Polarization memory in single quantum dots

We measured the polarization memory of excitonic and biexcitonic optical transitions from single quantum dots at either positive, negative or neutral charge states. Positive, negative and no circular or linear polarization memory was observed for various spectral lines, under the same quasi-resonant excitation below the wetting layer bandgap. We developed a model which explains both qualitatively and quantitatively the experimentally measured polarization spectrum for all these optical transitions. We consider quite generally the loss of spin orientation of the photogenerated electron–hole pair during their relaxation towards the many-carrier ground states. Our analysis unambiguously demonstrates that while electrons maintain their initial spin polarization to a large degree, holes completely dephase.     

Ultrafast nonadiabatic photodissociation dynamics of F2 in solid Ar

We explore the ultrafast spin-flip dynamics in a diatomic molecule imbedded in a rare gas matrix using the combination of a quantum mechanical and a semiclassical surface hopping method. Specifically, we investigate (1) the extent to which the phenomenon of electronically-localized eigenstates in strongly-coupled manifolds survives in the presence of rapid decay and a multitude of electronically coupled states; (2) the ability of the surface hopping method to predict the short time dynamics; and (3) the time range over which frozen lattice models are valid. Our results point to the active role played by a large number of coupled electronic states in the F₂/Ar dynamics while substantiating our confidence in the validity of the popular surface hopping approach for the system considered.     

Preparation of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles for superthermites by the detonation of an explosive nanocomposite material

This article reports on the preparation of chromium(III) oxide nanoparticles by detonation. For this purpose, a high explosive—hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)—has been solidified from a solution infiltrated into the macro- and mesoporosity of Cr₂O₃ powder obtained by the combustion of ammonium dichromate. The resulting Cr₂O₃/RDX nanocomposite material was embedded in a cylindrical charge of pure explosive and detonated in order to fragment the metallic oxide into nanoparticles. The resulting soot contains Cr₂O₃ nanoparticles, nanodiamonds, amorphous carbon species and inorganic particles resulting from the erosion by the blast of the detonation tank wall. The purification process consists in (i) removing the carbonaceous species by an oxidative treatment at 500 °C and (ii) dissolving the mineral particles by a chemical treatment with hydrofluoric acid. Contrary to what could be expected, the Cr₂O₃ particles formed during the detonation are twice larger than those of initial Cr₂O₃. The detonation causes the fragmentation of the porous oxide and the melting of resulting particles. Nanometric droplets of molten Cr₂O₃ are ejected and quenched by the water in which the charge is fired. Despite their larger size, the Cr₂O₃ nanoparticles prepared by detonation were found to be less aggregated than those of the initial oxide used as precursor. Finally, the Cr₂O₃ synthesized by detonation was used to prepare a superthermite with aluminium nanoparticles. This material possesses a lower sensitivity and a more regular combustion compared to the one made of initial Cr₂O₃.     

Proton transfer and dissociation of GlyLysH+ following O-H and N-H stretching mode excitations: dynamics simulations

Proton transfer and dissociation processes following excitation of the OH or NH stretching modes of the proton-bound complex GlyLysH(+) are studied by classical trajectories. "On the fly" simulations with the PM3 semiempirical electronic structure method for the potential surface are used. Initial conditions are sampled to correspond to the v=1 excited state of the OH or NH stretching modes. Five different conformers of the complex are studied as initial structures. The main findings are (1) Photoinduced proton transfer is on the picosecond time scale. (2) Proton transfer is much faster than the processes of dissociation. (3) Proton transfer involves different sites. Most trajectories show sequences of two proton transfer events. (4) The proton transfer events show high selectivity with regard to the initially excited vibration and the initial structure. (5) Photodissociation of the complex occurs on a typical time scale of 100 ps. (6) Conformational transitions are found to be often faster than proton transfer. These results have implications for the mass spectrometry of complexes, for dynamics of proton wires, and for proton migration in proteins.     

