Showing papers by "Richard J. Saykally published in 2007"

Tunable nanowire nonlinear optical probe

Abstract: One crucial challenge for subwavelength optics has been the development of a tunable source of coherent laser radiation for use in the physical, information and biological sciences that is stable at room temperature and physiological conditions. Current advanced near-field imaging techniques using fibre-optic scattering probes have already achieved spatial resolution down to the 20-nm range. Recently reported far-field approaches for optical microscopy, including stimulated emission depletion, structured illumination, and photoactivated localization microscopy, have enabled impressive, theoretically unlimited spatial resolution of fluorescent biomolecular complexes. Previous work with laser tweezers has suggested that optical traps could be used to create novel spatial probes and sensors. Inorganic nanowires have diameters substantially below the wavelength of visible light and have electronic and optical properties that make them ideal for subwavelength laser and imaging technology. Here we report the development of an electrode-free, continuously tunable coherent visible light source compatible with physiological environments, from individual potassium niobate (KNbO3) nanowires. These wires exhibit efficient second harmonic generation, and act as frequency converters, allowing the local synthesis of a wide range of colours via sum and difference frequency generation. We use this tunable nanometric light source to implement a novel form of subwavelength microscopy, in which an infrared laser is used to optically trap and scan a nanowire over a sample, suggesting a wide range of potential applications in physics, chemistry, materials science and biology.

The effects of dissolved halide anions on hydrogen bonding in liquid water.

Abstract: It is widely believed that the addition of salts to water engenders structural changes in the hydrogen-bond network well beyond the adjacent shell of solvating molecules. Classification of many ions as "structure makers" and "structure breakers" has been based in part on corresponding changes in the vibrational spectra (Raman and IR). Here we show that changes in O-H vibrational spectra induced by the alkali halides in liquid water result instead from the actions of ions' electric fields on adjacent water molecules. Computer simulations that accurately reproduce our experimental measurements suggest that the statistics of hydrogen-bond strengths are only weakly modified beyond this first solvation shell.

Infrared spectroscopy of cationized arginine in the gas phase: direct evidence for the transition from nonzwitterionic to zwitterionic structure.

Abstract: The gas-phase structures of protonated and alkali metal cationized arginine (Arg) and arginine methyl ester (ArgOMe) are investigated with infrared spectroscopy and ab initio calculations. Infrared spectra, measured in the hydrogen-stretch region, provide compelling evidence that arginine changes from its nonzwitterionic to zwitterionic form with increasing metal ion size, with the transition in structure occurring between lithium and sodium. For sodiated arginine, evidence for both forms is obtained from spectral deconvolution, although the zwitterionic form is predominant. Comparisons of the photodissociation spectra with spectra calculated for low-energy candidate structures provide additional insights into the detailed structures of these ions. Arg*Li+, ArgOMe*Li+, and ArgOMe*Na+ exist in nonzwitterionic forms in which the metal ion is tricoordinated with the amino acid, whereas Arg*Na+ and Arg*K+ predominately exist in a zwitterionic form where the protonated side chain donates one hydrogen bond to the N terminus of the amino acid and the metal ion is bicoordinated with the carboxylate group. Arg*H+ and ArgOMe*H+ have protonated side chains that form the same interaction with the N terminus as zwitterionic, alkali metal cationized arginine, yet both are unambiguously determined to be nonzwitterionic. Calculations indicate that for clusters with protonated side chains, structures with two strong hydrogen bonds are lowest in energy, in disagreement with these experimental results. This study provides new detailed structural assignments and interpretations of previously observed fragmentation patterns for these ions.

One water molecule stabilizes the cationized arginine zwitterion.

Abstract: Singly hydrated clusters of lithiated arginine, sodiated arginine, and lithiated arginine methyl ester are investigated using infrared action spectroscopy and computational chemistry. Whereas unsolvated lithiated arginine is nonzwitterionic, these results provide compelling evidence that attachment of a single water molecule to this ion makes the zwitterionic form of arginine, in which the side chain is protonated, more stable. The experimental spectra of lithiated and sodiated arginine with one water molecule are very similar and contain spectral signatures for protonated side chains, whereas those of lithiated arginine and singly hydrated lithiated arginine methyl ester are different and contain spectral signatures for neutral side chains. Calculations at the B3LYP/6-31++G** level of theory indicate that solvating lithiated arginine with a single water molecule preferentially stabilizes the zwitterionic forms of this ion by 25−32 kJ/mol and two essentially isoenergetic zwitterionic structure are most st...

