(HCN)(m)-M(n) (M = K, Ca, Sr): vibrational excitation induced solvation and desolvation of dopants in and on helium nanodroplets.

Infrared (IR) laser spectroscopy is used to probe the rotational and vibrational dynamics of the (HCN)(m)-M(n) (M = K, Ca, Sr) complexes, either solvated within or bound to the surface of helium nanodroplets. The IR spectra of the (HCN)(m)-K (m = 1-3), HCN-Sr, and HCN-Ca complexes have the signature of a surface species, similar to the previously reported spectra of HCN-M (M = Na, K, Rb, Cs) [Douberly, G. E.; Miller, R. E. J. Phys. Chem. A 2007, 111, 7292.]. A second band in the HCN-Ca spectrum is assigned to a solvated complex. The relative intensities of the two HCN-Ca bands are droplet size dependent, with the solvated species being favored in larger droplets. IR-IR double resonance spectroscopy is used to probe the interconversion of the two distinct HCN-Ca populations. While only a surface-bound HCN-Sr species is initially produced, CH stretch vibrational excitation results in a population transfer to a solvated state. Complexes containing multiple HCN molecules and one Sr atom are surface-bound, while the nu(1) (HCN)(2)Ca spectrum has both the solvated and surface-bound signatures. All HCN-(Ca,Sr)(n) (n > or = 2) complexes are solvated following cluster formation in the droplet. Density-functional calculations of helium nanodroplets interacting with the HCN-M show surface binding for M = Na with a binding energy of 95 cm(-1). The calculations predict a fully solvated complex for M = Ca. For M = Sr, a 2.2 cm(-1) barrier is predicted between nearly isoenergetic surface binding and solvated states.

High-resolution infrared spectroscopy of Mg-HF and Mg-(HF)2 solvated in helium nanodroplets.

High-resolution infrared (IR) spectroscopy is used to investigate the Mg-HF and Mg-(HF)(2) van der Waals complexes. Both complexes are formed and probed within helium nanodroplets. Rotationally resolved zero-field and Stark spectra are assigned to a linear binary complex composed of a Mg atom bound to the hydrogen end of the HF molecule. Although high level ab initio calculations predict a fluorine bonded complex, none of the observed IR bands can be assigned to this complex. The collocation method is employed to determine the bound states on the two-dimensional intermolecular Mg-HF potential energy surface. The ground and first excited state wave functions for this potential surface have zero amplitude in the well corresponding to the fluorine bonded complex, consistent with experiment. The two HF stretching bands of the Mg-(HF)(2) complex are observed and assigned using a combination of the spectral symmetry, ab initio calculations, pick-up cell pressure dependencies, and dipole moment measurements. Comparisons with the helium solvated HF dimer show large changes to the HF stretching frequencies upon the addition of a single Mg atom to the hydrogen side of (HF)(2).

Surface-enhanced Raman spectroscopy.

The ability to control the size, shape, and material of a surface has reinvigorated the field of surface-enhanced Raman spectroscopy (SERS). Because excitation of the localized surface plasmon resonance of a nanostructured surface or nanoparticle lies at the heart of SERS, the ability to reliably control the surface characteristics has taken SERS from an interesting surface phenomenon to a rapidly developing analytical tool. This article first explains many fundamental features of SERS and then describes the use of nanosphere lithography for the fabrication of highly reproducible and robust SERS substrates. In particular, we review metal film over nanosphere surfaces as excellent candidates for several experiments that were once impossible with more primitive SERS substrates (e.g., metal island films). The article also describes progress in applying SERS to the detection of chemical warfare agents and several biological molecules.

Infrared spectroscopy of prereactive aluminum-, gallium-, and indium-HCN entrance channel complexes solvated in helium nanodroplets.

