Thomas F. George

Structural properties of As-rich GaAs grown by molecular beam epitaxy at low temperatures

GaAs layers grown by molecular beam epitaxy (MBE) at substrate temperatures between 200 and 300 °C were studied using transmission electron microscopy (TEM), x‐ray diffraction, and electron paramagnetic resonance (EPR) techniques. High‐resolution TEM cross‐sectional images showed a high degree of crystalline perfection of these layers. For a layer grown at 200 °C and unannealed, x‐ray diffraction revealed a 0.1% increase in the lattice parameter in comparison with bulk GaAs. For the same layer, EPR detected arsenic antisite defects with a concentration as high as 5×1018 cm−3. This is the first observation of antisite defects in MBE‐grown GaAs. These results are related to off‐stoichiometric, strongly As‐rich growth, possible only at such low temperatures. These findings are of relevance to the specific electrical properties of low‐temperature MBE‐grown GaAs layers.

Analytic Continuation of Classical Mechanics for Classically Forbidden Collision Processes

Classically forbidden processes are those that cannot take place via ordinary classical dynamics. Within the framework of classical S‐matrix theory, however, classical mechanics can be analytically continued and classical‐limit approximations obtained for these classically forbidden, or weak transition amplitudes (i.e., S‐matrix elements). The most powerful and general way of analytically continuing classical mechanics for a complex dynamical system is to integrate the equations of motion themselves through the classically inaccessible regions of phase space. Success in calculating these analytically continued trajectories is reported in this work; with certain special features of these complex‐valued trajectories recognized and taken account of, it is seen that they are essentially as easy to deal with numerically as ordinary (i.e., real) classical trajectories. Application to the linear A+BC collision (vibrational excitation) gives excellent results; transition probabilities as small as 10−11 (the small...

Laser-induced explosion of gold nanoparticles: potential role for nanophotothermolysis of cancer

AIMS This article explores the laser-induced explosion of absorbing nanoparticles in selective nanophotothermolysis of cancer. METHODS This is realized through fast overheating of a strongly absorbing target during the time of a short laser pulse when the influence of heat diffusion is minimal. RESULTS On the basis of simple energy balance, it is found that the threshold laser fluence for thermal explosion of different gold nanoparticles is in the range of 25-40 mJ/cm(2). CONCLUSION Explosion of nanoparticles may be accompanied by optical plasma, generation of shock waves with supersonic expansion and particle fragmentation with fragments of high kinetic energy, all of which can contribute to the killing of cancer cells.

Vibrational motions of Buckminsterfullerene

Abstract A non-Cartesian coordinate system is developed which permits the vibrational motions of Buckminsterfullerene to be expressed in terms of four force constants. A 180 × 180 matrix is then derived which, when diagonalized, yields the complete vibrational spectrum. These results are compared with those obtained previously via a MNDO calculation.

The quantum damped harmonic oscillator

Abstract Starting with the quantization of the Caldirola–Kanai Hamiltonian, various phenomenological methods to treat the damped harmonic oscillator as a dissipative system are reviewed in detail. We show that the path integral method yields the exact quantum theory of the Caldirola–Kanai Hamiltonian without violation of Heisenbergs uncertainty principle. Through the dynamical invariant and second quantization methods together with the path integral, we also present systematically the exact quantum theories for the various dissipative harmonic oscillators, bound and unbound quadratic Hamiltonian systems, and the relation between the canonical and unitary transformations for the classical and quantum dissipative systems.

Classical S‐Matrix Theory of Reactive Tunneling: Linear H+H2 Collisions

Complex‐valued classical trajectories (computed by direct numerical integration of Hamiltons equations) are found for linear reaction collisions of H+H2→ H2+H (on the Porter‐Karplus potential surface) at collision energies for which all ordinary real trajectories are nonreactive, and from such trajectories classical S‐matrix elements are constructed. This analytically continued classical‐limit theory is seen to be an accurate description of reactive tunneling for the H+H2 system. At each collision energy there is only one classical trajectory that contributes to the reaction, so that various features of the reaction dynamics are easily elucidated by looking specifically at this one trajectory. It is also shown how a Boltzmann average of the reaction probability can be carried out semiclassically, and this leads to an interesting relation between the imaginary part of the time increment of the complex‐valued trajectory at a given energy and the absolute temperature at which this is the dominant energy in ...

Complex‐valued classical trajectories for reactive tunneling in three‐dimensional collisions of H and H2

Complex‐valued classical trajectories for three‐dimensional reactive collisions of H+H2 have been calculated at collision energies below the classical threshold for reaction, and from such trajectories classical S‐matrix elements for the 0 → 1 rotational transition have been constructed. Comparison with available quantum mechanical results for the same system are encouraging and suggest that this semiclassical theory is capable of accurately describing reactive tunneling in a physically realistic model of a chemical reaction. Ways of simplifying the practical aspects of applying classical S‐matrix theory to three‐dimensional reactive systems are also described.

Semiempirical study of rare gas and rare gas–hydrogen ionic clusters: R+n, (RnH)+, and (RnH2)+ for R≡Ar, Xe

The ionic rare gas clusters Ar+n and Xe+n and rare gas–hydrogen clusters (ArnH)+, (ArnH2)+, (XenH)+ and (XenH2)+ are studied by the semiempirical diatomics‐in‐ionic‐systems (DIIS) method. The Ar+n clusters (n>3) are seen to have a structure of a linear Ar+3 core surrounded by n−3 neutral or almost neutral Ar atoms. For Xe+n (n>3), a symmetrical Xe+4 ionic core with the geometry of regular pyramid is formed. The rare gas–hydrogen clusters with one H atom have a simple Rk(RH)+ structure with k neutral rare gas atoms attracted to the (RH)+ molecule by polarization forces. Two H atoms can bind with Ar atoms to form quasistable clusters ArnH+2 which dissociate to (n−1)Ar+H+(ArH)+ through a high barrier of roughly 0.75 eV. Two H atoms and one Xe+ ion are shown to form a collinear valence‐bound (XeHH)+ cluster whose dissociation energy is 0.46 eV.

The Hückel model for small metal clusters. I. Geometry, stability, and relationship to graph theory

The relative stabilities of alkali‐like metal clusters, Mn and M+n with 2≤n≤9, are calculated within the framework of the simple Huckel model. With the aid of graph theory, the binding energies for all possible Huckel structures are determined. With the exception of M+5 and M+6 , the Huckel model gives minimum energy structures which are the same as those predicted by recent local‐spin‐density and configuration interaction calculations. Since the Huckel method is independent of the mechanical details of the bonding, a close connection is inferred between a cluster’s stability and its topology. In the paper following this one, the Huckel results are extended to include absolute atomization energies and ionization potentials. In addition, it is shown that cluster energies may be quantitatively extrapolated to the bulk phase.

Thermal oxidation of porous silicon: Study on structure

The structural changes of porous silicon (PS) samples during oxidation are investigated and analyzed using various microscopy techniques and x-ray diffraction. It is found that the surface roughness of oxidized PS layers increases with the oxidation at 200–400°C and decreased at 600–800°C. At 800°C a partially fused surface is observed. The oxide formed on the wall of porous silicon skeleton is amorphous. The shifts of Si(400) peaks are observed in the x-ray diffraction patterns, which are correlated to the lattice deformation induced by thermal expansion coefficient mismatch between the grown SiO2 and the residual Si, and to the intrinsic stress caused by the Si–O bonds at the Si–SiO2 interface. These explanations are supported by thermomechanical modeling using three-dimensional finite element method.