Infrared Spectroscopy of Cationized Lysine and ε-N-methyllysine in the Gas Phase: Effects of Alkali-Metal Ion Size and Proton Affinity on Zwitterion Stability

Abstract: The gas-phase structures of protonated and alkali-metal-cationized lysine (Lys) and e-N-methyllysine (Lys(Me)) are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, in conjunction with ab initio calculations. IRMPD spectra of Lys·Li+ and Lys·Na+ are similar, but the spectrum for Lys·K+ is different, indicating that the structure of lysine in these complexes depends on the metal ion size. The carbonyl stretch of a carboxylic acid group is clearly observed in each of these spectra, indicating that lysine is nonzwitterionic in these complexes. A detailed comparison of these spectra to those calculated for candidate low-energy structures indicates that the bonding motif for the metal ion changes from tricoordinated for Li and Na to dicoordinated for K, clearly revealing the increased importance of hydrogen-bonding relative to metal ion solvation with increasing metal ion size. Spectra for Lys(Me)·M+ show that Lys(Me), an analogue ...

Evidence for water rings in the hexahydrated sulfate dianion from IR spectroscopy.

Abstract: Whereas isolated SO42- is unstable, hydrated clusters of this dianion have been formed and investigated using a variety of different methods. Several structures of [SO4(H2O)6]2- have been proposed that account for its high stability in the gas phase. Zhou et al. [J. Chem. Phys. 2006, 125, 111102] recently reported infrared spectra of [SO4(H2O)n]2- in the 540−1850 cm-1 region and assigned the spectrum of the hexahydrated ion to a Td symmetry structure in which all six water molecules donate two hydrogen bonds to the sulfate core. Here, an infrared spectrum of this ion in the hydrogen stretch region (2620−3840 cm-1) and B3LYP/AUG-cc-pVDZ calculations indicate that a significant population of these ions correspond to lower symmetry structures containing water rings in which each water molecule donates hydrogen bonds to both the sulfate dianion and a neighboring water molecule. These calculations indicate that inter-water hydrogen bonds are slightly favored over additional solvation of the dianion core. These...

Hydration of the calcium dication: direct evidence for second shell formation from infrared spectroscopy.

Abstract: Infrared laser action spectroscopy in a Fourier-transform ion cyclotron resonance mass spectrometer is used in conjunction with ab initio calculations to investigate doubly charged, hydrated clusters of calcium formed by electrospray ionization. Six water molecules coordinate directly to the calcium dication, whereas the seventh water molecule is incorporated into a second salvation shell. Spectral features indicate the presence of multiple structures of Ca(H 2 O) 2+ 7 in which outer-shell water molecules accept either one (single acceptor) or two (double acceptor) hydrogen bonds from inner-shell water molecules. Double-acceptor water molecules are predominately observed in the second solvent shells of clusters containing eight or nine water molecules. Increased hydration results in spectroscopic signatures consistent with additional second-shell water molecules, particularly the appearance of inner-shell water molecules that donate two hydrogen bonds (double donor) to the second solvent shell. This is the first reported use of infrared spectroscopy to investigate shell structure of a hydrated multiply charged cation in the gas phase and illustrates the effectiveness of this method to probe the structures of hydrated ions.

Nature of the aqueous hydroxide ion probed by X-ray absorption spectroscopy.

Abstract: X-ray absorption spectra of aqueous 4 and 6 M potassium hydroxide solutions have been measured near the oxygen K edge Upon addition of KOH to water, a new spectral feature (5325 eV) emerges at energies well below the liquid water pre-edge feature (535 eV) and is attributed to OH- ions In addition to spectral changes explicitly due to absorption by solvated OH- ions, calculated XA spectra indicate that first-solvation-shell water molecules exhibit an absorption spectrum that is unique from that of bulk liquid water It is suggested that this spectral change results primarily from direct electronic perturbation of the unoccupied molecular orbitals of first-shell water molecules and only secondarily from geometric distortion of the local hydrogen bond network within the first hydration shell Both the experimental and the calculated XA spectra indicate that the nature of the interaction between the OH- ion and the solvating water molecules is fundamentally different than the corresponding interactions of

