Collision mechanisms in one-electron capture by slow H and He- like ions of C, N and O in H and H-2

Translational energy spectroscopy (TES) has been used to study state-selective one-electron capture by H and He-like ions of C, N and O in both H and H-2 within the range 250-900 eV amu(- 1). The main collision mechanisms leading to state-selective electron capture have been identified, their relative importance assessed and compared, where possible, with theoretical predictions and with any previous measurements based on photon emission spectroscopy. For one-electron capture in H-2, the relative importance of contributions from non- dissociative and dissociative capture as well as from two- electron capture into autoionizing states is found to be strikingly different for the cases considered. Our TES measurements in atomic hydrogen provide an important extension of previous measurements to energies below 1000 eV amu(-1) and show that, as the impact energy decreases, electron capture becomes more selective until only a single n product channel is significant. These product main channels are well described by reaction windows calculated using a Landau-Zener approach. However, the same approach applied to the more complex energy- change spectra observed in H-2 is found to be less successful.

State-selective one-electron capture by slow state-prepared N2+(P-2) ground-state ions in collisions with hydrogen atoms

The technique of double translational energy spectroscopy.(DTES), recently successfully developed in this laboratory for use with targets of atomic hydrogen, has been used to study one-electron capture by ground-state N2+(2s22p)(2)p(0) ions in collisions with hydrogen atoms at energies within the range 0.8-6.0 keV. Cross sections for the formation of the main excited product channels have been determined. The measurements allow a re-evaluation of our previous TES measurements carried out with N2+ primary beams containing an admixture of metastable N2+(2s2p2)(4)p ions. The main findings of these earlier measurements are confirmed and the DTES measurements now remove any ambiguity in interpretation of the experimental data. While recent theoretical studies correctly predict the two main N+ D-3(0) and P-3(0) product channels, the quantitative agreement with experiment is only partially satisfactory.

Experimental investigation of the processes determining x-ray emission intensities from charge-exchange collisions

Absolute cross sections have been measured for single and double charge exchange and x-ray line emission for highly charged ions of C, N, 0, and Ne colliding with He, H-2 CO2, and H2O at collisions energies of 7q keV. Present results of charge exchange in He and H-2 compare favorably with previous results. For CO2 and H2O, where prior work is scarce, the classical overbarrier model is found to overestimate results by up to a factor of 3. An analysis of the relative intensities of the observed Lyman x-ray transitions indicates that capture into l states is not statistical, as collision velocities are insufficient to populate the highest angular-momentum states. The importance of autoionization following multiple capture is highlighted, and enhanced radiative stabilization following double capture is observed and compared to other studies. Present results are also discussed in terms of mechanisms likely to generate x-ray emission in comets.

Molecular Plasmonics with Tunable Exciton-Plasmon Coupling Strength in J-Aggregate Hybridized Au Nanorod Assemblies

Controlling coherent electromagnetic interactions in molecular systems is a problem of both fundamental interest and important applicative potential in the development of photonic and opto-electronic devices. The strength of these interactions determines both the absorption and emission properties of molecules coupled to nanostructures, effectively governing the optical properties of such a composite metamaterial. Here we report on the observation of strong coupling between a plasmon supported by an assembly of oriented gold nanorods (ANR) and a molecular exciton. We show that the coupling is easily engineered and is deterministic as both spatial and spectral overlap between the plasmonic structure and molecular aggregates are controlled. We think that these results in conjunction with the flexible geometry of the ANR are of potential significance to the development of plasmonic molecular devices.

Identical extraction behavior and coordination of trivalent or hexavalent f-element cations using ionic liquid and molecular solvents

The extraction of both UO22+ and trivalent lanthanide and actinide ions (Am3+, Nd3+, Eu3+) by dialkylphosphoric or dialkylphosphinic acids from aqueous solutions into the ionic liquid, 1-decyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide has been studied and compared to extractions into dodecane. Radiotracer partitioning measurements show comparable patterns of distribution ratios for both the ionic liquid/aqueous and dodecane/aqueous systems, and the limiting slopes at low acidity indicate the partitioning of neutral complexes in both solvent systems. The metal ion coordination environment, elucidated from EXAFS and UV-visible spectroscopy measurements, is equivalent in the ionic liquid and dodecane solutions with coordination of the uranyl cation by two hydrogen-bonded extractant dimers, and of the trivalent cations by three extractant dimers. This is the first definitive report of a system where both the biphasic extraction equilibria and metal coordination environment are the same in an ionic liquid and a molecular organic solvent.

Galactic Edge Clouds I. Molecular Line Observations and Chemical Modelling of Edge Cloud 2

Edge Cloud 2 (EC2) is a molecular cloud, about 35 pc in size, with one of the largest galactocentric distances known to exist in the Milky Way. We present observations of a peak CO emission region in the cloud and use these to determine its physical characteristics. We calculate a gas temperature of 20 K and a density of n(H2)~10^4 cm-3. Based on our CO maps, we estimate the mass of EC2 at around 10^4 Msolar and continuum observations suggest a dust-to-gas mass ratio as low as 0.001. Chemical models have been developed to reproduce the abundances in EC2, and they indicate that heavy element abundances may be reduced by a factor of 5 relative to the solar neighborhood (similar to dwarf irregular galaxies and damped Lya systems), very low extinction (A_V <4 mag) due to a very low dust-to-gas mass ratio, an enhanced cosmic-ray ionization rate, and a higher UV field compared to local interstellar values. The reduced abundances may be attributed to the low level of star formation in this region and are probably also related to the continuing infall of primordial (or low-metallicity) halo gas since the Milky Way formed. Finally, we note that shocks from the old supernova remnant GSH 138-01-94 may have determined the morphology and dynamics of EC2.

