CROSS-SECTIONS FOR RESONANT TRANSFER AND EXCITATION IN FEQ++H-2 COLLISIONS

Resonant transfer and excitation (RTE) is investigated for Fe(q+) ions (q=23, 24, and 25) colliding with H2. For each charge state, cross sections for RTE were obtained from measurements of K x rays, emitted from the doubly excited intermediate state, coincident with single-electron capture by the incident ion. Additionally, for Fe25+ cross sections were obtained from measurements of coincidences between the two K x rays emitted from the intermediate state. These latter measurements Provide information on the lifetimes of intermediate metastable states formed in the RTE process. In all cases, measured cross sections are in good agreement with calculations based on theoretical cross sections for dielectronic recombination (DR). Since RTE closely approximates DR, the results indicate that dielectronic-recombination cross sections involving K-shell excitation can be accurately predicted for highly charged iron ions. The results for Fe25+ show that metastable states are sufficiently short lived to be observable in the RTE (or DR) process for these hydrogenlike ions.

Measurement of absolute charge-exchange cross sections for He2+ collisions with He and H-2

Reported are total, absolute charge-exchange cross sections for collisions of 3He(2+) ions with He and H-2. Measurements are reported at fixed energies between 0.33 and 4.67 keV/amu. Both the present results and earlier results of others are analyzed in terms of available experimental small-angle differential cross sections as a function of collision energy, and hence the geometry of the exit aperture of the gas-collision cells used by the various experimental groups. In addition, the effective length of gas-collision cells is studied using fluid dynamic and molecular flow simulations to address the density patterns near the cell entrance and exit apertures. When small acceptance-angle corrections were applied, the results of present and previous measurements for the single electron capture in these systems were brought into good accord in the relevant energy ranges. Taken in their entirety, the present data for 3He(2+) with He and H-2 lend themselves to new theoretical calculations of the multichannel charge-exchange cross sections.

Time-dependent R-matrix theory applied to two-photon double ionization of He

We introduce a time-dependent R-matrix theory generalized to describe double-ionization processes. The method is used to investigate two-photon double ionization of He by intense XUV laser radiation. We combine a detailed B-spline-based wave-function description in an extended inner region with a single-electron outer region containing channels representing both single ionization and double ionization. A comparison of wave-function densities for different box sizes demonstrates that the flow between the two regions is described with excellent accuracy. The obtained two-photon double-ionization cross sections are in excellent agreement with other cross sections available. Compared to calculations fully contained within a finite inner region, the present calculations can be propagated over the time it takes the slowest electron to reach the boundary.

Competition between memory-keeping and memory-erasing decoherence channels

We study the competing effects of simultaneous Markovian and non-Markovian decoherence mechanisms acting on a single spin. We show the existence of a threshold in the relative strength of such mechanisms above which the spin dynamics becomes fully Markovian, as revealed by the use of several non-Markovianity measures. We identify a measure-dependent nested structure of such thresholds, hinting at a causality relationship among the various non-Markovianity witnesses used in our analysis. Our considerations are then used to argue the unavoidably non-Markovian evolution of a single-electron quantum dot exposed to both intrinsic and Markovian technical noise, the latter of arbitrary strength. 

New source of MeV negative ion and neutral atom beams

The scenario of "electron-capture and -loss" was recently proposed for the formation of negative ion and neutral atom beams with MeV kinetic energies. However, it does not explain why the formation of negative ions in a liquid spray is much more efficient than with an isolated atom. The role of atomic excited states in the charge-exchange processes is considered, and it is shown that it cannot account for the observed phenomena. The processes are more complex than the single electron-capture and -loss approach. It is suggested that the shell effects in the electronic structure of the projectile ion and/or target atoms may influence the capture/loss probabilities.

