Shaun M. Williams

Photoelectron spectra of hydrated electron clusters vs. cluster size: connecting to bulk

Connecting cluster properties to bulk can result in new insights, both for the bulk and for clusters, and this is especially the case for the hydrated electron, e–(aq), and its cluster counterparts. In bulk, e–(aq) can be viewed as an anionic defect state of pure water, which is itself a large-band-gap semiconductor. Thus, when properly extrapolated to bulk, the properties of clusters must reflect important energetic properties of bulk water. Several of these have been extracted and are presented here. As for the nature of clusters themselves, the topic has now inspired spirited debate for over two decades. Here, we offer our perspective on these intriguing entities. Among several isomeric groups of and clusters, the one having the highest electron binding energies (herein called bulk embryonts) has been extrapolated to deduce the implied photoelectron spectrum of the bulk hydrated electron. The legitimacy of the lineshape fitting procedures used here is supported by the successful extrapolation of cluster absorption data to its known bulk benchmarks.

Enhanced infrared absorption spectra of self-assembled alkanethiol monolayers using the extraordinary infrared transmission of metallic arrays of subwavelength apertures.

The surface-plasmon-mediated, extraordinary transmission of metallic arrays of subwavelength apertures has been used as the light source for absorption studies of self-assembled monolayers on metal. Enhanced infrared absorption spectra of a sequence of alkanethiol self-assembled monolayers on copper were recorded for carbon chain lengths varying from 8 to 18 atoms. Transition positions and intensities are presented over a large range of the infrared region. The connection between the vibrational modes of the CH(2) wagging progression and the infinite methylene chain is explored using a traditional coupled oscillator approach and a new cluster perspective.

Extraordinary infrared transmission of Cu-coated arrays with subwavelength apertures: Hole size and the transition from surface plasmonto waveguide transmission

The zero-order, infrared transmission spectra were recorded and studied at varying angles of incidence to study the dispersion of the resonances in both the Γ−X and Γ−M reciprocal lattice directions. Reduction of hole size shows dramatic effects on the intensity of transmission, the width of the resonances, the identity of the most prominent resonances, and the dispersion behavior of resonances.

Scaffolding for nanotechnology: extraordinary infrared transmission of metal microarrays for stacked sensors and surface spectroscopy

Self-assembled monolayers, phospholipid bilayers, and membrane bound proteins are assembled on a subwavelength metallic array. These assemblies are assayed with direct infrared absorption spectroscopy which is greatly enhanced due to the extraordinary infrared transmission of the arrays. Stacking the arrays, one upon another, accentuates the surface plasmon properties and provides the basis of a nanospaced capacitive sensor.

Carbon chains and the (5,5) single-walled nanotube: Structure and energetics versus length

Reliable thermochemistry is computed for infinite stretches of pure-carbon materials including acetylenic and cumulenic carbon chains, graphene sheet, and single-walled carbon nanotubes (SWCNTs) by connection to the properties of finite size molecules that grow into the infinitely long systems. Using ab initio G3 theory, the infinite cumulenic chain (:C[double bond]C[double bond]C[double bond]C:) is found to be 1.9+/-0.4 kcal/mol per carbon less stable in free energy at room temperature than the acetylenic chain (.C[triple bond]C-C[triple bond]C.) which is 24.0 kcal/mol less stable than graphite. The difference between carbon-carbon triple, double, and single bond lengths (1.257, 1.279, and 1.333 A, respectively) in infinite chains is evident but much less than with small hydrocarbon molecules. These results are used to evaluate the efficacy of similar calculations with the less rigorous PM3 semiempirical method on the (5,5) SWCNT, which is too large to be studied with high-level ab initio methods. The equilibrium electronic energy change for C(g)-->C[infinite (5,5) SWCNT] is -166.7 kcal/mol, while the corresponding free energy change at room temperature is -153.3 kcal/mol (6.7 kcal/mol less stable than graphite). A threefold alternation (6.866, 6.866, and 6.823 A) in the ring diameter of the equilibrium structure of infinitely long (5,5) SWCNT is apparent, although the stability of this structure over the constant diameter structure is small compared to the zero point energy of the nanotube. In general, different (n,m) SWCNTs have different infinite tube energetics, as well as very different energetic trends that vary significantly with length, diameter, and capping.