Session Q26: Advances in Spectroscopy

IR and Raman spectroscopy of water and ice by ab initio simulations

We use ab initio molecular dynamics to compute the IR and Raman spectra of a variety of heavy water systems, ranging from pure water and ice, to liquid water confined between graphene foils and D-terminated diamond surfaces. The analysis of the simulated spectra provides the fingerprints of different hydrogen bonding environments, giving access to the complex structural and dynamical properties of water in various conditions. In addition our results provide a detailed, microscopic interpretation of IR and Raman experiments, as they allow us to assign univocaly spectroscopic bands to specific vibrational modes, and to identify electro-dynamic coupling between water molecules and surfaces, in the case of confined water. Our MD simulations also give a quantitative estimate of the anharmonicities and lifetimes of various vibrational modes.   

Infrared spectra of ice and water from first principles: intra vs. intermolecular dipole correlations

We report simulated infrared (IR) spectra of deuterated ice and water using Car-Parrinello molecular dynamics with maximally localized Wannier functions. Experimental features of both ice and water are accurately reproduced within the harmonic approximation. Calculated line shapes are further decomposed in terms of intra and intermolecular dipole correlation functions with spatial resolution. This approach proves to be very useful to understand the origin of spectral features and the nature of the underlying hydrogen-bond (H-bond) network. We find that intermolecular dynamic charge fluctuations play a crucial role over the entire frequency range.   

Nuclear quantum effects in water

In this work, a path integral Car-Parrinello molecular dynamics\footnote{CPMD V3.11 Copyright IBM Corp 1990-2006, Copyright MPI fuer Festkoerperforschung Stuttgart 1997-2001.} simulation of liquid water is performed. It is found that the inclusion of nuclear quantum effects systematically improves the agreement of first-principles simulations of liquid water with experiment. In addition, the proton momentum distribution is computed utilizing a recently developed ``open'' path integral molecular dynamics methodology\footnote{J.A. Morrone, V. Srinivasan, D. Sebastiani, R. Car \emph{J. Chem. Phys.} \textbf{126} 234504 (2007).}. It is shown that these results, which are consistent with our computations of the liquid structure, are in good agreement with neutron Compton scattering data\footnote{G.F. Reiter, J.C. Li, J. Mayers, T. Abdul-Redah, P. Platzman \emph{Braz. J. Phys.} \textbf{34} 142 (2004).}. The remaining discrepancies between experiment and the present results are indicative of some degree of over-binding in the hydrogen bond network, likely engendered by the use of semi-local approximations to density functional theory in order to describe the electronic structure.   

Fluorescence correlation spectroscopy with Gaussian-Lorentzian volumes

Fluorescence correlation spectroscopy (FCS) is a fundamental technique of fluorescence microscopy used for many applications of chemical physics where molecular diffusion plays primary roles [see, for example, O. Krichevsky and G. Bonnet, Rep. Prog. Phys. 65, 251 (2002)]. The milestone of FCS is called three-dimensional Gaussian (3DG) approximation. According to this assumption, the observation volume is modeled by Gaussian profiles along the main three spatial directions. This simplification is necessary to achieve analytical treatment of FCS measurements. In this work, analytical solutions are shown for another geometry corresponding to the fundamental mode of laser beams, i.e. the Gaussian-Lorentzian distribution, where Gaussian profiles are associated with the two transverse directions while a Lorentzian dependence characterizes the axial direction (coincident with the optical axis of the microscope). Analytical solutions are guaranteed for both one-photon and two-photon excitations of diffusing molecules [one-photon excitation is considered in M. Marrocco, Chem. Phys. Lett. 449, 227 (2007)]. Similarities and differences with respect to the 3DG approximation are discussed.   

