Flexible attosecond beamline for high harmonic spectroscopy and XUV/near-IR pump probe experiments requiring long acquisition times.

We describe the versatile features of the attosecond beamline recently installed at CEA-Saclay on the PLFA kHz laser. It combines a fine and very complete set of diagnostics enabling high harmonic spectroscopy (HHS) through the advanced characterization of the amplitude, phase, and polarization of the harmonic emission. It also allows a variety of photo-ionization experiments using magnetic bottle and COLTRIMS (COLd Target Recoil Ion Momentum Microscopy) electron spectrometers that may be used simultaneously, thanks to a two-foci configuration. Using both passive and active stabilization, special care was paid to the long term stability of the system to allow, using both experimental approaches, time resolved studies with attosecond precision, typically over several hours of acquisition times. As an illustration, applications to multi-orbital HHS and electron-ion coincidence time resolved spectroscopy are presented.

A zero dead-time multi-particle time and position sensitive detector based on correlation between brightness and amplitude.

A new multi-particle time and position sensitive detector using only a set of microchannel plates, a waveform digitizer, a phosphor screen, and a CMOS camera is described. The assignment of the timing information, as taken from the microchannel plates by fast digitizing, to the positions, as recorded by the camera, is based on the COrrelation between the BRightness of the phosphor screen spots, defined as their integrated intensity and the Amplitude of the electrical signals (COBRA). Tests performed by observing the dissociation of HeH, the fragmentation of H3 into two or three fragments, and the photo-double-ionization of Xenon atoms are presented, which illustrate the performances of the COBRA detection scheme.

Theory of dynamical electron channeling contrast images of near-surface crystal defects.

This paper describes the dynamical simulation of electron channeling contrast images (ECCIs) of dislocations. The approach utilizes both the Bloch wave formalism and the scattering matrix formalism to generate electron channeling patterns (ECPs). The latter formalism is then adapted to include the effect of lattice defects on the back-scattered electron yield, resulting in a computational algorithm for the simulation of ECCIs. Dislocations of known line direction and Burgers vector are imaged experimentally by ECCI and match well with simulated ECCIs for various channeling conditions. Experiment/simulation comparisons for ECPs and ECCIs are demonstrated for metals (Al), semiconductors (Si), and ceramics (SrTiO3).

The role of Rydberg states in photoionization of NO2 and (NO+, O-) ion pair formation induced by one VUV photon.

We report a combined experimental and theoretical study of photoionization (PI) of the NO2 molecule into the NO2(+) (X (1)Σg(+)) ground state and the photodissociation of NO2 into the NO(+)((1)Σ(+)) + O(-)((2)P) ion pair. These processes were induced by 10.9 eV-13 eV synchrotron radiation and the products were detected using electron-ion or O(-)-NO(+) coincident momentum spectroscopy. The results demonstrate the strong influence of [R(∗)(4b2)(-1), nlα(i), v2(')] Rydberg states vibrationally resolved in the v2(') bending modes for both processes. In particular, we emphasize two regions around 11.5 eV and 12.5 eV that were studied in more detail for their relevance to 400 nm multiphoton ionization induced by femtosecond pulses. The photoelectron energy spectra and asymmetry parameters support the existence of two PI mechanisms, as probed with the help of fixed-nuclei frozen-core Hartree-Fock calculations. We found significant deviations from Franck-Condon ionization predictions which may be assigned to vibronic coupling of NO2(∗) states such as that induced by a conical intersection. The limited agreement between theory and experiment, even for the non-resonant processes, indicates the need for calculations that go beyond the approximations used in the current study. Ion pair formation leads to strong vibrational and rotational excitation of the NO(+)((1)Σ(+),v) product, with an ion fragment angular anisotropy depending on both the v2(') bending quantum number of the excited parent molecule and the v vibrational level of the fragment.

Influence of gold coating and interplate voltage on the performance of chevron micro-channel plates for temporally and spatially resolved single particle detection.