The relative contribution of adsorption to the overall sorption and transport of small molecules in amber

In 1987, Berner and Landis reported that upon vacuum grinding of 80 million year old amber, a gas mixture was released which suggested an oxygen-rich prehistoric environment. Fundamental to their argument was the assumption that amber, an organic glass formed during the fossilization of tree sap, is a perfect sealant. Their assumption was challenged by three technical comments which collectively concluded that gases diffuse readily through amber. In order to defend their key assumption that gases are perfectly trapped in amber, Berner and Landis dismissed the data obtained through gravimetric sorption experiments as only a measure of surface adsorption rather than bulk absorption in and concomitant diffusion through the amber matrix. The validity of interpreting these gravimetric experiments as a measure of bulk diffusion is demonstrated by exploring the physical basis for interpreting gravimetric sorption data. Most importantly, new experimental gravimetric sorption data are presented which demonstrate an explicit separation of adsorption from diffusion-controlled absorption and also reveal that adsorption accounts for a very small fraction of the total sorption in amber.     

Synthesis and Chemistry of Bicyclo[4.1.0]hept-1,6-ene

Bicyclo[4.1.0]hept-1,6-ene has been generated by elimination of 1-chloro-2-(trimethysilyl)bicyclo[4.1.0]heptane in the gas phase over solid fluoride at 25 degrees C. The cyclopropene dimerizes by a rapid ene reaction forming two diastereomeric cyclopropenes. In tetrahydrofuran or chloroform the ene dimers couple to form a single crystalline triene tetramer, whereas a mixture of tricyclohexane tetramers is formed when the neat dimers are allowed to warm to room temperature. Oxidation by dimethyldioxirane or dioxygen gives carbonyl products. Quantum mechanical calculations yielded an increase in strain of approximately 17 kcal/mol over that for 1,2-dimethylcyclopropene. The potential enegy barrier to flexing (folding) along the fused double bond of bicyclo[4.1.0]hept-1,6-ene is only approximately 1 kcal/mol at the highest level of theory investigated.     

Inclusion of exact exchange for self-interaction corrected H3 density functional potential energy surface

The effect of the inclusion of the exact exchange into self-interaction corrected generalized gradient approximation density functional theory (GGA-DFT) for the simplest hydrogen abstraction reaction, H + H₂ → H₃ → H₂ + H, is presented using a triple-zeta augmented 6-311++G(d,3pd) basis set. The introduction of the self-interaction correction has a considerably larger effect on molecular geometry and vibrational frequencies than the inclusion of the exact exchange. We investigate the influence of the self-interaction error on the shape of the potential energy surface around the transition state of the hydrogen abstraction reaction. The decomposition of the self-interaction error into correlation and exchange parts shows that the exchange self-interaction error is the main component of the energy barrier error. The best agreements with the experimental barrier height were achieved by self-interaction corrected B3LYP, B-LYP and B3PW functionals with errors of 1.5, 2.9 and 3.0 kcal/mol, respectively. Received: 13 August 1997 / Accepted: 14 November 1997     

Beam profile measurements and simulations for ultrasonic transducers operating in air

This paper outlines a method that has been implemented to predict and measure the acoustic radiation generated by ultrasonic transducers operating into air in continuous wave mode. Commencing with both arbitrary surface displacement data and radiating aperture, the transmitted pressure beam profile is obtained and includes simulation of propagation channel attenuation and where necessary, the directional response of any ultrasonic receiver. The surface displacement data may be derived directly, from laser measurement of the vibrating surface, or indirectly, from finite element modeling of the transducer configuration. To validate the approach and to provide experimental measurement of transducer beam profiles, a vibration-free, draft-proof scanning system that has been installed within an environmentally controlled laboratory is described. A comparison of experimental and simulated results for piezoelectric composite, piezoelectric polymer, and electrostatic transducers is then presented to demonstrate some quite different airborne ultrasonic beam-profile characteristics. Good agreement between theory and experiment is obtained. The results are compared with those expected from a classical aperture diffraction approach and the reasons for any significant differences are explained.