Rydberg structure associated with core-excited autoionizing states of the LiH radical

We have investigated inner-shell excitation of the LiH + molecular ion by electron impact within several different collision models to delineate Rydberg autoionizing resonance structure associated with the LiH + (1σ2σ 2 2 Σ + ) core-excited threshold. The minimal representation requires only the retention of the 1σ and 2σ molecular orbitals, in which the core-excited state involves the promotion of a single electron into the 2σ orbital. This model is extended to include two further representations, in which both the 3σ and 4σ orbitals obtained from a self-consistent field calculation improve target representation, correlation and support additional autoionization channels. This affects the autoionization widths and to a lesser degree the positions of the LiH (1σ2σ 2 n s, n p 1,3 Σ + ) resonance series. Comparing our work with calculations on the counterpart atomic Be system assists in the assignment of the core-excited molecular resonance states. The results from our investigation provide helpful insights into the study of inner-shell transitions produced by electron or photon impact in more complex diatomic molecules.

Dielectronic recombination of W20 + (4d104f8): Addressing the half-open f shell

A recent measurement of the dielectronic recombination (DR) of W20+ [Schippers et al., Phys.Rev.A 83, 012711 (2011)] found an exceptionally large contribution from near-threshold resonances (1 eV). This still affected the Maxwellian rate coefficient at much higher temperatures. The experimental result was found to be higher by a factor of 4 or more than that currently in use in the 100- to 300-eV range, which is of relevance for modeling magnetic fusion plasmas. We have carried out DR calculations with AUTOSTRUCTURE which include all significant single-electron promotions. Our intermediate-coupling (IC) results are more than a factor of 4 larger than our LS-coupling ones at 1 eV but still lie a factor of 3 below experiment here. If we assume complete (chaotic)mixing of near-threshold autoionizing states, then our results come into agreement (to within 20%)with experiment below 2 eV. Our total IC Maxwellian rate coefficients are 50%–30% smaller than those based on experiment over 100–300 eV.

Single-shot characterisation of independent femtosecond extreme ultraviolet free electron andf infrared laser pulses

Two-color above threshold ionization of helium and xenon has been used to analyze the synchronization between individual pulses of the femtosecond extreme ultraviolet (XUV) free electron laser in Hamburg and an independent intense 120 fs mode-locked Ti:sapphire laser. Characteristic sidebands appear in the photoelectron spectra when the two pulses overlap spatially and temporally. The cross-correlation curve points to a 250 fs rms jitter between the two sources at the experiment. A more precise determination of the temporal fluctuation between the XUV and infrared pulses is obtained through the analysis of the single-shot sideband intensities. ©2007 American Institute of Physics

Imaging domains in BaTiO3 single crystal nanostructures: comparing information from transmission electron microscopy and piezo-force microscopy

This article compares and contrasts information
obtained, using transmission electron microscopy (TEM)
and piezo-force microscopy (PFM), on domain configurations
adopted in single crystal lamellae of BaTiO3, that had
been cut directly from bulk using a focused ion beam
microscope with top and bottom surfaces parallel to
{100}pseudocubic. Both forms of imaging reveal domain
walls parallel to {110}pseudocubic, consistent with sets of 90
domains with dipoles oriented parallel to the two
\001[pseudocubic directions in the plane of the lamellae.
However, the domain width was observed to be dramatically
larger using PFM than it was using TEM. This suggests
significant differences in the surface energy densities
that drive the domain formation in the first place, that could
relate to differences in the boundary conditions in the two
modes of imaging (TEM samples are imaged under high
vacuum, whereas PFM imaging was performed in air).
Attempts were made to map local dipole orientations
directly, using a form of ‘vector’ PFM. However, information
inferred was largely inconsistent with the known
crystallography of the samples, raising concern about the
levels of care needed for accurate interpretation of PFM
images.