Studying hydrogen bond by Quantum Monte Carlo: binding energy and dispersion curve of the water dimer

We present a variational MonteCarlo (VMC) and lattice regularized diffusion MonteCarlo (LRDMC) study of the binding energy and dispersion curve of the water dimer. One the aim of the present work is to investigate how the bonding of two water molecules, as a prototype of the hydrogen-bonded complexes, could be described by a JAGP wave function, an implementation of the resonating valence bond idea.Using a pseudopotential for the inert core of the Oxygen, with a full optimization of the variational parameters, we obtain at the VMC level a binding energy of -4.5(0.1) Kcal/mol, while LRDMC gives -4.9(0.1)Kcal/mol (exp. 5 Kcal/Mol). The calculated dispersion curve reproduces both at the VMC and LRDMC level the miminum position and the right curvature.The quality of the WF gives us the possibility to dissect the binding energy in different contributions by appropriately switching off determinantal and Jastrow terms in the JAGP: we estimate the dynamical contribution to the binding energy of the order of 1.4(0.2) Kcal/Mol whereas the covalent one about 1.0(0.2) Kcal/Mol. JAGP reveales thus a promising WF for describing systems where dispersive and covalent forces play an important role   

Hydrogen bond network ordering of liquid water confined between two metallic plates studied by ab initio molecular dynamics

We present an {\it ab initio} molecular dynamics study of liquid water confined between two palladium $\langle 111\rangle$ surface slabs, at room temperature. We analyze in detail the layering and ice-Ih-type ordering of water molecules close to the metal interfaces. In particular we show how water molecules next to the metal surface display a very different structural and dynamic behavior as compared those in the ``bulk" regions, which can be easily characterized using infra-red spectroscopy. Hydrogen bonds near the metallic interfaces are strengthen, inducing a characteristic ordering which decays with de distance from the surfaces. Our preliminary results show that this confined water presents an asymmetrical and stable structure as a function of Z (axis perpendicular to the surfaces) which results in a characteristic system with an overall ordering of the water molecules resembling that of ferroelectric systems.   

Quasiparticle lifetime and edge localized states of graphite studied by high-resolution ARPES

We have performed an ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES) of high-quality graphite single crystal (kish graphite) to elucidate the band structure and many-body interaction. We clearly observed an extremely small hole-like Fermi surface centered at the K(H) point, a sharp quasiparticle peak in the vicinity of the Fermi level ($E_{F})$, and a kink in the dispersin at 0.18 eV.$^{1}$ We also found first evidence for the edge localized states near $E_{F}$, whose energy dispersion is markedly different from that of the bulk band. We will discuss the energy- and temperature-dependence of quasiparticle lifetime in relation to the strong electron-phonon coupling, the electron-plasmon interaction, and the electron-hole pair excitations. $^{1}$K. Sugawara\textit{ et. al., }Phys. Rev. Lett. \textbf{98} (2007) 036801. $^{2}$K. Sugawara \textit{et. al.,} Phys. Rev. B \textbf{73} (2006) 045124.   

Fully ab-initio study of the optical response of charged rare gas clusters

Charged rare-gas clusters are markedly different from their neutral, van der Waals bonded counterparts. The removal of an electron from a strongly antibonding orbital causes the bonding to become much stronger and shifts the optical absorption to the visible region. We report a fully ab-initio determination of the geometry, electronic structure, and optical response of small singly charged Ne, Ar, Kr and Xe clusters. All calculations were performed using a pseudopotential based real space implementation of Time-Dependent Density-Functional Theory. We find that GGA leads, in general, to much better results than LDA, even though it predicts some absorption peaks at slightly higher energies than those found experimentally. The lighter elements show a single absorption peak but in the heavier elements spin-orbit interaction induces a splitting of the absorption peak, in good agreement with experiment.   

Continuous fluorescence from single colloidal semiconductor nanocrystals

Photoluminescence (PL) intermittency, or ``blinking'', first discovered for single CdSe colloidal nanocrystals (NCs) a decade ago, has been established as an intrinsic and unavoidable property of all colloidal semiconductor NCs. Indeed, fluorescence blinking is generally accepted as the hallmark of single fluorophore emission. By judicious synthesis of a semiconductor shell of ZnSe around a CdSe NC, we were able to completely suppress PL blinking in these NCs on time scales from milliseconds to hours. Interestingly, these NCs have a radiative lifetime of about 5 ns, 3-4 times smaller than the value routinely measured from traditional CdSe NCs. Finally, single particle PL spectra are highly unusual, and display three peaks separated by about 160 meV. Possible mechanisms for the non-blinking behavior, the short radiative lifetime, and the multiple emission peaks will be discussed.   

