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Time-resolved second harmonic generation (SHG) spectroscopy is used to investigate acetaminophen (APAP)-induced changes in the adsorption and transport properties of malachite green isothiocyanate (MGITC) dye to the surface of unilamellar 1,2-dioleoyl

The in situ growth dynamics of colloidal silver–gold core–shell (Ag@Au CS) nanoparticles (NPs) in water are monitored in a stepwise synthesis approach using time-dependent second harmonic generation (SHG) and extinction spectroscopy. Three sequential additions of chloroauric acid, sodium citrate, and hydroquinone are added to the silver nanoparticle solution to grow a gold shell around a silver core. The first addition produces a stable urchin-like surface morphology, while the second and third additions continue to grow the gold shell thickness as the surface becomes more smooth and uniform, as determined using transmission electron microscopy. The extinction spectra after each addition are compared to finite-difference time-domain (FDTD) calculations, showing large deviations for the first and second additions due to the bumpy surface morphology and plasmonic hotspots while showing general agreement after the third addition reaches equilibrium. The in situ SHG signal is dominated by the NP surface, providing complementary information on the growth time scales due to changes to the surface morphology. This combined approach of synthesis and characterization of Ag@Au CS nanoparticles with in situ SHG spectroscopy, extinction spectroscopy, and FDTD calculations provides a detailed foundation for investigating complex colloidal nanoparticle growth mechanisms and dynamics in developing enhanced plasmonic nanomaterial technologies.

A fundamental understanding of the kinetics and thermodynamics of chemical interactions at the phospholipid bilayer interface is crucial for developing potential drug-delivery applications. Here we use molecular dynamics (MD) simulations and surface-sensitive second harmonic generation (SHG) spectroscopy to study the molecular adsorption and transport of a small organic cation, malachite green (MG), at the surface of 1,2-dioleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (DOPG) liposomes in water at different temperatures. The temperature-dependent adsorption isotherms, obtained by SHG measurements, provide information on adsorbate concentration, free energy of adsorption, and associated changes in enthalpy and entropy, showing that the adsorption process is exothermic, resulting in increased overall entropy. Additionally, the molecular transport kinetics are found to be more rapid under higher temperatures. Corresponding MD simulations are used to calculate the free energy profiles of the adsorption and the molecular orientation distributions of MG at different temperatures, showing excellent agreement with the experimental results.

Synthesis, characterization, and ultrafast dynamics of porphyrin- and zinc porphyrin-based nanomaterials are reported. Spherical nanoparticles composed of a group of uniform materials based on orga...

In non-centrosymmetric crystals, the surface contributions to second harmonic generation are mainly overwhelmed by the strong bulk contributions. We demonstrate that a femtosecond amplifier laser can enhance the surface contributions to second harmonic signal.

Transition metal oxide thin films and heterostructures have attracted remarkable and continuous attention due to the incredible variety of electronic and magnetic properties. A significant challeng...

In situ second harmonic generation (SHG) coupled with extinction spectroscopy is used for real-time monitoring of seed-mediated growth dynamics of colloidal citrate-stabilized gold nanoparticles in water. The time-dependent in situ SHG results capture an early stage of the growth process where a large enhancement in the SHG signal is observed, which is attributed to the formation of plasmonic hot spots from a rough and uneven nanoparticle surface. The temporal peak in the SHG signal is followed by a decay that is fit to an exponential function to characterize the size-dependent nanoparticle growth lifetime, which varies from 0.45 to 1.7 min for final nanoparticle sizes of 66 and 94 nm, respectively. This early growth stage also corresponds to a broadening of the plasmon spectra, as monitored using time-dependent in situ extinction spectroscopy. Over the course of the seed-mediated growth reaction, the nanoparticle becomes more thermodynamically stable through surface reconstruction resulting in a smoother...

Organic/inorganic hybrid nanoscale materials possess fascinating optical, electronic, magnetic, and catalytic properties that are promising for a variety of practical applications. Such properties can be dramatically affected by the hierarchical structure and molecular organization in the nanomaterials. Herein, we employed surface-initiated Kumada catalyst-transfer polymerization to prepare hybrid materials consisting of shells of conjugated polymers (CPs)—polythiophene or poly(p-phenylene)—and their block copolymers covalently attached to the surface of silica nanoparticles. Because of the controlled chain-growth mechanism of surface-initiated polymerization, we obtained structurally well-defined CP blocks in the diblock copolymer shells, which produced distinct spectroscopic properties related to the intraparticle excitation energy transfer between the nanoscale polymer shell components, as well as the formation of interfacial exciplex states. The spectroscopic phenomena were further understood via time...

