Localized surface plasmon resonance (LSPR) is a key optical property of metallic nanoparticles. The peak position of the LSPR for noble-metal nanoparticles is highly dependent upon the refractive index of the surrounding media and has therefore been used for chemical and biological sensing. In this work, we explore the influence of resonant adsorbates on the LSPR of bare Ag nanoparticles (lambda(max,bare)). Specifically, we study the effect of rhodamine 6G (R6G) adsorption on the nanoparticle plasmon resonance because of its importance in single-molecule surface-enhanced Raman spectroscopy (SMSERS). Understanding the coupling between the R6G molecular resonances and the nanoparticle plasmon resonances will provide further insights into the role of LSPR and molecular resonance in SMSERS. By tuning lambda(max,bare) through the visible wavelength region, the wavelength-dependent LSPR response of the Ag nanoparticles to R6G binding was monitored. Furthermore, the electronic transitions of R6G on Ag surface were studied by measuring the surface absorption spectrum of R6G on an Ag film. Surprisingly, three LSPR shift maxima are found, whereas the R6G absorption spectrum shows only two absorption features. Deconvolution of the R6G surface absorption spectra at different R6G concentrations indicates that R6G forms dimers on the metal surface. An electromagnetic model based on quasi-static (Gans) theory reveals that the LSPR shift features are associated with the absorption of R6G monomer and dimers. Electronic structure calculations of R6G under various conditions were performed to study the origin of the LSPR shift features. These calculations support the view that the R6G dimer formation is the most plausible cause for the complicated LSPR response. These findings show the extreme sensitivity of LSPR in elucidating the detailed electronic structure of a resonant adsorbate.

L-phenylalanine (Phe), one of the aromatic amino acids, and its hydrated clusters were generated in supersonic expansion and investigated by resonant two-photon ionization. Excitation spectra of Phe and Phe–(H2O)1 were obtained near their S0–S1 origins. We found that, by comparing the experimental results with the density functional theory and ab initio calculations, the water in Phe–(H2O)1 tends to form a cyclic hydrogen bond at the carboxyl group while inducing little change in the corresponding monomer structure. No sign of water making bridged hydrogen bonds with both polar groups was found. In order to form the cyclic hydrogen bond, hydration takes place only with the conformers whose carboxyl hydrogen is free, i.e., not occupied in the intramolecular hydrogen bonding with the amino nitrogen in the monomer.

Articles you may be interested inElectronic spectra of the jet-cooled DNA base adenine were obtained by the resonant two-photon ionization ͑R2PI͒ and the laser induced fluorescence ͑LIF͒ techniques. The 0-0 band to the lowest electronically excited state was found to be located at 35 503 cm Ϫ1 . Well-resolved vibronic structures were observed up to 1100 cm Ϫ1 above the 0-0 level, followed by a slow rise of broad structureless absorption. The lowest electronic state was proposed to be of n * character, which lies ϳ600 cm Ϫ1 below the onset of the * state. The broad absorption was attributed to the extensive vibronic mixing between the n * state and the high-lying * state.

Frequency-scanned excitation profiles of coherent second harmonic generation (SHG) were measured for silver nanoparticle arrays prepared by nanosphere lithography. The frequency of the fundamental beam did not coincide with the localized surface plasmon resonance (LSPR) of the nanoparticles and was tuned so that the coherent second harmonic (SH) emission was in the region of the LSPR at 720−750 nm. The SH emission from the arrays was compared with a smooth silver film to identify an enhancement of SH emission efficiency that peaks near ∼650 nm for nanoparticles 50 nm in height. The polarization and orientation dependence of this enhancement suggests that it is related to a dipolar LSPR mode polarized normal to the plane of the substrate. Linear extinction spectra are dominated by in-plane dipoles and do not show this weak out-of-plane LSPR mode. The nanoparticle arrays are truncated tetrahedrons symmetrically oriented by nanosphere lithography to cancel SH from in-plane dipoles which allows observation of the weak out-of-plane component.

In recent years there has been an outburst of research interest in small biological molecules in the gas phase. [1] By placing these molecules on the ™transparent cover glass∫ of an isolated environment, one can unravel intrinsic properties usually hidden in the complex medium of a real biological system. The need for such gas-phase studies arises from the anticipation that many biological phenomena can be traced to the fundamental properties of the molecular constituents. Both laser spectroscopy and theoretical methods have made great contributions to elucidating the structures and dynamics of nonrigid biomolecules and their solvated complexes in the gas phase. [2] Amino acids are known to exist in various conformations resulting from the flexibility of their structures, which comprise a backbone and a side chain or residue. The conformational variety of amino acids plays a crucial role in determining the three-dimensional structure of proteins and controlling their dynamics. [3] The energy barrier that separates different conformers is typically rather small so that thermal energy at room temperature enables the molecule to freely change from one conformation to another. Therefore, it is not generally feasible to isolate a specific conformer experimentally at room temperature. By employing a supersonic expansion, however, one can cool down the molecule to a temperature low enough to isolate it in various frozen forms, in other words, as individual conformers. Numerous exper-