Plasmon Mapping in Metallic Nanostructures and its Application to Single Molecule Surface Enhanced Raman Scattering: Imaging Electromagnetic Hot-Spots and Analyte Location

A major component of this proposal is to elucidate the connection between optical and electron excitation of plasmon modes in metallic nanostructures. These accomplishments are reported: developed a routine protocol for obtaining spatially resolved, low energy EELS spectra, and resonance Rayleigh scattering spectra from the same nanostructures.; correlated optical scattering spectra and plasmon maps obtained using STEM/EELS.; and imaged electromagnetic hot spots responsible for single-molecule surface-enhanced Raman scattering (SMSERS)

Wafer-scale metasurface for total power absorption, local field enhancement and single molecule Raman spectroscopy

The ability to detect molecules at low concentrations is highly desired for applications that range from basic science to healthcare. Considerable interest also exists for ultrathin materials with high optical absorption, e.g. for microbolometers and thermal emitters. Metal nanostructures present opportunities to achieve both purposes. Metal nanoparticles can generate gigantic field enhancements, sufficient for the Raman spectroscopy of single molecules. Thin layers containing metal nanostructures (“metasurfaces”) can achieve near-total power absorption at visible and near-infrared wavelengths. Thus far, however, both aims (i.e. single molecule Raman and total power absorption) have only been achieved using metal nanostructures produced by techniques (high resolution lithography or colloidal synthesis) that are complex and/or difficult to implement over large areas. Here, we demonstrate a metasurface that achieves the near-perfect absorption of visible-wavelength light and enables the Raman spectroscopy of single molecules. Our metasurface is fabricated using thin film depositions, and is of unprecedented (wafer-scale) extent

O(^3P) +CO_2 Collisions at Hyperthermal Energies: Dynamics of Nonreactive Scattering, Oxygen Isotope Exchange, and Oxygen-Atom Abstraction

The dynamics of O(^3P) + CO_2 collisions at hyperthermal energies were investigated experimentally and theoretically. Crossed-molecular-beams experiments at Ecoll = 98.8 kcal mol^(–1) were performed with isotopically labeled ^(12)C^(18)O_2 to distinguish products of nonreactive scattering from those of reactive scattering. The following product channels were observed: elastic and inelastic scattering (^(16)O(^3P) + ^(12)C^(18)O^2), isotope exchange (^(18)O + ^(16)O^(12)C^(18)O), and oxygen-atom abstraction (^(18)O^(16)O + ^(12)C^(18)O). Stationary points on the two lowest triplet potential energy surfaces of the O(^3P) + CO_2 system were characterized at the CCSD(T)/aug-cc-pVTZ level of theory and by means of W4 theory, which represents an approximation to the relativistic basis set limit, full-configuration-interaction (FCI) energy. The calculations predict a planar CO_3(C_(2v),^3A″) intermediate that lies 16.3 kcal mol^(–1) (W4 FCI excluding zero point energy) above reactants and is approached by a C_(2v) transition state with energy 24.08 kcal mol^(–1). Quasi-classical trajectory (QCT) calculations with collision energies in the range 23–150 kcal mol^(–1) were performed at the B3LYP/6-311G(d) and BMK/6-311G(d) levels. Both reactive channels observed in the experiment were predicted by these calculations. In the isotope exchange reaction, the experimental center-of-mass (c.m.) angular distribution, T(θ_(c.m.)), of the ^(16)O^(12)C^(18)O products peaked along the initial CO_2 direction (backward relative to the direction of the reagent O atoms), with a smaller isotropic component. The product translational energy distribution, P(E_T), had a relatively low average of E_T = 35 kcal mol^(–1), indicating that the ^(16)O^(12)C^(18)O products were formed with substantial internal energy. The QCT calculations give c.m. P(E_T) and T(θ_(c.m.)) distributions and a relative product yield that agree qualitatively with the experimental results, and the trajectories indicate that exchange occurs through a short-lived CO_3^* intermediate. A low yield for the abstraction reaction was seen in both the experiment and the theory. Experimentally, a fast and weak ^(16)O^(18)O product signal from an abstraction reaction was observed, which could only be detected in the forward direction. A small number of QCT trajectories leading to abstraction were observed to occur primarily via a transient CO_3 intermediate, albeit only at high collision energies (149 kcal mol^(–1)). The oxygen isotope exchange mechanism for CO_2 in collisions with ground state O atoms is a newly discovered pathway through which oxygen isotopes may be cycled in the upper atmosphere, where O(^3P) atoms with hyperthermal translational energies can be generated by photodissociation of O_3 and O_2

Crossed-Beams and Theoretical Studies of Hyperthermal Reactions of O(\u3csup\u3e3\u3c/sup\u3eP) with HCl†

The reaction of O(3 P) with HCl at hyperthermal collision energies (45-116 kcal mol-1 ) has been investigated with crossed-molecular beams experiments and direct dynamics quasi-classical trajectory calculations. The reaction may proceed by two primary pathways, (1) H-atom abstraction to produce OH and Cl and (2) H-atom elimination to produce H and ClO. The H-atom abstraction reaction follows a stripping mechanism, in which the reagent O atom approaches the HCl molecule at large impact parameters and the OH product is scattered in the forward direction, defined as the initial direction of the reagent O atoms. The H-atom elimination reaction is highly endoergic and requires low-impact-parameter collisions. The excitation function for ClO increases from a threshold near 45 kcal mol-1 to a maximum around 115 kcal mol-1 and then begins to decrease when the ClO product can be formed with sufficient internal energy to undergo secondary dissociation. At collision energies slightly above threshold for H-atom elimination, the ClO product scatters primarily in the backward direction, but as the collision energy increases, the fraction of these products that scatter in the forward and sideways directions increases. The dependence of the angular distribution of ClO on collision energy is a result of the differences in collision geometry. Collisions where the H atom on HCl is oriented away from the incoming reagent O atom lead to backward-scattered ClO and those where the H atom is oriented toward the incoming O atom lead to forward-scattered ClO. The latter trajectories do not follow the minimum energy path and involve larger translational energy release. Therefore, they become dominant at higher collision energies because they lead to lower internal energies and more stable ClO products. The H-atom abstraction and elimination reactions have comparable cross sections for hyperthermal O(3 P) + HCl collisions

Plasmon Mapping in Metallic Nanostructures and its Application to Single Molecule Surface Enhanced Raman Scattering: Imaging Electromagnetic Hot-Spots and Analyte Location

A major component of this proposal is to elucidate the connection between optical and electron excitation of plasmon modes in metallic nanostructures. These accomplishments are reported: developed a routine protocol for obtaining spatially resolved, low energy EELS spectra, and resonance Rayleigh scattering spectra from the same nanostructures.; correlated optical scattering spectra and plasmon maps obtained using STEM/EELS.; and imaged electromagnetic hot spots responsible for single-molecule surface-enhanced Raman scattering (SMSERS)

Quasiclassical Trajectory Studies of the Hyperthermal N(⁴S) + D₂ → Nd(x³Σˉ) + D Reaction

Hyperthermal reactions, such as O(3P) + HCl, are known to occur within rocket plumes and around space vehicles in low earth orbit and have been widely studied using quasiclassical trajectory calculations (QCT) among other theoretical methods. The reaction of N(2D) + H2 has been widely studied using various experimental and theoretical techniques, however little attention has been given to reactions with the ground state N atom, N(4S), due to its low reactivity. In this study, we present QCT calculations on the reaction of N(4S) + D2 and comparisons to current experimental work will also be discussed