Femtosecond broadband sum frequency generation (SFG) spectroscopy is applied to surface studies of the archetypical non-centrosymmetric semiconductor GaAs (001). Azimuthal angular dependence studies in reflection geometry under eight possible polarization configurations reveal strong surface-bulk interference owing to heterodyne amplification. The crystal symmetry and the surface quadrupole contributions need to be considered to properly interpret the resulting nonlinear spectroscopic signals. In addition, over bandgap excitation by one of the incident beams brings the semiconductor surface to a transient excited state, enabling enhanced sensitivity of broadband SFG to probe the surface electronic properties of non-centrosymmetric semiconductors. These findings suggest that this technique can be generally applied to surface studies of other non-centrosymmetric crystals.

A multiscale hybrid quantum/classical approach using classical electrodynamics and a collection of discrete three-level quantum systems is used to simulate the coupled dynamics and spectra of a malachite green monolayer adsorbed to the surface of a spherical gold nanoparticle (NP). This method utilizes finite difference time domain (FDTD) to describe the plasmonic response of the NP within the main FDTD framework and a three-level quantum description for the molecule via a Maxwell/Liouville framework. To avoid spurious self-excitation, each quantum molecule has its own auxiliary FDTD subregion embedded within the main FDTD grid. The molecular parameters are determined by fitting the experimental extinction spectrum to Lorentzians, yielding the energies, transition dipole moments, and the dephasing lifetimes. This approach can be potentially applied to modeling thousands of molecules on the surface of a plasmonic NP. In this paper, however, we first present results for two molecules with scaled oscillator strengths to reflect the optical response of a full monolayer. There is good agreement with experimental extinction measurements, predicting the plasmon and molecule depletions. Additionally, this model captures the polariton peaks overlapped with a Fano-type resonance profile observed in the experimental extinction measurements. This technique can be generalized to any nanostructure/multichromophore system, where the molecules can be treated with essentially any quantum method.

It is of great importance to interrogate the impact of local environment on the transport of small molecules across lipid bilayers, as they are key to the function and capabilities of eukaryotic cells and liposome-based delivery systems. Herein are described real-time studies of the molecular adsorption and transport kinetics of positively charged small-molecule organic dyes at the surface of liposomes under different buffer and salt conditions, made possible by application of second harmonic generation (SHG). The molecular transport of malachite green (MG) within the liposome bilayer is more rapid in 1,2-dioleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) and 1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPG and DOPS, respectively) liposomes in citrate buffer without added salts, whereas no adsorption or transport of MG is observed in trimethyl quinone-1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (QPADOPE) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes. While the transport rate constant increa...

The growth dynamics of gold-silver core-shell (Au@Ag) nanoparticles are studied using in situ time-dependent second harmonic generation (SHG) and extinction spectroscopy to investigate the nanoparticle shell formation. The silver shell is grown by reduction of silver cations onto a 14 nm gold core using ascorbic acid in colloidal aqueous solution under varying reaction concentrations producing Au@Ag nanoparticles of final sizes ranging from 51 to 78 nm in diameter. The in situ extinction spectra show a rapid increase in intensity on the timescale of 5-6 s with blue shifting and narrowing of the plasmonic peak during the silver shell formation. The in situ SHG signals show an abrupt rise at early times of the reaction, followed by a time-dependent biexponential decrease, where the faster SHG lifetime corresponds to the timescale of the shell growth, and where the slower SHG lifetime is attributed to changes in the nanoparticle surface charge density. A large enhancement in the SHG signal at early stages of the reaction is caused by plasmonic hot spots due to the nanoparticle surface morphology, which becomes smoother as the reaction proceeds. The final extinction spectra are compared to finite-difference time-domain (FDTD) calculations, showing general agreement with experiment, where the plasmon peak red shifts and increases in spectral width as the silver shell thickness increases. These in situ SHG and extinction spectroscopy results, combined with FDTD calculations, help characterize the complicated processes involved in colloidal nanoparticle shell formation in real time for developing potential plasmon-enhanced nanomaterial applications.

A fundamental understanding of the factors that determine the interactions with and transport of small molecules through phospholipid membranes is crucial in developing liposome-based drug delivery systems. Here we combine time-dependent second harmonic generation (SHG) measurements with molecular dynamics simulations to elucidate the events associated with adsorption and transport of the small molecular cation, malachite green isothiocyanate (MGITC), in colloidal liposomes of different compositions. The molecular transport of MGITC through the liposome bilayer is found to be more rapid in 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) and 1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPG and DOPS, respectively) liposomes, while the molecular transport is slower in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes. Interestingly, MGITC is observed to neither adsorb nor transport in trimethyl quinone-1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (QPADOPE) liposomes due to shielding by the quinone group. The modified Langmuir adsorption isotherm model is used to determine the free energy of adsorption for MGITC, which is found to be less negative in DOPC than in DOPG and DOPS, caused by lower electrostatic interactions between the positively charged dye and the zwitterionic DOPC liposome surface. The results are compared to our previous investigations, which showed that malachite green (MG) adsorbs and transports in DOPG and DOPS liposomes but not in DOPC and QPADOPE liposomes. Molecular dynamics simulations are used to investigate the adsorption and transport properties of MG and MGITC in DOPC and DOPG liposomes using umbrella sampling to determine the free energy profiles and interfacial molecular orientations. Together, these time-resolved SHG studies and corresponding molecular dynamics simulations characterize the complicated chemical interactions at different lipid membranes to provide key molecular-level insights for potential drug delivery applications. The results also point toward understanding the role of chemical functional groups, in this case isothiocyanate, in controlling molecular adsorption at and transport through lipid bilayers.

