Noncentrosymmetric gold nanoparticle structures were prepared in porous anodic aluminum oxide films via a modified template synthesis procedure. The noncentrosymmetric structures are defined by two gold particles in close proximity (ca. 22 ( 8 nm end to end) within a single host oxide pore, one having average dimensions of a ) 37 ( 6 nm and b ) 26 ( 3, and the other having dimensions of a ) 27 ( 5 nm and b ) 26 ( 3 nm, where a is the axial length and b is the diameter of the quasi-cylindrical structures. Linear UV/vis polarization spectra of the gold particle/porous alumina film composites show plasmon resonance bands whose λ max incidence angle dependence is similar to composites containing centrosymmetric gold structures, where the incidence angle is defined with respect to the surface normal. Second harmonic generation (SHG) studies using an incident wavelength of 780 nm indicate that SHG intensities under s-polarization are low and independent of incidence angle (θ) for composites containing centrosymmetric and noncentrosymmetric gold nanostructures. However, in p-polarization, both composites show an increase in SHG counts with θ, with the noncentrosymmetric structures showing a higher SHG signal than their centrosymmetric counterparts. These results are consistent with local-field enhancements arising from long particle axis dipolar plasmon resonances.

An approach for detecting the vibrational spectrum of transient species is demonstrated on the vinyl radical. Photodissociation of carefully chosen precursors at selected photolysis wavelengths produce highly vibrationally excited radicals. Infrared (IR) emission from these radicals is then measured by time-resolved Fourier transform spectroscopy with nanosecond time resolution. All nine vibrational bands of the vinyl radical, generated from four different precursors, are obtained and reported here for the first time.

The rotational distributions of CO products from the dissociation of ketene at photolysis energies 10 cm- ' below, 56, 110,200,325,425,1107, 1435, 1720, and 2500 cm-' above the singlet threshold (30 116.2 cm-'), are measured in a supersonic free jet of ketene. The CO(v!' = 0) rotational distributions at 56, 110,200,325, and 425 cm-' are bimodal. The peaks at low J's, which are due to CO from the singlet channel, show that the product rotational distribution of CO product from ketene dissociation on the singlet surface is well described by phase space theory (PST). For CO( u" = 0) rotational distributions at higher excess energies (1107, 1435, 1720, and 2500 cm-'), the singlet and triplet contributions are not clearly resolved, and the singlet/triplet branching ratios are estimated by assuming that PST accurately predicts the CO rotational distribution from the singlet channel and that the distribution from the triplet channel changes little from that at 10 cm-' below the singlet threshold. The singlet yield shows a rapid increase in the low excess energy region (O-300 cm -'>, and a slower increase above. The singlet and triplet rate constants are derived from the directly measured total rate constants using the singlet yields. The triplet rate constant increases monotonically with increasing photolysis energy through the singlet threshold region. The singlet rate constant is accurately established in the threshold region and found to increase much less rapidly than predicted by phase space theory. At 2500 cm -' excess energy, the CO( u" = 1) rotational distribution is obtained, and the ratio of CO( 0" = 1) to CO(u" = 0) products for the singlet channel is measured to be 0.045 f 0.017. This ratio is close to the variational Rice-Ramsberger-Kassei-Marcus (RRKM) calculation 0.038, and the separate statistical ensembles (SSE) prediction 0.041, but much greater than the PST prediction, 0.016. 1954

The vinyl radical (C2H3)Ã 2A″←X̃ 2A′ spectrum has been measured between 530 and 385 nm using cavity-ringdown spectroscopy. The active vibrational progressions involve C–C stretching and alpha H–C–C bending vibrations. Optimal rotational constants and linewidths were determined for the first four vibrational bands by modeling the spectrum as an asymmetric top. The best-fit rotational constants obtained for the excited electronic state are consistent with the molecular geometry predicted by ab initio calculations. The lifetime of the vibrationless level in the excited electronic state is estimated to be a few picoseconds, and increasing vibrational excitation leads to a decrease in the lifetime, based upon an increasing linewidth. Various possibilities for the predissociation mechanism are discussed. The most likely is judged to be a conical intersection or seam of intersections. A preliminary CASSCF calculation has found the point on the relevant potential energy surfaces at which the ground and electronically excited states are closest. While the geometry and other properties of this crossing point are in accord with the experimental results, the calculated position of the point of closest approach of the two electronic states lies considerably (>1 eV, including zero-point energy) above the already predissociative Ã 2A″ state origin. Other mechanisms are also discussed to account for the observed rapid predissociation. Clearly there is a need for a higher level theoretical work on this problem.