We present a study of two different sets of Micro-Channel Plates used for time and space resolved single particle detection. We investigate the effects of the gold coating and that of introducing an interplate voltage between the spatially separated plates. We find that the gold coating increases the count rate of the detector and the pulse amplitude as previously reported for non-spatially resolved setups. The interplate voltage also increases count rates. In addition, we find that a non-zero interplate voltage improves the spatial accuracy in determining the arrival position of incoming single particles (by ~20%) while the gold coating has a negative effect (by ~30%).

Circular dichroism in photoionization of H2.

Circular dichroism is a consequence of chirality. However, nonchiral molecules can also exhibit it when the measurement itself introduces chirality, e.g., when measuring molecular-frame photoelectron angular distributions. The few such experiments performed on homonuclear diatomic molecules show that, as expected, circular dichroism vanishes when the molecular-frame photoelectron angular distributions are integrated over the polar electron emission angle. Here we show that this is not the case in resonant dissociative ionization of H2 for photons of 30-35 eV, which is the consequence of the delayed ionization from molecular doubly excited states into ionic states of different inversion symmetry.

Ion pair formation in multiphoton excitation of NO(2) using linearly and circularly polarized femtosecond light pulses: kinetic energy distribution and fragment recoil anisotropy.

The NO(2) ion pair photodissociation dynamics leading to NO(+)(X(1)Sigma(+),v) + O(-)((2)P(3/2) or (2)P(1/2)), induced by a 1 kHz femtosecond laser with wavelengths near 400 nm, has been characterized using the coincidence vector correlation method. The ion pair production after four-photon absorption reaches more than 15% of the primary ionization. The kinetic energy release of the fragments demonstrates a significant vibrational excitation of the NO(+)(X(1)Sigma(+),v) molecular fragment. Recoil ion fragment emission is strongly aligned along the polarization axis of linearly polarized light or preferentially emitted in the plane perpendicular to the propagation axis of circularly polarized light. The formalism describing the recoil anisotropy for bound-to-bound n-photon transition inducing prompt axial recoil dissociation of a nonlinear molecule has been developed to interpret the measured anisotropies in terms of excitation pathways via near-resonant intermediate states of specific symmetries. Possible reaction pathways are discussed that are consistent with the data and supported by calculations of potential energy surfaces and transition moments.

Cathodoluminescence studies of the inhomogeneities in Sn-doped Ga2O3 nanowires.

Cathodoluminescence real-color imaging and spectroscopy were employed to study the properties of Ga(2)O(3) nanowires grown with different Sn/Ga ratios. The structures grown under Sn-rich conditions show large spectral emission variation, ranging from blue to red, with a green transition zone. Spectral emission changes correlate with changes in the chemical composition and structure found by energy dispersive spectroscopy and electron diffraction. A sharp transition from green to red emission correlates with a phase transition of beta-Ga(2)O(3) to polycrystalline SnO(2). The origin of the green emission band is discussed based on ab initio calculation results.

Photodissociation dynamics of Ar2(+) and Ar3(+) excited by 527 nm photons.

The photofragmentation dynamics of Ar(2)(+) and Ar(3)(+) clusters has been investigated at a 527 nm wavelength (2.35 eV) using a setup that allows simultaneous detection of the ionic and neutral fragments in a coincidence experiment. Measurement of positions and times of flight enables in principle a complete description of the fragmentation dynamics. The photofragmentation dynamics of Ar(3)(+) clusters is similar to that of Ar(2)(+) with, in addition, the ejection of a third fragment that can be neutral or ionized via a resonant electron capture. This is attributed to the triangular geometry of the Ar(3)(+) ion.

Polyphasic characterization of Xanthomonas axonopodis pv. allii associated with outbreaks of bacterial blight on three Allium species in the Mascarene archipelago.