The photocleaving dynamics of colloidal micro-RNA-functionalized nanoparticles are studied using time-dependent second harmonic generation (SHG) measurements. Model drug-delivery systems composed of oligonucleotides attached to either silver nanoparticles or polystyrene nanoparticles using a nitrobenzyl photocleavable linker are prepared and characterized. The photoactivated controlled release is observed to be most efficient on resonance at 365 nm irradiation, with pseudo-first-order rate constants that are linearly proportional to irradiation powers. Additionally, silver nanoparticles show a 6-fold plasmon enhancement in photocleaving efficiency over corresponding polystyrene nanoparticle rates, while our previous measurements on gold nanoparticles show a 2-fold plasmon enhancement compared to polystyrene nanoparticles. Characterizations including extinction spectroscopy, electrophoretic mobility, and fluorimetry measurements confirm the analysis from the SHG results. The real-time SHG measurements are shown to be a highly sensitive method for investigating plasmon-enhanced photocleaving dynamics in model drug delivery systems.

Photothermal release of oligonucleotides from the surface of plasmonic nanoparticles represents a promising platform for spatiotemporal controlled drug delivery. Here we demonstrate the use of novel gold–silver–gold core–shell–shell (CSS) nanoparticles to study the photothermal cleaving and release of micro-RNA (miRNA) mimics or small interfering RNA (siRNA) under nearinfrared (NIR) irradiation. The furan–maleimide-based Diels–Alder adduct cleaves thermally above 60 °C and is used to bind siRNA to the colloidal nanoparticle surface in water. We investigate the photothermal cleaving kinetics of siRNA under different NIR laser powers using surface-sensitive time-dependent second-harmonic generation (SHG) spectroscopy. The photothermal release of siRNA from the surface of CSS nanoparticles is significantly higher than that from the surface of gold nanoparticles (GNPs) under similar experimental conditions. These results demonstrate that plasmonic CSS nanoparticles with photothermal cleaving linkers have important potential applications for nanoparticle-based NIR-mediated drug-delivery systems.

The present study explores alternate pericyclic chemistries for tethering amine-terminal biomolecules onto silver nanoparticles. Employing the versatile tool of the retro-Diels-Alder (rDA) reaction, three thermally-labile cycloadducts are constructed that cleave at variable temperature ranges. While the reaction between furan and maleimide has widely been reported, the current study also evaluates the reverse reaction kinetics between thiophene-maleimide, and pyrrole-maleimide cycloadducts. Density Functional Theorem (DFT) calculations used to model and plan the experiments, predict energy barriers for the thiophene-maleimide reverse reaction to be greatest, and the pyrrole-maleimide barriers the lowest. Based on the computational analyses, it is projected that the cycloreversion rate would occur slowest with the thiophene, followed by furan, and finally pyrrole would yield the promptest release. These thermally-responsive linkers, characterized by Electrospray Ionization Mass Spectrometry, 1H and 13C NMR, are thiol-linked to silver nanoparticles and conjugate single stranded siRNA mimics with 5′ fluorescein tag. Second harmonic generation spectroscopy (SHG) and fluorescence spectroscopy are used to measure release and rate of release. The SHG decay constants and fluorescence release profiles obtained for the three rDA reactions confirm the trends obtained from the DFT computations.

The synthesis, characterization, and ultrafast spectroscopy of size-selected nanospheres of ruthenium bipyridine–bis(pentafluoroethylsulfonyl)imide ([Ru(bipy)3][BETI]2) in water are reported. These studies represent the first experimental evidence of phonons with nanosecond lifetimes in organometallic nanomaterials. Thermally stable, crystalline nanoparticles of [Ru(bipy)3][BETI]2 are derived from a group of uniform materials based on organic salts (GUMBOS). Excited-state relaxation dynamics are studied using pump–probe time-resolved transient absorption spectroscopy, and the results are compared to corresponding measurements of aqueous Ru(bipy)3Cl2. The nanoGUMBOS show spectral shifts and size-dependent relaxation dynamics for nanoparticle diameters varying from 20 to 100 nm, characterized by excited-state decay dynamics similar to those of the precursor dye at higher pump pulse energies with an additional pathway attributed to intermolecular energy transfer, where all lifetimes increase with increasing ...

