Kazi A. Motakabbir

Structure and Dynamics of Water at the Pt(111) Interface: Molecular Dynamics Study

We prescribe an analytical form of the interaction potential between rigid water and a rigid platinum metal surface, which takes into account the surface symmetry and corrugation. Using this potential we perform a molecular dynamics computer simulation on water lamina restricted by two Pt(111) surfaces and investigate the structure and dynamics of water at the Pt interface. At 300 K the water layer adjacent to the metal surface displays solid‐like properties. Patches of ice‐like structure embedded in this layer are observed in the simulation. The next two layers of water display ordering similar to ice‐I. Beyond these three layers the structure and dynamics of water are bulk‐like.

Liquid—vapor interface of TIP4P water: comparison between a polarizable and a nonpolarizable model

Abstract A computer simulation study of TIP4P water using a polarizable model and the standard TIP4P model was performed. We have found that polarization effects are of secondary importance in predicting the orientational structure in the liquid—vapor interface.

On the nature of pre-existing states for an excess electron in water

Abstract The nature of the electronic eigenstates for an excess electron in pure water in the absence of solvent reorganization is investigated using solvent configurations generated from molecular dynamics simulation of liquid water at 283 K and a previously developed electron-water molecule pseudopotential. The electronic density of states is found to exhibit a significant tail at lower energies that is comprised of physically localized states, although these states are, at best, only shallowly trapped with respect to the free electron. The electronic absorption spectrum associated with the set of ground electronic states is found to be a diffuse infrared band in general accord with the spectrum observed experimentally at the earliest times following creation of an excess electron in water. However, this spectrum is insensitive to the degree of localization of the initial state, indicating that this result does not discriminate strongly among alternative descriptions of the electronic state present experimentally.

Transient photophysical hole‐burning spectroscopy of the hydrated electron: A quantum dynamical simulation

Results for the time‐dependent adiabatic eigenspectrum of an electron in water evolving in dynamic equilibrium have been obtained via quantum molecular dynamics simulation and used to evaluate the results expected from time‐resolved transient optical hole‐burning experiments. The dependence on excitation frequency and pulse length have been explored. The calculated results indicate that a relatively broad hole is created, but that, for ultrashort pump–probe time delays (≤100 fs) and comparably short pulses, the shape is distinctly different from the equilibrium spectrum. A slower component in the spectral evolution is also present, but appears likely to be difficult to distinguish experimentally. The shape of the absorption deficit is characteristic of the inhomogeneously broadened 1s, 2p‐type electronic state structure found previously to underlie the equilibrium spectrum, and distinguishes between this description and a number of proposed alternatives. With pulse durations comparable to the best now ava...

A comparison of classical and quantum analyses of electron localization sites in liquid water

The results of an earlier study [Schnitker, Rossky, and Kenney‐Wallace, J. Chem. Phys. 85, 2986 (1986)] in which likely sites for electron localization in pure liquid water were identified and characterized via a physically motivated purely classical analysis are statistically compared to a corresponding fully quantum mechanical treatment of the excess electronic ground state. It is shown that the most energetically favorable localization sites identified by the classical treatment correspond reasonably to the quantum mechanical result both energetically and spatially. It is found that the existence and location of a physically localized ground state can be determined from the classical results if both the minimum of the estimated absolute total electronic energy and the difference between this minimum and the alternative local minima identified within a solvent configuration are considered. Further, the results confirm that the concentration of such effective sites is relatively high in the liquid (∼0.01 M). Hence, the classical approach has merit as a qualitative tool for the analysis of the electronic states supported by the preexisting configurational order in a liquid.

The effect of lead, gold, and silver backings on dose near 125I seeds

Brachytherapy for ocular melanoma uses 125I seeds backed by a gold shield. Conflicting results are reported in the literature on the effect of the gold on dose close to the seeds. In this work, a small lucite jig was constructed such that the seed-to-detector separation remained fixed as high-Z materials of lead, silver, and gold were moved in and out of position behind the seed. The jig was clamped in place in the water filled tank of a beam scanning system. The response of two p-type silicon diodes was measured at several distances from the seed with and without the high-Z backings. The response with the high-Z backing relative to water, found to be the same for each diode and the same for lead and gold, decreased from about 1.01 at 1.5 mm to about 0.92 at 20 mm. It has been suggested in the literature that L-shell fluorescent x rays of approximately 10 keV from the gold backing might contribute significantly to the dose within 7 mm of the seed. To test this, the response with the gold backing relative to water was measured with an aluminum cap of 1-mm wall thickness covering the diode. The cap transmits about 70% of the 125I influence but is essentially infinitely thick to 10-keV photons. The relative response (gold/water) was the same with and without the cap showing that the contribution of 10-keV x rays is negligible. Compared to water, the silver backing was found to enhance the diode response by about 14% between 5 to 10 mm from the seed.