Based on the number of new reports during the last two decades, bacterial blight of onion (Allium cepa) is considered an emerging disease. The causal agent, Xanthomonas axonopodis pv. allii, is pathogenic to several Allium species after inoculation, but outbreaks worldwide have been primarily reported on onion. We describe a unique epidemiological situation in Réunion Island, France, with concomitant outbreaks on three Allium species, onion, leek (A. porrum), and garlic (A. sativum). There was no host specialization within Allium spp. among strains associated with the three host species. Based on amplified fragment length polymorphism (AFLP) and restriction fragment length polymorphism, strains associated with these outbreaks in Réunion Island were highly related genetically to strains isolated from diseased plant samples and contaminated seed lots in the neighboring island of Mauritius, where the disease has occurred since 1984. All AFLP haplotypes were identified as X. axonopodis pv. allii based on polymerase chain reaction analysis using specific primers, biochemical tests, and/or pathogenicity tests. Two genetically related groups of strains (A and B) that can be distinguished by AFLP, differential utilization of three carbon sources, and xanthomonadin pigment production were detected initially after establishment of the pathogen. In less than 10 years after the establishment of the pathogen there was nearly an extinction of group A strains in Réunion Island, suggesting differences in fitness between strains in the two groups.

Characterization of neutral fragments issued from the photodissociation of protonated tryptophane.

New information on the photo-fragmentation of biomolecules is obtained from the detection of neutral and ionic fragments using a time and position resolved coincidence technique that reveals whether an ionic photofragment is associated with one or more neutral fragments. In the case of a sequential dissociation, both fragmentation channels are identified as well as their time ordering.

Dissociative charge transfer and collision induced dissociation of Ar2+ and Ar3+ clusters in collisions with argon atoms at keV energies.

The dynamics of dissociative charge transfer and collision induced dissociation of Ar(2) (+) and Ar(3) (+) clusters colliding with Ar atoms at 4.8 keV has been investigated using a novel multifragment detection scheme that maps the postcollision vectors of all particles simultaneously. Estimation of internal energies and measurement of pre- and postcollision vectors enables a full description of reaction dynamics. The prominence of electronic excitation in defining the dynamics of these collision systems is demonstrated. The dissociation dynamics of Ar(3) (+) clusters is distinctly different from that of Ar(2) (+). This is attributed to a combination of lower internal energies and predominantly triangular T-shape structure of the Ar(3) (+) ion.

Quantitative and molecular epidemiology of bacterial blight of onion in seed production fields.

ABSTRACT Onion, a biennial plant species, is threatened by the emerging, seed-borne, and seed-transmitted Xanthomonas axonopodis pv. allii. Bacterial blight epidemics were monitored in seed production fields over two seasons. Temporal disease progress was different between the two seasons, with final incidence ranging from 0.04 to 0.06 in 2003 and from 0.44 to 0.61 in 2004. The number of hours with temperatures above 24 degrees C was the best descriptor for predicting the number of days after inoculation for bacterial blight development on inoculated plants. Fitting the beta-binomial distribution and binary power law analysis indicated aggregated patterns of disease incidence data. The beta-binomial distribution was superior to the binomial distribution for 97% of the examined data sets. Spatial dependency ranged from 5.9 to 15.2 m, as determined by semivariance analysis. Based on amplified fragment length polymorphism (AFLP) analysis, it was concluded that plots predominantly were infected by the inoculated haplotype. A single other haplotype was identified by AFLP in all plots over the 2 years, and its detection in the field always followed wind-driven rains. X. axonopodis pv. allii-contaminated seed were detected by semiselective isolation and a nested polymerase chain reaction assay at levels up to 0.05% when final disease incidence was 0.61. Contaminated seed originated from both diseased and asymptomatic plants.

Lifetime and yield of metastable Ar2(+) ions.

Ar2(+) ions produced in a cooled supersonic expansion by electron-impact ionization are accelerated at 2.5 keV and kept during few milliseconds inside a linear electrostatic trap. The lifetime of the metastable Ar2(+) ion is determined from the measurement of the rate of the argon atoms escaping the trap. The lifetime and the relative metastable populations are measured as a function of the pressure and temperature in the supersonic expansion, i.e., of the mean cluster size. Possible mechanisms responsible for the metastable formation are discussed.