Photoactivated drug delivery systems using gold nanoparticles provide the promise of spatiotemporal control of delivery that is crucial for applications ranging from regenerative medicine to cancer therapy. In this study, we use second harmonic generation (SHG) spectroscopy to monitor the light-activated controlled release of oligonucleotides from the surface of colloidal gold nanoparticles. MicroRNA is functionalized to spherical gold nanoparticles using a nitrobenzyl linker that undergoes photocleaving upon ultraviolet irradiation. The SHG signal generated from the colloidal nanoparticle sample is shown to be a sensitive probe for monitoring the photocleaving dynamics in real time. The photocleaving irradiation wavelength is scanned to show maximum efficiency on resonance at 365 nm, and the kinetics are investigated at varying irradiation powers to demonstrate that the nitrobenzyl photocleaving is a one-photon process. Additional characterization methods including electrophoretic mobility measurements, extinction spectroscopy, and fluorimetry are used to verify the SHG results, leading to a better understanding of the photocleaving dynamics for this model oligonucleotide therapeutic delivery system.

The synthesis, characterization, and excited-state dynamics of colloidal gold–silver–gold core–shell–shell nanoparticles are reported. These plasmonic nanoparticles are spherical in shape with uniform shells. The plasmonic extinction peak wavelengths can be controlled over the visible and near-infrared regions by varying the thicknesses of the gold and silver shells. These unique spectroscopic properties make these nanoparticles potential candidates for biologically relevant applications including photothermal cancer therapy and biosensing. The ratio of the gold shell thickness to the overall particle size shows a linear dependence with the position of the plasmon extinction peak wavelength. Temperature measurements after laser irradiation show that the colloidal core–shell–shell nanoparticles have a higher photothermal effect compared to spherical gold nanoparticles and gold nanorods. Transient absorption measurements determine that the phonon–phonon scattering lifetime is considerably faster in the core...

Second harmonic generation is used to investigate the surface charge density of 50 nm colloidal gold nanoparticles in water. The gold nanoparticles are thiolated with mercaptosuccinic acid and are dialyzed in ultrapure water to remove excess salts and reactants. The second harmonic generation signal from the nanoparticle sample is measured as a function of added sodium chloride and magnesium chloride salt concentrations using the χ(3) technique. The experimental results are fit to the Gouy–Chapman model and to numerical solutions to the spherical Poisson–Boltzmann equation that account for the nanoparticle surface curvature, the different salt valences, and ion adsorption to the Stern layer interface. The best fits use the numerical solutions including ion adsorption and determine the initial surface charge density to be (−2.0 ± 0.1) × 10–3 C/m2 at the gold nanoparticle surface, in agreement with electrophoretic mobility measurements. In addition, the sodium ion is observed to adsorb with a higher surface...

Optically excited carrier dynamics are observed for self-assembled Lai-xSrxMnO3/SrTiO3 heterostructures using pump-probe reflectivity.

Energy Frontier Research Centers have been developed by the Department of Energy to accelerate research synergism among experimental and theoretical scientists in catalysis. The overall goal is to advance tools of synthesis, characterization, and computation of solid catalysts to design and predict catalytic properties at the atomic level. The Center for Atomic-Level Catalyst Design (CALC-D) has the goal of significantly advancing: (a) the tools of materials synthesis, allowing catalysts identified by computation to be prepared with atomic-level precision, (b) characterization methods such as advanced spectroscopy to understand surface structures of the working catalyst unambiguously, and (c) the ability of computational catalysis to accurately model reactions at working conditions.

Second harmonic generation is used to investigate the adsorption properties of malachite green, brilliant green, and methyl green to the surface of 80 nm colloidal gold nanoparticles capped with mercaptosuccinic acid in water. The experimental results are fit using the modified Langmuir model to obtain the free energies of adsorption and the adsorbate site densities for each cationic triphenylmethane molecular dye. Malachite green is observed to bind more strongly than brilliant green or methyl green to the nanoparticle surface but has a lower adsorbate site density, indicating differences in image-charge effects, adsorbate–adsorbate repulsions, and adsorption tilt angles. Complementary measurements from extinction spectroscopy show plasmonic and molecular resonance coupling leading to the formation of new polaritonic states and Fano-type resonances with corresponding plasmon and molecular spectral depletions as the adsorbate concentration is increased. The changes in the resonance coupling spectra are co...

We have utilized second harmonic generation (SHG) to disentangle the coupled first-order charge order/structural transition at in the transition-metal dichalocogenide IrTe2, an exceptional layered material with 3D properties. The data from SHG shows extremely sharp transition in both the cooling and warming processes with less than 0.2 K transition window. Surface electronic symmetries of and S 2 are observed in the high temperature and low temperature phases, respectively. Compared to neutron diffraction data for the structural transition (Phys. Rev. B 88 (2013) 115122) and to the electrical resistivity for the microscopic transition (Phys. Rev. B 95 (2017) 035148), our data indicates the electronic transition at the surface is the precursor to the structural transition.

