ALBERT

Notes: The equilibrium between clusters and dopant in solution was studied on silicon on insulator specimens uniformly doped with As at concentrations CAs from 1 to 7.6×1020 cm−3. The values of the carrier density n* after equilibration at 700, 800, and 900 °C are reported. With increasing dopant concentration n* rapidly saturates to the limiting value of the carrier density ne, thus simulating a precipitation process. It is shown that the values of n* at different temperatures and dopant concentrations can be calculated by an equation derived in the Appendix by using a simple cluster model. The deactivation was analyzed by isothermal annealing of the specimens at temperatures in the range 550–800 °C. At high temperature the kinetics accurately complies with the rate equation −dn/dt=A{exp[−(E−αn)/kT]−(n0−n)/(n0−n*)exp[−(E−αn*)/kT]} which is the one reported in [D. Nobili, S. Solmi, M. Merli, and J. Shao, J. Electrochem. Soc. 146, 4246 (1999)] complemented by the second term on the right to account for the declustering process. Deviations leading to rates lower than predicted by the above equation are presented by the most heavily doped compositions after partial deactivation at temperatures ≤ 700 °C. The analysis of this phenomenon puts into evidence that clustering presents a limiting rate which only depends on temperature and carrier density, and is insensitive to As concentration. © 2001 American Institute of Physics.

Notes: The use of manipulative experiments to study directly the effects of climate change on natural and managed systems is expensive if performed on a large scale: it also relies upon accurate predictions of future climatic conditions. Predictive modelling is less expensive and permits a range of scenarios to be considered but this is only as reliable as the underpinning model.Predicting the suitability of England and Wales for the cultivation of forage maize under climate change was attempted using an established crop growth model by Muchow et al. (1990) integrated with climate data. The biological and climatic inputs to the model were the thermal requirements for the stages of crop growth, leaf number, leaf area, harvest index, the daily maximum and minimum air temperature, precipitation and solar radiation. These values were obtained from a daily meteorological database for ninety-three sites in England and Wales for the period 1951–80. Model outputs as point values of potential crop yield predicting current production -‘baseline’- were statistically validated using actual crop yield data collated from bibliographic analysis. The baseline results indicated that pans of the south-east appeared to be too dry and the north too cold. The model was run again using an artificial sensitivity test (temperature +2°C. precipitation ±10%). Increased precipitation led to a predicted increase in geographical suitability of the UK for forage maize production to the north and west. Under reduced precipitation there was a decrease in suitability in the south-east, possibly owing to moisture stress.

Notes: The reverse annealing phenomena and the nature of the As clusters were studied on silicon on insulator samples uniformly doped with As at concentrations up to 7.6×1020 cm−3. Carrier concentration and electron mobility were determined by Hall and resistivity measurements after annealing at temperatures in the range 550–800 °C. The amplitude of transient reactivation, which can involve up to 20% of the clustered As, depends on dopant concentration and on the annealing temperatures and times. The results of a detailed study of the influence of these parameters are reported. The occurrence of reverse annealing confirms the existence of different types of As clusters, whose distribution and features depend on the experimental conditions. The effect of clustered As on the electron mobility was measured and a quantitative relationship for this phenomenon is reported. The amplitude of this effect is in agreement with the view that at room temperature As clusters are electrically neutral. © 2000 American Institute of Physics.

Notes: Absolute state-selected total cross sections σv', v'=0 and 1, for the reaction N+2(X˜,v'=0,1) +Ar(1S0)→N2(X,v)+Ar+(2P3/2,1/2) [reaction (1)] over the center-of-mass collisional energy (Ec.m.) range of 1.2–140 eV have been measured using the photoionization mass spectrometric and radio frequency ion guide methods. These measurements, together with the relative values for σv', v'=0–2, and spin-orbit-state distributions of product Ar+ ions determined using the crossed ion-neutral beam photoionization apparatus, allow the determination of the absolute values for σ2 and partial state-to-state cross sections σv'→J, v'=0–2, for reaction (1). Absolute values for σv', v'=0–2, at Ec.m.=8 and 20 eV are in good agreement with those determined previously by the threshold photoelectron secondary ion coincidence method. Absolute values for σv'→J, v'=0–2, at Ec.m.=8 and 20 eV are also found to be in satisfactory accord with the predictions of the semiclassical multistate calculation which uses the ab initio potential energy surfaces of the [N2+Ar]+ system. Experimental state-to-state cross sections obtained in this study are consistent with those for the reaction Ar+(2P3/2)+N2(X,v=0)→Ar(1S0)+N+2 (X˜,v') from the consideration of microscopic reversibility.