Ultrafast coherent control and quantum encoding of molecular vibrations in D2+ using intense laser pulses

Intense, few-femtosecond pulse technology has enabled studies of the fastest vibrational relaxation processes. The hydrogen group vibrations can be imaged and manipulated using intense infrared pulses. Through numerical simulation, we demonstrate an example of ultrafast coherent control that could be effected with current experimental facilities, and observed using high-resolution time-of-flight spectroscopy. The proposal is a pump-probe-type technique to manipulate the D2+ ion with ultrashort pulse sequences. The simulations presented show that vibrational selection can be achieved through pulse delay. We find that the vibrational system can be purified to a two-level system thus realizing a vibrational qubit. A novel scheme for the selective transfer of population between these two levels, based on a Raman process and conditioned upon the delay time of a second control-pulse is outlined, and may enable quantum encoding with this system.

Intense-field dissociation dynamics of D2+ molecular ions using ultrafast laser pulses

The dynamics of dissociation of pre-ionized D2+ molecules using intense (10^12–10^15 W cm-2), ultrashort (50 fs), infrared (? = 790 nm) laser pulses are examined. Use of an intensity selective scan technique has allowed the deuterium energy spectrum to be measured over a broad range of intensity. It is found that the dominant emission shifts to lower energies as intensity is increased, in good agreement with corresponding wavepacket simulations. The results are consistent with an interpretation in terms of bond softening, which at high intensity (approximately >3 × 10^14 W cm-2) becomes dominated by dissociative ionization. Angular distribution measurements reveal the presence of slow molecular dissociation, an indication that vibrational trapping mechanisms occur in this molecule.

Energy analysis of hyperthermal hydrogen atoms generated through surface neutralisation of ions

Hydrogen ions (H+, H-2(+) and H-3(+)) are produced in a magnetically confined inductively coupled radio frequency plasma. Ions are accelerated in the plasma boundary sheath potential, of several hundred volts, in front of a biased metal electrode immersed in the plasma. Backscattered hyperthermal hydrogen atoms are investigated by optical emission spectroscopy and an energy-resolved mass spectrometer. Ionisation of fast neutrals through electron stripping of atoms in the plasma allows energy analysis of the resulting ions. Thereby, the energy distribution function of the hyperthermal atoms can be deduced. The energy spectra can be explained as a superposition of individual spectra of the various ion species. The measured spectra also shows contributions of negative ions created at the electrode surface. In addition to experimental measurements, simulations of the neutral flux of backscattered atoms are carried out.

Space and time resolved rotational state populations and gas temperatures in an inductively coupled hydrogen RF discharge

Gas temperature is of major importance in plasma based surface treatment, since the surface processes are strongly temperature sensitive. The spatial distribution of reactive species responsible for surface modification is also influenced by the gas temperature. Industrial applications of RF plasma reactors require a high degree of homogeneity of the plasma in contact with the substrate. Reliable measurements of spatially resolved gas temperatures are, therefore, of great importance. The gas temperature can be obtained, e.g. by optical emission spectroscopy (OES). Common methods of OES to obtain gas temperatures from analysis of rotational distributions in excited states do not include the population dynamics influenced by cascading processes from higher electronic states. A model was developed to evaluate this effect on the apparent rotational temperature that is observed. Phase resolved OES confirmed the validity of this model. It was found that cascading leads to higher apparent temperatures, but the deviation (similar or equal to 25 K) is relatively small and can be ignored in most cases. This analysis is applied to investigate axially and radially resolved temperature profiles in an inductively coupled hydrogen RF discharge.

The electrochemical reduction of hydrogen sulfide on platinum in several room temperature ionic liquids

The electrochemical reduction of I atm hydrogen sulfide gas (H2S) has been studied at a platinum microelectrode (10 mu m diameter) in five room temperature ionic liquids (RTILs): [C(2)mim][NTf2], [C(4)mpyrr][NTf2], [C(4)mim][OTf], [C(4)mim][NO3] and [C(4)mim]][PF6] (where [C(n)mim](+) = 1-alkyl-3-methylimidazolium, [NTf2](-) = bis(trifluoromethylsulfonyl)imide, [C(4)mpyrr](+) = N-butyl-N-methylpyrrolidinium, [OTf](-) = trifluoromethlysulfonate, [NO3](-) = nitrate, and [PF6](-) = hexafluorophosphate). In all five RTILs, a chemically irreversible reduction peak was observed on the reductive sweep, followed by one or two oxidative peaks on the reverse scan. The oxidation peaks were assigned to the oxidation of SH- and adsorbed hydrogen. In addition, a small reductive peak was observed prior to the large wave in [C(2)mim]][NTf2] only, which may be due to the reduction of a sulfur impurity in the gas. Potential-step chronoamperometry was carried out on the reduction peak of H2S, revealing diffusion coefficients of 3.2, 4.6, 2.4, 2.7, and 3.1 x 10(-11) m(2) s(-1) and solubilities of 529, 236, 537, 438, and 230 mM in [C(2)mim][NTf2], [C(4)mpyrr][NTf2], [C(4)mim][OTf], [C(4)mim][NO3], and [C(4)mim]][PF6], respectively. The solubilities of H2S in RTILs are much higher than those reported in conventional molecular solvents, suggesting that RTILs may be very favorable gas sensing media for H2S detection.