Time-resolved anisotropy parameters and cross-section ratios of the positive and negative sidebands from two-colour two-photon above threshold ionization of helium atoms are measured using photoelectron velocity map imaging with the selected 19th high-order harmonic at 29.1 eV in an 810 nm perturbative dressing field. The intensities of both the sidebands and the single-photon ionization depletion follow a Gaussian correlation function where the photoelectron angular distributions and cross-section ratios of the sidebands do not change as a function of the temporal delay between the extreme ultraviolet and infrared pulses. The experimental results are compared with theoretical predictions using the soft-photon approximation, showing poor agreement, and analytical expressions are derived using second-order perturbation theory to determine the relative magnitudes of the resulting S and D partial waves of the above threshold ionization features.

Superexcited states of neutral nitrogen molecules, with excited ion cores, are prepared above the N2 ionization limit with femtosecond pulses of 23.1 eV photons, produced by high-order harmonic generation. Time-resolved predissociation of the superexcited nitrogen molecules is observed by detection of electronically excited atomic N*(4p) and/or N*(3d) product formation, by photoionization of the excited atoms with a delayed 805 nm pulse. An upper limit lifetime of 25 fs is determined for the superexcited states, attributed to the sum of autoionization and predissociation pathways, based on the risetime of predissociation products. The observation of dissociation products from the short-lived states shows that the predissociation decay pathways compete with autoionization pathways on an ultrafast timescale.

Dynamic etching methods for fabricating fibre optic tips are explored and modelled. By vertically translating the fibre during etching by an HF solution under an organic protective layer, a variety of tip shapes were created. The probe taper lengths, cone angles and geometrical probe shapes were measured in order to evaluate the dynamic meniscus etching process. Fibre motion, etching rate, meniscus distortion and etching time were all found to be important variables that can be used to control the final probe shape.

The reduced zero-field mobilities of t -C 4 H 9 + drifting in He, N 2 , O 2 and H 2 O are measured by ion depletion arrival time analysis to be 14.8 ± 0.6, 3.7 ± 0.8, 3.3 ± 0.8 and 0.04 ± 0.02 cm 2 V −1 s −1 , respectively. The mobilities are relatively constant with varying drift fields in the three nonpolar gases, but increase significantly with increasing drift field in water vapor, demonstrating the large difference between polar and nonpolar interaction potentials on the mobility of t -C 4 H 9 + . The results are compared to locked–dipole calculations. The relevance towards better modeling of hypersonic combustion and ion-injection processes is also discussed.

The electronic structure of conjugated polymer films is of current interest due to the wide range of potential applications for such materials in optoelectronic devices. A central outstanding issue is the significance of interchain electronic species in films of these materials. In this paper, we investigate the nature of interchain species in films of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) both before and after thermal annealing. Our investigation employs a combination of third harmonic generation (THG) and near-field scanning optical microscopy to measure the wavelength and spatial dependence of the THG efficiency. These chemically selective imaging measurements reveal new, low-energy absorption features in nanometer-scale spatially distinct regions of annealed films that are only infrequently observed prior to annealing. This suggests that the polymer strands in annealed MEH-PPV films pack together closely enough that significant ground-state wave function overlap can occ...

The nature of interchain electronic species in conjugated polymers has been the subject of much debate. In this paper, we exploit a novel near-field scanning optical microscopy (NSOM)-based solvatochromism method to spatially image the difference in dipole moment, and hence the difference in degree of charge separation, between the ground and electronic excited states of the emissive interchain species in films of poly(2-methoxy5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV). The method uses NSOM to collect emission from near the surface of solid samples that are placed into contact with liquids of varying polarity. The solvatochromic spectral shifts of the interfacial luminescence are measured as a function of solvent polarity; the results are analyzed with an interfacial dielectric continuum model to determine the dipole moment of emissive excited states. Experiments performed on films of the laser dye trans-4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) in poly(methyl methacrylate) (PMMA) demonstrate that our interfacial NSOM solvatochromic method and analysis can successfully reproduce the known dipole change of DCM upon photoexcitation. With the method calibrated, we then apply it to the interchain luminescence from the surface of thermally annealed MEH-PPV films. The interfacial solvatochromic analysis reveals that the dominant interchain species in annealed MEH-PPV films is “excimer-like”, exhibiting an 4-7 D decrease in dipole moment upon optical excitation. In a few highly localized regions of the film (ca. 1-2 Im in diameter), however, the interchain excited state exhibits a large (9-13 D) increase in dipole moment upon excitation, indicative of minority interchain species with a large degree of charge separation, such as exciplexes or polaron pairs. The large variation in excited-state dipole moments observed throughout the film is suggestive of an entire family of interchain species, each characterized by a different degree of charge separation. The fact that the large-dipole interchain species are found in spatially segregated domains implies that interchain charge separation in conjugated polymer films is associated with the presence of defects. When the molecular weight of the polymer is lowered, the large excited-state dipole regions increase in spatial extent, suggesting that the defects that promote charge separation are intrinsic and may be associated with the chain ends.

