Showing papers by "Richard J. Saykally published in 1997"

Reversible Tuning of Silver Quantum Dot Monolayers Through the Metal-Insulator Transition

Abstract: The linear and nonlinear (χ(2)) optical responses of Langmuir monolayers of organically functionalized silver quantum dots were measured as a continuous function of interparticle separation under near-ambient conditions. As the distance between metal surfaces was decreased from 12 to ∼5 angstroms, both quantum and classical effects were observed in the optical signals. When the separation was less than 5 angstroms, the optical second-harmonic generation (SHG) response exhibited a sharp discontinuity, and the linear reflectance and absorbance began to resemble those of a thin metallic film, indicating that an insulator-to-metal transition occurred. This transition was reversible.

The Water Dipole Moment in Water Clusters

Abstract: The average dipole moment of a water molecule in the condensed phase is enhanced by around 40 percent relative to that of an isolated monomer. This enhancement results from the large polarization caused by the electric field induced by surrounding monomers. A quantitative molecular description of this polarization is essential for modeling aqueous solvation phenomena. This combined theoretical and experimental study of dipole moments in small water clusters provides such a description and also gives insights into the structure of liquid water.

Cavity Ringdown Laser Absorption Spectroscopy: History, Development, and Application to Pulsed Molecular Beams.

Abstract: The measurement of electronic spectra of supersonically cooled molecules and clusters is a widely used approach for addressing many problems in chemistry. The most established techniques for making such measurements are laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI), and both have been employed very successfully in a large number of studies. However, both methods often fail for systems containing more than a few atoms, due to rapid internal conversion, predissociation, or other dynamical processes. Even for small systems, the vibronic band intensities are often contaminated by intramolecular relaxation dynamics; in such cases, these techniques cannot be used for reliable intensity measurements. For clusters that exhibit rapid photofragmentation, depletion spectroscopy can be employed quite effectively to measure their vibronic structure, but again, dynamic effects complicate the interpretation of spectra. The same considerations apply to other types of “action” spectroscopy. It would often be preferable to measure the electronic spectra of molecules and clusters in direct absorption, as this approach is the most straightforward and accurate means of determining absolute vibronic band intensities and for accessing states that are invisible to LIF or REMPI. The problem, of course, is that direct absorption methods are generally orders of magnitude less sensitive than the “action” techniques and are, therefore, difficult to apply to transient species, such as clusters or radicals. In this review, we describe a relatively new direct absorption technique that we have developed for measuring the electronic spectra of jet-cooled molecules and clusters with both high sensitivity and high spectral resolution. The method is based on measurement of the time rate of decay of a pulse of light trapped in a high reflectance optical cavity; we call it cavity ringdown laser absorption spectroscopy (CRLAS). In practice, pulsed laser light is injected into an optical cavity that is formed by a pair of highly reflective (R > 99.9%) mirrors. The small amount of light that is now trapped inside the cavity reflects back and forth between the two mirrors, with a small fraction (∼1 R) transmitting through each mirror with each pass. The resultant transmission of the circulating light is monitored at the output mirror as a function of time and allows the decay time of the cavity to be determined. A simple picture of the cavity decay event for the case where the laser pulse is temporally shorter than the cavity round trip transit time is presented in Figure 1. In this case, the intensity envelope of these discrete transmitted pulses exhibits a simple exponential decay. The time required for the cavity to decay to 1/e of the initial output pulse is called the “cavity ringdown” time. Determination of the ringdown time allows the absolute single pass transmission coefficient of the cavity to be determined with high accuracy, given the mirror spacing. The apparatus is converted to a sensitive absorption spectrometer simply by placing an absorbing medium between the two mirrors and recording the frequency dependent ringdown time of the cavity. Ideally, the ringdown time is a function of only the mirror reflectivities, cavity dimensions, and sample absorption. Absolute absorption intensities are obtained by subtracting the base-line transmission of the cavity, which is determined when the laser wavelength is off-resonance with all molecular transitions. † IBM Predoctoral Fellow. Current address: Sandia National Laboratories, M/S 9055, Livermore, CA 94551-0969. ‡ Los Gatos Research. 25 Chem. Rev. 1997, 97, 25−51

Direct Measurement of Water Cluster Concentrations by Infrared Cavity Ringdown Laser Absorption Spectroscopy

Abstract: The recently developed technique of infrared cavity ringdown laser absorption spectroscopy (IR-CRLAS) has been employed in the 3.0 μm region to determine the absolute concentrations of water dimers, trimers, tetramers, and pentamers in a pulsed supersonic expansion for the first time. Additional spectral features are reported, one of which we assign to the bound O−H stretching bands of the hexamer. Additionally, by simple variation of the jet stagnation pressure, the collective O−H stretching absorption from all clusters produced in the expansion was observed to change from that of discrete features of small clusters to band profiles of liquid water and finally to amorphous ice.