Electrokinetic Hydrogen Generation from Liquid Water Microjets

Abstract: We describe a method for generating molecular hydrogen directly from the charge separation effected via rapid flow of liquid water through a metal orifice, wherein the input energy is the hydrostatic pressure times the volume flow rate. Both electrokinetic currents and hydrogen production rates are shown to follow simple equations derived from the overlap of the fluid velocity gradient and the anisotropic charge distribution resulting from selective adsorption of hydroxide ions to the nozzle surface. Pressure-driven fluid flow shears away charge balancing hydronium ions from the diffuse double layer and carries them out of the aperture. Downstream neutralization of the excess protons at a grounded target electrode produces gaseous hydrogen molecules. The hydrogen production efficiency is currently very low (ca. 10-6) for a single cylindrical jet but can be improved with design changes.

Interpreting the H/D Isotope Fractionation of Liquid Water during Evaporation without Condensation

Abstract: A theoretical model of liquid water evaporation has been developed to interpret results from a recent experimental investigation of isotope fractionation during free evaporation [Cappa et al. J. Phys. Chem. B 2005, 109 (51), 24391]. It is established that the free evaporation isotope fractionation factors (αevap) are primarily influenced by the nature of the intermolecular interactions between water molecules, namely, the condensed phase hindered translational and librational frequencies at the surface. The dependence of αevap on the isotopic composition of the liquid can be understood in terms of small variations in these frequencies with isotopic composition. This result suggests that the explicit nature of the solvation environment directly influences evaporation rates from liquids. The sensitivity of the calculated evaporation coefficient for liquid water to both temperature and isotope composition is also explored.

Toward a precise determination of the acceptor switching splitting in the water dimer

Abstract: Precise measurement of all tunneling splittings in the ground vibrational state of the water dimer is essential for a complete and rigorous determination of the intermolecular potential energy surface. Here, accurate experimentally determined rotational constants and tunneling splittings were combined with estimates of the ground state acceptor switching (AS) splittings for both (H2O)2 and (D2O)2 in order to exactly predict the fingerprints of the weakly allowed E 2 ↔ E 1 transitions and to approximately predict their absolute frequencies. While these transitions are predicted to be quite weak, current technology guided by the now fairly complete water dimer data should permit their observation. The measurement of these weak transitions would permit the direct determination of AS splittings, which has so far eluded experiments.

Refinements in the description of excited VRT states of the water dimer

Abstract: Extensive new spectroscopic measurements are combined with a global analysis of the ground state data in order to re-examine and to refine the description of the excited vibration rotation tunneling (VRT) states of the water dimer. Notably, six new ‘donor torsion’ subbands are analytically identified, current vibrational assignments of the K a = 1 stacks are reassessed, the previously reported (H2O)2 donor torsion overtone (DT)2 and hydrogen bond stretch (S) data sets are augmented, and four new (S) subbands have been measured. Unusually large Coriolis effects are predicted, excited state E2 ↔ E1 assignments are reinforced, and possibilities of experimentally determining ground state AS splitting in (H2O)2 from excited state data are discussed.

Chirped coherent anti-Stokes Raman scattering as a high-spectral- and spatial-resolution microscopy

Abstract: Coherent anti-Stokes Raman scattering (CARS) microscopy is a promising tool for chemically selective imaging based on molecular vibrations. While CARS is currently used as a biological imaging tool, many variations are still being developed, perhaps the most important being multiplex CARS microscopy. Multiplex CARS has the advantage of comparing images based on different molecular vibrations without changing the excitation wavelengths. Here we demonstrate both high-spectral- and spatial-resolution multiplex CARS imaging of polymer films, using a simple scheme for chirped CARS with a spectral bandwidth of 300 cm(-1).

Terahertz vibration-rotation-tunneling spectroscopy of the ammonia dimer. II. A-E states of an out-of-plane vibration and an in-plane vibration.

Abstract: Terahertz vibration−rotation−tunneling transitions have been measured between ca. 78.5 and 91.9 cm-1, and assigned to A−E (ortho−para) combinations of NH3 monomer states. The spectrum is complicate...

Evidence for an Enhanced Proton Concentration at the Liquid Water Surface from SHG Spectroscopy

Abstract: Using resonant UV SHG spectroscopy, we have observed surface-enhanced concentrations of several ions in aqueous solutions, confirming theoretical predictions from several groups. Our experiments also support recent predictions of enhanced proton concentrations at aqueous surfaces.