We demonstrate the combination of coherent anti-Stoke Raman scattering (CARS) with near-field scanning optical microscopy (NSOM) for chemically selective imaging via intrinsic vibrational resonances with spatial resolution below the diffraction limit. Femtosecond, near-IR pulses are used to produce CARS signals from human hepatocytes, and by tuning the CARS signal to be resonant with C−H stretching frequencies, image contrast is observed with an optical spatial resolution of ∼128 nm.

The emerging field of coherent nonlinear near-field scanning optical microscopy (NSOM) is reviewed. Second harmonic, third harmonic, and sum frequency generation (SHG, THG, and SFG) are explored as means of providing chemically and environmentally selective probes with nanometer spatial resolution provided by scanning probe microscopy. Chemical selectivity is generated via resonant enhancement of the nonlinear signals, whereas interface vs bulk contrast is achieved by the order (even vs odd) of the optical process. A method of producing higher spectral resolution, and thus increased chemical selectivity, is also demonstrated in the form of near-field detected multiplex (or “broad-bandwidth”) SFG (MSFG). Applications to biological and material samples are described.

The synthesis, characterization, ultrafast dynamics, and nonlinear spectroscopy of 30 nm nanospheres of brilliant green-bis(pentafluoroethylsulfonyl)imide ([BG][BETI]) in water are reported. These thermally stable nanoparticles are derived from a group of uniform materials based on organic salts (nanoGUMBOS) that exhibit enhanced near-infrared emission compared with the molecular dye in water. The examination of ultrafast transient absorption spectroscopy results reveals that the overall excited-state relaxation lifetimes of [BG][BETI] nanoGUMBOS are longer than the brilliant green molecular dye in water due to steric hindrance of the torsional degrees of freedom of the phenyl rings around the central carbon. Furthermore, the second harmonic generation signal of [BG][BETI] nanoGUMBOS is enhanced by approximately 7 times and 23 times as compared with colloidal gold nanoparticles of the same size and the brilliant green molecular dye in water, respectively. A very clear third harmonic generation signal is observed from the [BG][BETI] nanoGUMBOS but not from either the molecular dye or the gold nanoparticles. Overall, these results show that [BG][BETI] nanoGUMBOS exhibit altered ultrafast and nonlinear spectroscopy that is beneficial for various applications including nonlinear imaging probes, biomedical imaging, and molecular sensing.

Ultraviolet lasing from single zinc oxide nanowires is demonstrated at room temperature. Near-field optical microscopy images quantify the localization and the divergence of the laser beam. The linewidths, wavelengths, and power dependence of the nanowire emission characterize the nanowire as an active optical cavity. These individual nanolasers could serve as miniaturized light sources for microanalysis, information storage, and optical computing.

The mechanism of tuning fork-based shear-force near-field scanning optical microscopy is investigated to determine optimal experimental conditions for imaging soft samples immersed in liquid. High feedback sensitivity and stability are obtained when only the fiber probe, i.e., excluding the tuning fork prongs, is immersed in solution, which also avoids electrical shorting in conductive (i.e., buffer) solutions. Images of MEH-PPV were obtained with comparable spatial resolution in both air and water. High optical resolution (approximately160 nm fwhm) was observed.

It is becoming increasingly clear that the way in which a conjugated polymer film is cast affects the interactions between polymer chains and thus the optical and electrical properties of the film. Given that conjugated polymer films cast in different ways also show different nanometer-scale surface topographies, the question that arises is: What is the correlation between surface topography, local chain packing, and the local electronic properties of a conjugated polymer film? In this paper, we address this question using fluorescence near-field scanning optical microscopy (NSOM) to examine films of poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) that were prepared in different ways. The spatially resolved photoluminescence (SRPL) spectra collected on top of the nanometer-scale topographic features (“bumps”) exhibited by spin-cast MEH-PPV films show an enhancement of the red portion of the emission relative to spectra collected from flat regions of the film. Moreover, photooxidativ...

The ultrafast excited-state dynamics of size-dependent TiO2-Au nanocomposites synthesized by reducing gold nanoclusters to the surface of colloidal TiO2 nanoparticles are studied using pump-probe transient absorption spectroscopy with 400 nm excitation pulses. The results show that the relaxation processes of the plasmon depletion band, which are described by electron-phonon and phonon-phonon scattering lifetimes, are independent of the gold nanocluster shell size surrounding the TiO2 nanoparticle core. The dynamics corresponding to interfacial electron transfer between the gold nanoclusters and the TiO2 bandgap are observed to spectrally overlap with the gold interband transition signal, and the electron transfer lifetimes are shown to significantly decrease as the nanocluster shell size increases. Additionally, size-dependent periodic oscillations are observed and are attributed to acoustic phonons of a porous shell composed of aggregated gold nanoclusters around the TiO2 core, with frequencies that decrease and damping times that remain constant as the nanocluster shell size increases. These results are important for the development of improved catalytic nanomaterial applications.