Bright Subcycle Extreme Ultraviolet Bursts from a Single Dense Relativistic Electron Sheet

Double-foil targets separated by a low density plasma and irradiated by a petawatt-class laser are shown to be a copious source of coherent broadband radiation. Simulations show that a dense sheet of relativistic electrons is formed during the interaction of the laser with the tenuous plasma between the two foils. The coherent motion of the electron sheet as it transits the second foil results in strong broadband emission in the extreme ultraviolet, consistent with our experimental observations.

Electron-impact single ionization of Mg and Al+

Theory and experiment are compared for the electron-impact single ionization of Mg and Al+. Nonpertur- bative R matrix with pseudostates RMPS and time-dependent close-coupling TDCC calculations have been carried out that exhibit large reductions from perturbative distorted-wave results of 38% for Mg and 20% for Al+. Experimental single-ionization data available for Mg and Al+ are in reasonable accord with distorted-wave data and lie substantially above the new theoretical results. Rate coefficients, necessary for the collisional- radiative modeling of Mg and Al plasmas were generated from the RMPS ionization cross sections. In the collisional-ionization region near the ionization threshold, the resulting rates were found to be up to two times lower for Mg and three times lower for Al+ than the rates generated from experimental data.

Synthesis of N3- and 2-NH2-substituted 6,7-diphenylpterins and their use as intermediates for the preparation of oligonucleotide conjugates designed to target photooxidative damage on single-stranded DNA representing the bcr-abl chimeric gene

Two 17-mer oligodeoxynucleotide-5'-linked-(6,7-diphenylpterin) conjugates, 2 and 3, were prepared as photosensitisers for targeting photooxidative damage to a 34-mer DNA oligodeoxynucleotide (ODN) fragment 1 representing the chimeric bcr-abl gene that is implicated in the pathogenesis of chronic myeloid leukaemia (CML). The base sequence in the 17-mer was 3'G G T A G T T A T T C C T T C T T5'. In the first of these ODN conjugates (2) the pterin was attached at its N3 atom, via a -(CH2)3OPO(OH)- linker, to the 5'-OH group of the ODN. Conjugate 2 was prepared from 2-amino-3-(3-hydroxypropyl)-6,7-diphenyl-4(3H)-pteridinone 10, using phosphoramidite methodology. Starting material 10 was prepared from 5-amino-7-methylthiofurazano[3,4-d]pyrimidine 4 via an unusual highly resonance stabilised cation 8, incorporating the rare 2H,6H-pyrimido[6,1-b][1,3]oxazine ring system. In the characterisation of 10 two pteridine phosphazenes, 15 and 29, were obtained, as well as new products containing two uncommon tricyclic ring systems, namely pyrimido[2,1-b]pteridine (20 and 24) and pyrimido[1,2-c]pteridine (27). In the second ODN conjugate the linker was -(CH2)5CONH(CH2)6OPO(OH)- and was attached to the 2-amino group of the pterin. In the preparation of 3, the N-hydroxysuccinimide ester 37 of 2-(5-carboxypentylamino)-6,7-diphenyl-4(3H)-pteridinone was condensed with the hexylamino-modified 17-mer. Excitation of 36 with near UV light in the presence of the single-stranded target 34-mer, 5'T G A C C A T C A A T A A G14 G A A G18 A A G21 C C C T T C A G C G G C C3' 1 caused oxidative damage at guanine bases, leading to alkali-labile sites which were monitored by polyacrylamide gel electrophoresis. Cleavage was observed at all guanine sites with a marked preference for cleavage at G14. In contrast, excitation of ODN-pteridine conjugate 2 in the presence of 1 caused oxidation of the latter predominantly at G18, with a smaller extent of cleavage at G15 and G14 (in the double-stranded portion) and G21. These results contrast with our previous observation of specific cleavage at G21 with ruthenium polypyridyl sensitisers, and suggest that a different mechanism, probably one involving Type 1 photochemical electron transfer, is operative. Much lower yields were found with the ODN-pteridine conjugate 3, perhaps as a consequence of the longer linker between the ODN and the pteridine in this case.