Molecular dynamics simulations of forced atomic mixing in crystalline binary alloys during plastic deformation at 100 K are performed. Nearly complete atomic mixing is observed in systems that have a large positive heat mixing and in systems with a large lattice mismatch. Only systems that contained a hard precipitate in a soft matrix do not mix. The amount of mixing is quantified by defining a mean square relative displacement of pairs of atoms, sigma(2)(R,t), that were initially separated by a distance R. Analysis of sigma(2)(R,t) and visual inspection of the displacement fields reveal that forced mixing results from dislocation glide, and that it resembles the forced mixing of a substance advected by a turbulent flow. Consideration of sigma(2)(R,t) also provides a rationalization of compositional self-organization during plastic deformation at higher temperatures.

Two designs for incorporating multiple solenoidal microcoils into a single probe head are presented to increase the throughput of high-resolution NMR. Through a combination of radio frequency switches and low-noise amplifiers, multiple NMR spectra can be acquired in the same time as a single spectrum from a conventional probe consisting of one coil. Since this method does not compromise sensitivity with regard to the individual microcoils, throughput increases linearly with the number of coils. Only one receiver is needed, and data acquisition parameters can be optimized for each sample. Specifically, a four-coil system has been implemented for proton NMR at 250 MHz using a wide-bore magnet, with an observe volume of 28 nL for each microcoil. Signal cross-contamination was approximately 0.2% between individual coils, and simultaneous one- and two-dimensional spectra have been obtained from samples of fructose, galactose, adenosine triphosphate, and chloroquine (7 nmol of each compound). A more compact two-coil configuration has also been designed for operation at 500 MHz, with observe volumes of 5 and 31 nL for the two coils. One- and two-dimensional spectra were acquired from samples of 1-butanol (55 nmol) and ethylbenzene (250 nmol).

The effect of pressure on both the steady-state intensity and the lifetime of poly[2-methoxy-5-(2′-ethylhexoxy)p-phenylenevinylene] (MEH-PPV) has been measured for the neat polymer and at various dilutions in polystyrene (PS) and poly(methyl methacrylate) (PMMA). At 1 atm the efficiency increases with increasing dilution, especially in PS blends, and new peaks appear in the blends at higher energy. The lower energy peaks decrease in efficiency with pressure. The behavior of the higher energy bands is more complex. The decay rate is very rapid in the neat polymer. In general the decays in the blends are multiexponential. The differences in behavior in PS and PMMA blends can be explained by the greater compatibility of MEH-PPV in PS. In PMMA the MEH-PPV apparently tends to curl up to minimize the area of contact with the medium, leading to dissipation of energy by long-range intramolecular electron transfer.

We report the pressure effect on one- and two-photon-excited fluorescence from three organic molecules dissolved in solid polymers. The molecules studied are 4-(p-nitrophenyl)-3,4-dihydropyrazo[c]benzo[b]morpholine (NDPB), 1-phenyl-3-nitrophenylpyrazoline (PNP), and bis[4-(dimethylaminophenyl)]methylide ammonium chlorideAuramine O (AO). All these molecules exhibit strong fluorescence when subjected to visible or infrared laser light. We determine the pressure dependencies of fluorescence intensity, as well as energy and lifetime of the emitting state for one- and two-photon excitation. The pressure dependence of the last two parameters reveals that the fluorescence, for all molecules, originates in the same state regardless of the mode of excitation. In contrast to this, the emission intensity may change with pressure differently for one- and two-photon excitation. We introduce a parameter defined as a ratio of the emission intensity following two-photon excitation to the emission intensity following one-photon excitation (( (p)/ (p)). This parameter, with increasing pressure, shows almost no change for NDPB but a significant decrease for AO and PNP. Thus we postulate that absorption transitions may proceed for one- and two-photon excitation to the same state in NDPB but to different states in AO and PNP. Moreover, for AO and PNP, the two locally excited states for both modes of excitation may relax through different pathways to the same emitting state. In the case of NDPB and PNP, a large Stokes shift indicates that the emitting state has a distinctly different charge distribution than the initially excited state and that this distribution is strongly pressure dependent.

Abstract. Iodotrifluoromethane (CF 3 I) has been considered to be a candidate replacement for bromotrifluoromethane (CF 3 Br), which is used in aircraft for fuel inerting and for fire fighting. In this study, the chemical effects of aircraftreleased CF 3 I on atmospheric ozone were examined with the University of Illinois at Urbana-Champaign two-dimensional chemical-radiative-transport (UIUC 2-D CRT) model. Using an earlier estimate of the aircraft emission profile for tank inerting in military aircraft, the resulting equivalent Ozone Depletion Potentials (ODPs) for CF 3 I were in the range of 0.07 to 0.25. As a sensitivity study, we also analyzed CF 3 I emissions associated with fuel inerting if it were to occur at lower altitudes using an alternative estimate. The model calculations of resulting effects on ozone for this case gave ODPs≤0.05. Furthermore, through interactions with the National Institute of Standards and Technology (NIST), we analyzed the potential effects on ozone resulting from using CF 3 I in fire fighting connected with engine nacelle and auxiliary power unit applications. The scenarios evaluated using the NIST estimate suggested that the ODPs obtained by assuming aircraft flights occurring in several different latitude regions of the Northern Hemisphere are extremely low. According to the model calculation, the altitude where CF 3 I is released from aircraft is a dominant factor in its ozone depletion effects. On the assumption that the CF 3 I emission profile is representative of actual release characteristics, aircraftreleased CF 3 I has much lower impacts than CF 3 Br.