Pump-probe photoelectron velocity-map imaging, using 27-eV high-harmonic excitation and 786-nm ionization, is used to resolve overlapping autoionizing resonances in atomic krypton, obtaining two-photon photoelectron angular distributions (PADs) for singly and doubly excited states. Two features in the photoelectron spectrum are assigned to singly excited 4${s}^{1}$4${p}^{6}$$n{p}^{1}$ ($n$ $=$ 7,8) configurations and four features provide information about double excitation configurations. The anisotropy parameters for the singly excited 7$p$ configuration are measured to be \ensuremath{\beta}${}_{2}$ $=$ 1.61 \ifmmode\pm\else\textpm\fi{} 0.06 and \ensuremath{\beta}${}_{4}$ $=$ 1.54 \ifmmode\pm\else\textpm\fi{} 0.16 while the 8$p$ configuration gives \ensuremath{\beta}${}_{2}$ $=$ 1.23 \ifmmode\pm\else\textpm\fi{} 0.19 and \ensuremath{\beta}${}_{4}$ $=$ 0.60 \ifmmode\pm\else\textpm\fi{} 0.15. These anisotropies most likely represent the sum of overlapping PADs from states of singlet and triplet spin multiplicities. Of the four bands corresponding to ionization of doubly excited states, two are assigned to 4${s}^{2}$4${p}^{4}$5${s}^{1}$6${p}^{1}$ configurations that are probed to different $J$-split ion states. The two remaining doubly excited states are attributed to a previously observed, but unassigned, resonance in the vacuum-ultraviolet photoabsorption spectrum. The PADs from each of the double excitation states are also influenced by overlap from neighboring states that are not completely spectrally resolved. The anisotropies of the observed double excitation states are reported, anticipating future theoretical and experimental work to separate the overlapping PADs into the state resolved PADs. The results can be used to test theories of excited state ionization.

Energy-dependent photoelectron angular distributions from two-color two-photon above threshold ionizations are investigated to determine the partial-wave characteristics of free-free electronic transitions in helium. Sideband photoelectron energies ranging from 0.18 to 13.0 eV are measured with different wavelengths of the perturbative infrared dressing field as well as different individually selected high-order harmonics. Using the experimentally measured cross-section ratios and anisotropy parameters together with analytical expressions derived from second-order perturbation theory, the partial-wave branching fractions going to the $S$ and $D$ waves in the positive and negative sidebands are determined as a function of photoelectron kinetic energy. The results provide a sensitive test for theoretical models of two-color two-photon above threshold ionization in atoms and molecules.

Photoelectron angular distributions (PADs) are obtained for a pair of 4s(1)4p(6)6p(1) (a singlet and a triplet) autoionizing states in atomic krypton. A high-order harmonic pulse is used to excite the pair of states and a time-delayed 801 nm ionization pulse probes the PADs to the final 4s(1)4p(6) continuum with femtosecond time resolution. The ejected electrons are detected with velocity map imaging to retrieve the time-resolved photoelectron spectrum and PADs. The PAD for the triplet state is inherently separable by virtue of its longer autoionization lifetime. Measuring the total signal over time allows for the PADs to be extracted for both the singlet state and the triplet state. Anisotropy parameters for the triplet state are measured to be β(2)=0.55 ± 0.17 and β(4)=-0.01 ± 0.10, while the singlet state yields β(2)=2.19 ± 0.18 and β(4)=1.84 ± 0.14. For the singlet state, the ratio of radial transition dipole matrix elements, X, of outgoing S to D partial waves and total phase shift difference between these waves, Δ, are determined to be X=0.56 ± 0.08 and Δ=2.19 ± 0.11 rad. The continuum quantum defect difference between the S and D electron partial waves is determined to be -0.15 ± 0.03 for the singlet state. Based on previous analyses, the triplet state is expected to have anisotropy parameters independent of electron kinetic energy and equal to β(2)=5∕7 and β(4)=-12∕7. Deviations from the predicted values are thought to be a result of state mixing by spin-orbit and configuration interactions in the intermediate and final states; theoretical calculations are required to quantify these effects.

Ultraviolet lasing from single zinc oxide nanowires is demonstrated at room temperature. Near-field optical microscopy images quantify the localization and the divergence of the laser beam. The linewidths, wavelengths, and power dependence of the nanowire emission characterize the nanowire as an active optical cavity. These individual nanolasers could serve as miniaturized light sources for microanalysis, information storage, and optical computing.