Notes: The vibrational state distributions of N+2(X˜,v') ions resulting from the reactions, Ar+(2P3/2)+N2(X˜,v=0)→Ar(1S0) +N+2(X˜,v') [reaction (1)] and Ar+(2P1/2)+N2(X˜,v=0)→Ar(1S0) +N+2(X˜,v') [reaction (2)], over the center-of-mass collisional energy (Ec.m.) range of 0.25–41.2 eV in a crossed ion–neutral beam experiment have been probed by the charge exchange method. The experimental results obtained for reaction (1) are in accord with the predictions of the semiclassical multistate calculation of Spalburg and Gislason that N+2 ions are formed predominantly ((approximately-greater-than)85%) in the v'=1 state and that the production of N+2(X˜,v'=0) becomes more important as Ec.m. is increased. The experiment also supports the theoretical results for reaction (2) at Ec.m.=1.2 and 4.1 eV showing that (approximately-greater-than)80% of N+2 product ions are in the v'=2 state. However, the calculation is found to either over-estimate the populations for N+2(v'〈2) or underestimate the populations for N+2(v'〉2) resulting from reaction (2) at Ec.m.=10.3and 41.2 eV. Absolute spin-orbit-state-selected total cross sections for reactions (1) and (2), σ3/2 and σ1/2, respectively, at the Ec.m. range of 0.25–115.3 eV have also been measured using a tandem photoionization mass spectrometer which is equipped with a radio frequency (RF) octopole ion guide reaction gas cell. The measured values for σ3/2 at Ec.m.=4.1, 10.3, and 41.2 eV and σ1/2 at 41.2 eV are in reasonable agreement with the theoretical cross sections. However, the experimental values for σ3/2 at 1.2 eV and σ1/2 at 1.2, 4.1, and 10.3 eV are approximately a factor of 2 higher than the theoretical predictions. A model analysis, which takes into account possible collision-induced spin-orbit mixings of the reactant Ar+ states in the RF octopole gas cell, shows that the values for σ1/2/σ3/2 and σ1/2 determined using the ion beam–RF octopole gas cell arrangement can be strongly susceptible to gas cell pressure effects whereas the experimental values for σ3/2 are reliable. The values for σ1/2 deduced by multiplying the values for σ3/2 and the ratios σ1/2/σ3/2 determined in the crossed ion–neutral beam experiment are in agreement with the theoretical cross sections. Both σ3/2 and σ1/2 are found to increase as Ec.m. is increased from 41.2 eV. This observation is interpreted as due to the formation of N+2 in the A˜ 2Πu state at high Ec.m. . Combining the measured vibrational state distributions of product N+2(X˜,v') ions and the absolute state-selected total cross sections, absolute state-to-state total cross sections for reactions (1) and (2) at selected Ec.m. are determined.

Notes: A new ion–molecule rection apparatus, which consists of a photoionization source, a tandem mass spectrometer, and a radio frequency octopole reaction cell is described. Using a quadrupole mass filter to reject H+3 background ions formed at the photoionization source, absolute total cross sections for the reaction H+2 (v'0) +H2 (v'0 =0)→H+3 +H, have been measured as a function of the vibrational state of reactant H+2, where v'0 =0–4, over the center-of-mass collision energy (Ec.m.) range of 0.04–15 eV. The experimental results are compared with phenomenological cross sections obtained in previous single gas cell studies, the quasiclassical trajectory calculations of Stine and Muckerman, and the recent similar calculations of Eaker and Schatz. The absolute total cross sections measured for v0 =0 and 3 at Ec.m. =0.5, 1, 3, and 5 eV are found to be in agreement with "trajectory surface hopping'' calculations which include nonadiabatic surface hopping throughout the reaction.

Notes: The angular variation of the median destructive field (MDF) of magnetite particles has been determined by subjecting anisotropic samples carrying an isothermal remanent magnetization (IRM) to static AF demagnetization, with the AF at an angle to the IRM. The alignment was produced by dispersing the magnetite particles in resin in the presence of a magnetic field thus making the sample anisotropic. After tumble demagnetization, an IRM in 80 mT was given to each sample along the alignment axis. 11 different particle sizes were used covering the range 0.7–82 μm. The results for all the samples are remarkably similar in that the MDF is smallest when the AF is applied parallel to the alignment axis and greatest when the AF is perpendicular to the axis. The results are also very similar to those found experimentally for single-domain ‘tape’ particles and indicate that the reversal mechanism even in the smallest particles is not by coherent reversal as in the Stoner-Wohlfarth model. The angular variation of the MDF for all the particle sizes is described quite well by a function derived by Aharoni for single-domain particles. The MDF at θ= 0 is found to be inversely proportional to the square root of the particle size.