Terahertz Laser Vibration−Rotation Tunneling Spectroscopy and Dipole Moment of a Cage Form of the Water Hexamer

Abstract: Tunable terahertz laser vibration−rotation-tunneling spectroscopy has been employed to characterize the structure and hydrogen bond network rearrangement dynamics of a cage form of the water hexamer having eight hydrogen bonds. The isolated clusters are produced in a pulsed supersonic slit jet. Striking similarities are found between the structure and the average interoxygen distance RO-O (2.82 A) of the hexamer cage and those of the basic unit of ice VI. The hybrid perpendicular band of b- and c-types is observed near 2.491 THz (83.03 cm-1) and rationalized to originate from the torsional motions of the two single-donor single-acceptor monomers about their donor hydrogen bonds, thereby causing changes in the dipole moments from each monomer to be orthogonal to each other as well as to be perpendicular to the approximate symmetry a-axis. Triplet spectral patterns accompanying each rovibrational transition with line spacings of 1.9 MHz and intensity ratios of 9:6:1 are attributed to the degenerate quantum ...

Fully coupled six-dimensional calculations of the water dimer vibration-rotation-tunneling states with a split Wigner pseudo spectral approach

Abstract: A novel and efficient pseudospectral method for performing fully coupled six-dimensional bound state dynamics calculations is presented, including overall rotational effects. A Lanczos based iterative diagonalization scheme produces the energy levels in increasing energies. This scheme, which requires repetitively acting the Hamiltonian operator on a vector, circumvents the problem of constructing the full matrix. This permits the use of ultralarge molecular basis sets (up to over one million states for a given symmetry) in order to fully converge the calculations. The Lanczos scheme was conducted in a symmetry adapted spectral representation, containing Wigner functions attached to each monomer. The Hamiltonian operator has been split into different terms, each corresponding to an associated diagonal or nearly diagonal representation. The potential term is evaluated by a pseudospectral scheme of Gaussian accuracy, which guarantees the variational principle. Spectroscopic properties are computed with this...

Terahertz Laser Vibration-Rotation Tunneling Spectroscopy of the Water Tetramer

Abstract: A vibration−rotation-tunneling (VRT) spectrum of (H2O)4 has been recorded near 2.04 THz (67.9 cm-1). The band origin of this ΔK = 0 symmetric rotor spectrum is shifted by 0.1 cm-1 to the blue of that of a (D2O)4 band reported previously [Science, 1996, 271, 59]. Similar to that spectrum, each transition in the (H2O)4 spectrum exhibits a regular doublet splitting. The (D2O)4 and (H2O)4 doublet spacings, 5.6 and 2260 MHz, respectively, are constant as a function of rotation, indicative of degenerate tunneling splittings. They have been rationalized in terms of a concerted flipping motion of the H atoms that do not participate in hydrogen bonding in the S4 equlibrium structure, analogous to ammonia inversion. Both bands have been assigned to the lowest Ag→ Bg intermolecular vibration in the molecular symmetry group C4h(M). Additional K = 2 splittings, which increase nonlinearly with increasing J, were found in both D2O and H2O tetramer spectra. Evidence for the bifurcation tunneling motion observed in the wa...

Pseudorotation in Water Trimer Isotopomers Using Terahertz Laser Spectroscopy

Abstract: We report the first observation of five water trimer isotopomers using terahertz laser spectroscopy coupled with a pulsed slit jet expansion technique. A single c-type vibration−rotation-tunneling (VRT) band has been observed for each isotopomer between 40 and 50 cm-1. By considering all the experimental data, including results from isotopic substitution experiments, analyses of bifurcation tunneling splittings, and rotational analyses of the VRT bands, it has been possible to determine unambiguously the isotopic composition and structure of each isotopomer. We have also extended the measurements of the 41.1 cm-1 band in (D2O)3, first reported by Suzuki and Blake [Suzuki, S.; Blake, G. A. Chem. Phys. Lett. 1994, 229, 499]. All six VRT bands have been assigned to pseudorotational transitions, which principally involve flipping of the “free” hydrogen or deuterium atoms in the trimer from above to below the plane of the OOO ring. Interestingly, four of the six bands have been assigned to the same transition....

Terahertz Laser Spectroscopy of the Water Pentamer: Structure and Hydrogen Bond Rearrangement Dynamics

Abstract: The detailed analysis of a parallel vibration−rotation-tunneling (VRT) band of the isolated cyclic perdeuterated (d10) water pentamer measured near 2.4 THz (81.2 cm-1) is presented. The vibrational...

Cavity ringdown measures trace concentrations

Abstract: Intensity decay of pulsed laser sources in a highly reflective cavity ringdown laser absorption spectroscopy to measure sub-parts-per-million levels of gaseous species.