Anisotropy parameters and cross section ratios of two-color two-photon above threshold ionization sidebands from argon are measured using photoelectron velocity map imaging with the selected 13th or 15th high-order harmonics in a perturbative 800 nm dressing field. A new data analysis technique determines accurate anisotropy parameters of the photoelectron angular distributions for each sideband by subtracting a sequence of percentages of the single-photon ionization background from the above threshold ionization signal to correct for the angular averaging of overlapping photoelectron energies. The results provide a fundamental test of theoretical predictions based on second-order perturbation theory with a one-electron model and the soft-photon approximation and show agreement with theory for the cross section ratios. However, discrepancies between the theoretically predicted and experimentally determined photoelectron angular distributions demonstrate the need for a more comprehensive theoretical description of two-color two-photon above threshold ionization.

Phase shift differences and ratios of radial dipole matrix elements of the outgoing $S$ and $D$ continuum waves from state-selected helium atoms are directly measured from the photoelectron angular distributions using pump-probe velocity map imaging. Aligned $1s3p$ $^{1}P_{1}$ and $1s4p$ $^{1}P_{1}$ states in helium are prepared by high-order harmonics and ionized with either 800, 400, or $267\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ light. The results allow for the determination of energy-dependent quantum defect differences and ratios of partial cross sections and agree favorably with theoretical calculations on electron scattering and photoionization.

Mobility and reactivity are studied for the tertiary butyl cation (t-C 4 H 9 + ), a major and stable product of fuel alkanes in ion-enhanced hydrocarbon combustion. The reduced zero-field mobilities of t-C 4 H 9 + in He, N 2 , O 2 , and H 2 O are determined to be K 0 (0) = 14.8 ± 0.6, 3.7 ± 0.8, 3.3 ± 0.8, and 0.04 ± 0.02 cm 2 V -1 s -1 , respectively. While the mobility values in the nonpolar gases are in general agreement with theoretical calculations, the mobility in H 2 O is extraordinary low. Collisioninduced dissociation of t-C 4 H 9 + indicates that the ion is stable with the dissociation threshold energy (E cm ) of approximately 2 eV. Compared to other hydrocarbon cations, the t-C 4 H 9 + ion is totally non-reactive with O 2 , N 2 O, NO, and NO 2 (k < 10 -12 cm 3 s -1 ) in a drift field of up to E/N ! 60 Td (E cm ! 0.7 eV). The study has also been extended to gas phase chemistry of deprotonated nitroalkane anions (R 2 C=NO 2 - ), key intermediates in ion-enhanced combustion/detonation of nitroalkanes.

Techniques of coherent nonlinear spectroscopy (second harmonic generation and third harmonic generation) are combined with near-field scanning optical microscopy for imaging selected chemical and physical environments in biological matter on a nanoscopic scale. Resonant enhancement of nonlinear signals is utilized as a method of producing chemically selective contrast while the order of the process provides environmental selectivity. Systems studied incl ude natural killer cells and erythrocytes.

Neutral superexcited states in molecular oxygen converging to the O(2)(+) c (4)Σ(u)(-) ion state are excited and probed with femtosecond time-resolved photoelectron spectroscopy to investigate predissociation and autoionization relaxation channels as the superexcited states decay. The c (4)Σ(u)(-) 4sσ(g) v=0, c (4)Σ(u)(-) 4sσ(g) v=1, and c (4)Σ(u)(-) 3dσ(g) v=1 superexcited states are prepared with pulsed high-harmonic radiation centered at 23.10 eV. A time-delayed 805 nm laser pulse is used to probe the excited molecular states and neutral atomic fragments by ionization; the ejected photoelectrons from these states are spectrally resolved with a velocity map imaging spectrometer. Three excited neutral O* atom products are identified in the photoelectron spectrum as 4d(1) (3)D(J)°, 4p(1) (5)P(J)° and 3d(1) (3)D(J)° fragments. Additionally, several features in the photoelectron spectrum are assigned to photoionization of the transiently populated superexcited states. Using principles of the ion core dissociation model, the atomic fragments measured are correlated with the molecular superexcited states from which they originate. The 4d(1) (3)D(J)° fragment is observed to be formed on a timescale of 65 ± 5 fs and is likely a photoproduct of the 4sσ(g) v = 1 state. The 4p(1) (5)P(J)° fragment is formed on a timescale of 427 ± 75 fs and correlated with the neutral predissociation of the 4sσ(g) v = 0 state. The timescales represent the sum of predissociation and autoionization decay rates for the respective superexcited state. The production of the 3d(1) (3)D(J)° fragment is not unambiguously resolved in time due to an overlapping decay of a v = 1 superexcited state photoelectron signal. The observed 65 fs timescale is in good agreement with previous experiments and theory on the predissociation lifetimes of the v = 1 ion state, suggesting that predissociation may dominate the decay dynamics from the v = 1 superexcited states. An unidentified molecular state is inferred by the detection of a long-lived depletion signal (reduction in autoionization) associated with the B (2)Σ(g)(-) ion state that persists up to time delays of 105 ps.