (400 nm, ~3.1 eV) is found to be a factor of 10(2) to 10(3) times greater than that produced by the flat silver thin film. High-resolution, multi-photon PEEM images of single silver nanoparticles reveal that the greatest enhancement in photoelectron yield is localized at distinct regions near the surface of the nanoparticle whose magnitude and spatial extent is dependent on the incident electric field polarization. In conjunction with correlated scanning electron microscopy (SEM), nanoparticles that deviate from nominally spherical shapes are found to exhibit irregular spatial distributions in the multi-photon PEEM images that are correlated with the unique shape and topology of the nanoparticle.

Near-field focused photoemission from polystyrene microspheres studied with photoemission electron microscopy

We use photoemission electron microscopy (PEEM) to image 3 μm diameter polystyrene spheres supported on a metal thin film illuminated by 400 nm (∼3.1 eV) and 800 nm (∼1.5 eV) femtosecond (fs) laser pulses. Intense photoemission is generated by microspheres even though polystyrene is an insulator and its ionization threshold is well above the photon energies employed. We observe intense photoemission from the far side (the side opposite the incident light) of the illuminated microsphere that is attributed to light focusing within the microsphere. For the case of p-polarized, 800 nm fs laser pulses, we observe photoemission exclusively from the far side of the microsphere and additionally resolve sub-50 nm hot spots in the supporting Pt∕Pd thin film that are located only within the focal region of the microsphere. We compare the PEEM images with finite difference time domain (FDTD) electrodynamic simulations to model our experimental results. The FDTD simulations predict light focusing in the microsphere and subsequent interaction with the supporting metal surface that is consistent with the experimental observations.

Preliminary observation of nonrelaxational inertial motion in CS2 liquid by femtosecond time‐resolved impulsive stimulated scattering

Preliminary femtosecond time‐resolved impulsive stimulated scattering data from CS2 liquid are shown which clearly indicate an inertial component of the short‐time motion which cannot be described in terms of Debye relaxational dynamics. A discussion of time‐domain and frequency‐domain light scattering techniques is given to illustrate the comparative difficulty of characterizing this type of motion by conventional methods.

Femtosecond time resolved absorption spectroscopy of Cr(CO)6 and Mn2(CO)10: Observation of elementary bond breakage and formation

Femtosecond transient absorption measurements have been used to study CO dissociation in three simple transition metal carbonyls, Cr(CO)6 and Mn2(CO)10 [M=Mn, Re[ in solution. In all compounds, CO dissociation is seen to occur within one picosecond. Cr(CO)6 shows subsequent solvent coordination within 2.5 ps which is consistent with previous results. All compounds show slow spectral evolution on a 10 to 50 ps timescale which is interpreted as relaxation of the resulting photoproducts.