ESR study on radiation chemical processes in polyvinyl alcohol films at 77°k

Abstract

Trapped electrons (e_t ^?) were detected in irradiated PVA films at 77°K. Yield of e_t ^? was decreased as evaporating water from the films, indicating that water molecules contribute to construct pre-existing traps in the polymer. The radical produced at 77°K was identified as a precursor of the radical produced at room temperature.     

In situ observation of the ultrafast lattice dynamics of graphite under ion irradiation

We develop a pump-probe experiment system, in which vibrational dynamics of a solid sample under ion irradiation can be measured in real time. In situ observation enables us to monitor small changes induced by ion irradiation, without being influenced by the irreproducibility of the sample quality or the experimental configuration. We apply the experimental system to investigate the femtosecond dynamics of the coherent E_2g1 phonon of graphite under 5 keV He⁺ irradiation. A slight decrease in the dephasing rate of the phonon at the initial stage, as well as a downshift followed by an upshift of the phonon frequency, are clearly demonstrated, all of which were ambiguous in the ex situ experiment due to the poor reproducibility of the surface quality. This technique could also be applied to study femtosecond vibrational dynamics in real time during thermal annealing, film deposition with e.g. ablation and sputter, and molecular adsorption on substrates.     

Dynamics and kinetics of heat transfer at the interface of model diamond [111] nanosurfaces

A molecular dynamics simulation was performed to study the effect of an applied force on heat transfer at the interface of model diamond [111] nanosurfaces. The force was applied to a small, hot nanosurface at 800, 1000, or 1200 K brought into contact with a larger, colder nanosurface at 300 K. The relaxation of the initial nonequilibrium interfacial force occurs on a subpicosecond time scale, much shorter than that required for heat transfer. Heat transfer occurs with exponential kinetics and a rate constant that increases linearly with the interfacial force according to 7 x 10(-4) ps(-1)/nN. This rate constant only increases by at most 10% as the temperature of the hot surface is increased from 800 to 1200 K. Replacing the interfacial H-atoms on both surfaces by D atoms also has a very small effect on the heat transfer. However, if one nanosurface has H atoms on its interface and the other nanosurface's interface has D atoms, then there is a marked 25% decrease in the rate constant for heat transfer. Increasing the size of the hot surface, and, thus, the interfacial contact area, increases the rate of heat transfer but not the rate constant. For the same interfacial force, different anharmonic models for the nanosurfaces' potential energy function give the same heat transfer rate constant. The possibility of quantum effects for heat transfer across the diamond interface is considered.     

Ultrafast Fano resonance between optical phonons and electron-hole pairs at the onset of quasiparticle generation in a semiconductor

Based on the many-body time-dependent approach applied to the ultrafast time region, we investigate the dynamics of creation of an optical phonon incorporating with the electron-hole continuum in a semiconductor. In the transient Fano resonance, due to an interference between those sharp (optical phonon) and continuum (electron-hole pair) quasiparticles, we find the robust destructive interference at birth of them, i.e., tau approximately 0 if the created phonon is coherent under the irradiation of ultrashort optical pulses. The origin is found to be the potential scattering of the electron-hole pair by the q=0 coherent phonon. This finding agrees well with the recent experiment.     

Structural transition of actin filament in a cell-sized water droplet with a phospholipid membrane

Actin filament, F-actin, is a semiflexible polymer with a negative charge, and is one of the main constituents of cell membranes. To clarify the effect of cross talk between a phospholipid membrane and actin filaments in cells, we conducted microscopic observations on the structural changes in actin filaments in a cell-sized (several tens of micrometers in diameter) water droplet coated with a phospholipid membrane such as phosphatidylserine (PS; negatively charged head group) or phosphatidylethanolamine (PE; neutral head group) as a simple model of a living cell membrane. With PS, actin filaments are distributed uniformly in the water phase without adsorption onto the membrane surface between 2 and 6 mM Mg2+, while between 6 and 12 mM Mg2+, actin filaments are adsorbed onto the inner membrane surface. With PE, the actin filaments are uniformly adsorbed onto the inner membrane surface between 2 and 12 mM Mg2+. With both PS and PE membranes, at Mg2+ concentrations higher than 12 mM, thick bundles are formed in the bulk water droplet accompanied by the dissolution of actin filaments from the membrane surface. The attraction between actin filaments and membrane is attributable to an increase in the translational entropy of counterions accompanied by the adsorption of actin filaments onto the membrane surface. These results suggest that a microscopic water droplet coated with phospholipid can serve as an easy-to-handle model of cell membranes.     

In-situ Al0.24Ga0.24In0.52As surface cleaning procedure using hydrogen radicals for molecular beam epitaxy regrowth

In-situ hydrogen radical treatment in a processing chamber attached to a molecular beam epitaxy (MBE) system is successfully applied for the native oxide removal from AlGaInAs surfaces exposed to air prior to MBE regrowth. The influence of cleaning conditions is studied on regrown GaInAs layers and AlGaInAs/GaInAs single quantum well (SQW) structures. At optimum cleaning conditions no deterioration of the quality of the treated material appears. In addition, reduction of the interface carbon concentration is achievable.