Prereactive metal atom-HCN entrance channel complexes [M-HCN (M=Al, Ga, In)] have been stabilized in helium nanodroplets. Rotationally resolved infrared spectra are reported for the CH stretching vibration of the linear nitrogen-bound HCN-Ga and HCN-In complexes that show significant perturbation due to spin-orbit coupling of the 2Pi1/2 ground state with the 2Sigma1/2 state which are degenerate at long range. Six unresolved bands are also observed and assigned to the linear hydrogen-bound isomers of Al-HCN, Ga-HCN, and In-HCN corresponding to the fundamental CH stretching vibration and a combination band involving the CH stretch plus intermolecular stretch for each isomer. A nitrogen-bound HCN-Al complex is not observed, which is attributed to reaction, even at 0.37 K. This conclusion is supported by the observation of a weakly bound complex containing two HCN's and one Al atom which, from the analysis of its rotationally resolved zero-field and Stark spectra is assigned to a weakly bound complex of a HCNAl reaction product and a second HCN molecule. Theoretical calculations are presented to elucidate the reaction mechanisms and energetics of these metal atom reactions with HCN.

High-resolution infrared spectroscopy of HCN-Agn (n = 1-4) complexes solvated in superfluid helium droplets.

High-resolution infrared spectroscopy has been used to determine the structures, C-H stretching frequencies, and dipole moments of the HCN-Agn (n = 1-3) complexes formed in superfluid helium droplets. The HCN-Ag4 cluster was tentatively assigned based upon pick-up cell pressure dependencies and harmonic vibrational shift calculations. Ab initio and density functional theory calculations were used in conjunction with the high-resolution spectra to analyze the bonding nature of each cluster. All monoligated species reported here are bound through the nitrogen end of the HCN molecule. The HCN-Agn complexes are structurally similar to the previously reported HCN-Cun clusters, with the exception of the HCN-Ag binary complex. Although the interaction between the HCN and the Agn clusters follows the same trends as the HCN-Cun clusters, the more diffuse nature of the electrons surrounding the silver atoms results in a much weaker interaction.

Structures and bonding nature of small monoligated copper clusters (HCN-Cun, n = 1-3) through high-resolution infrared spectroscopy and theory.

The structures, C-H stretching frequencies, and dipole moments of HCN-Cun (n = 1-3) clusters are determined through high-resolution infrared spectroscopy. The complexes are formed and probed within superfluid helium droplets, whereby the helium droplet beam is passed over a resistively heated crucible containing copper shot and then through a gas HCN pickup cell. All complexes are found to be bound to the nitrogen end of the HCN molecule and on the "atop site" of the copper cluster. Through the experimental C-H vibrational shifts of HCN-Cun and ab initio calculations, it was found that the HCN-metal interaction changes from a strong van der Waals bond in n = 1 to a partially covalent bond in HCN-Cu3. Comparisons with existing infrared data on copper surfaces show that the HCN-Cun bond must begin to weaken at very large copper cluster sizes, eventually returning to a van der Waals bond in the bulk copper surface case.

High-resolution infrared spectroscopy of HCN-Znn (n = 1-4) clusters: structure determination and comparisons with theory.

High-resolution infrared laser spectroscopy has been used to obtain rotationally resolved spectra of HCN-Zn(n) (n = 1-4) complexes formed in helium nanodroplets. In the present study the droplets passed through a metal oven, where the zinc vapor pressure was adjusted until one or more atoms were captured by the droplets. A second pickup cell was then used to dope the droplets with a single HCN molecule. Rotationally resolved infrared spectra are obtained for all of these complexes, providing valuable information concerning their structures. Stark spectra are reported and used to determine the corresponding permanent electric dipole moments. Ab initio calculations are also reported for these complexes for comparison with the experimental results.

Dipole moments of molecules solvated in helium nanodroplets.

Stark spectra are reported for hydrogen cyanide and cyanoacetylene solvated in helium nanodroplets. The goal of this study is to understand the influence of the helium solvent on measurements of the permanent electric dipole moment of a molecule. We find that the dipole moments of the helium solvated molecules, calculated assuming the electric field is the same as in vacuum, are slightly smaller than the well-known gas-phase dipole moments of HCN and HCCCN. A simple elliptical cavity model quantitatively accounts for this difference, which arises from the dipole-induced polarization of the helium.