Cubic Nonlinearity of Ag/Au Coreshells

Cubic nonlinearity of Ag/Au spherical coreshells in toluene were investigated by polarization-resolved degenerate four-wave mixing with 6-ns laser pulse at 532 nm with 10-Hz repetition. The average diameter of Ag core was $\sim $6.7 nm. The overall diameter of Ag/Au was changed from 6.1 to 9.1 nm by adding more mole concentration of HAuCl$_{4}$, which resulted in the change of surface plasmon resonance peaks from 411 to 492 nm. The hyperpolarizability of Ag/Au coreshells with parallel and orthogonal excitations were changed from $\sim $3.4$\times $10$^{-38}$ to $\sim $2.7$\times $10$^{-40}$ m$^{5}$/V$^{2}$ and from $\sim $2.5$\times $10$^{-38}$ to $\sim $1.1$\times $10$^{-40}$ m$^{5}$/V$^{2}$, respectively, as the shell thickness of Au was increased. It implies that dephase or decay rates of materials have main contributions on cubic nonlinearity rather than excitation cross-section. This work at Hampton University was supported by Army Research Office (W911NF-07-1-0608) and National Science Foundation (HRD-0734635, HRD-0630372, ESI-0426328/002, and EEC-0532472).   

Tracking Nanocars Using Single Molecule Spectroscopy

Nanocars belong to an exciting new class of molecules known as molecular machines. They consist of four fullerene or carborane wheels attached to a chassis consisting of a stiff aromatic backbone. The nanocars are designed to roll over a solid surface making them potential candidates for nano-cargo transporters. Here, we present our results on tracking of nanocars by single molecule fluorescence spectroscopy. By attaching the fluorescent tag tetramethylrhodamin isothiocyanate to the nanocars, we were able to visualize and track individual nanocars using confocal sample scanning microscopy. Fluorescence images were analyzed for directional movement as opposed to random diffusion or stage drift. We had to overcome 2 major problems in our image analysis: 1) fluorescence photo-blinking and 2) photo-bleaching. We developed routines that are capable of tracking individual fluorescent molecules while accounting for photo-blinking and photo-bleaching. The ability to track individual nanocars is checked independently by simulations. Our method is not limited to tracking of nanocars however, and can be extended to follow individual molecules in biological or mechanical systems as well.   

Time-resolved photoluminescence Studies of CdSe Core and CdSe/ZnS Core/Shell colloidal nanoparticles as function of temperature and concentration

We report the results of time-resolved photoluminescence studies (TRPL) of CdSe Core and CdSe/ZnS Core/Shell colloidal nanoparticles dissolved in toluene in the temperature range of 10-300 K. The integrated PL intensity of nanoparticles in liquid changes little between 10 K and 300 K, whereas the intensity of the ``dry'' nanoparticles quenches dramatically as temperature is increased. The PL exhibits biexponential decay characteristics; the longer decay component is affected by the presence of the solvent. In particular, the phase transitions (the glass-solid and the solid-liquid) of the solvent are clearly detected by our experiment. In addition, the PL efficiency and decay times are studied as a function of nanoparticle concentration. Our findings suggest that the PL quantum yield as well as the decay times strongly depend on the solvent temperature, as well as nanoparticles concentration. These results are discussed in terms of reabsorption and reemission between nanoparticles. We acknowledge support of NSF IGERT, NYSTAR and ONR.   

Photon-Correlation Fourier Spectroscopy on CdSe Nanocrystals

The emission spectrum of a single emitter can be artificially widened and blurred due to fluctuations in emission energy, i.e. spectral diffusion. ~This spectral diffusion can be much more rapid than the time required to collect sufficient photons to measure a spectrum. ~We use a new method, Photon-Correlation Fourier Spectroscopy (PCFS), to ``freeze'' spectral diffusion and obtain spectral information of single CdSe nanocrystals on timescales comparable to the lifetime of the emitter.~ This method cross-correlates the two outputs of a Michelson interferometer, providing a histogram of frequency shifts between two photons separated by a given amount of time. ~~We apply PCFS to single nanocrystals in a confocal geometry.~ We also combine PCFS with Fluorescence Correlation Spectroscopy (FCS) to resolve single nanocrystal linewidths from a solution of nanocrystals diffusing under a microscope objective.~