A novel mutation in the anti-müllerian hormone gene as cause of persistent müllerian duct syndrome.

UNLABELLED: Persistent müllerian duct syndrome is a relatively rare inherited defect of sexual differentiation characterised by failure of regression of the müllerian ducts in males. In affected individuals, uterus and tubes are present because of defects of synthesis or action of anti-müllerian hormone (AMH), normally produced by the Sertoli cells of the testis. Patients are normally virilised, although mono- or bilateral cryptorchidism may be present. We observed two brothers (chromosomes 46 XY), aged 11 years and 2 months and 8 years and 3 months respectively, with bilateral cryptorchidism. The diagnosis of persistent müllerian duct syndrome was made on the basis of laparoscopic evidence of uterus and tubes, undetectable plasma levels of AMH and a 23 base pair duplicative insertion in exon 5 of the AMH gene, causing the introduction of a premature stop codon, homozygous in the two brothers. The surgical correction of the genital abnormalities was successfully carried out by laparoscopic orchidopexy according to Fowler-Stephens. CONCLUSION: Persistent müllerian duct syndrome should be taken into consideration in all cases of bilateral cryptorchidism. Laparoscopy is the elective procedure for diagnosis of this disease and laparoscopic surgery for orchidopexy of intra-abdominal testes. Mutation analysis of the anti-müllerian hormone gene in these patients helps to understand the structure-function relationship of the anti-müllerian hormone protein, although it is not clear at present whether anti-müllerian hormone is necessary to maintain normal testicular function.

Motion correction of PET images using multiple acquisition frames.

Positron emission tomography (PET) is a relatively lengthy brain imaging method. Because it is difficult for the subject to stay still during the data acquisition, head motion during scans is a source of image degradation. A simple data acquisition technique to reduce the effect of this problem is described. The technique associates the incoming data with the real-space position of the head. During the PET scan, the head position is constantly monitored with two video cameras and compared to its initial position. Every time the displacement for a region within the field of view (FOV) is larger than a specified threshold displacement, the PET data acquisition system starts to save the PET data in a new frame. The total number of frames required for a complete study depends on the magnitude of the head motion during the study and on the threshold displacement. At the end of the study, all the acquired frames are reconstructed independently and each image is rotated and translated to coincide with the initial position. When these images are summed, they produce a final image with fewer motion artefacts.

PETSIM: Monte Carlo simulation of all sensitivity and resolution parameters of cylindrical positron imaging systems.

Monte Carlo simulation techniques are applied to track the annihilation photons from positron decay, and store the photon histories. Reasonably realistic models of the isotope distribution in the brain and heart during typical PET studies, as well as the traditional phantoms used for measuring PET scanner performance can be built out of up to 10 hollow or solid cylinders. Separate programs model the source distribution and its attenuation characteristics, the collimators and the detectors. These modules are connected by compact gamma history files which are stored on disc or tape. Over 50 million gamma ray histories can be saved on a 1 Gbyte disc, representing the decay of several billion atoms. This allows for good precision even for single thin slices in scanners with wide axial acceptance. The simulation results include spectrum analysis, sensitivity to true coincident events, scattered coincident and single rays, and the effects on these parameters of detector dead time. The storage of intermediate results on tape reduces simulation time, since most common source geometries need be generated only once. The sensitivities in multi-slice systems are presented as matrices of coincident crystal planes. The matrix shows the true count sensitivity and the scatter fraction together for each valid combination of planes. This presentation is very useful for assessing the effects of various degrees of inter-plane collimation. The spatial resolution analysis includes the effects of positron range, non-collinearity of the gamma rays, multiple interaction within the detectors, and the effects of quantization into single crystals in multiple-crystal block detectors. Each of these effects can be turned on or off without repeating the simulation. Both in-plane and axial resolutions are calculated as a function of location of the positron-emitting nucleus and the angle of incidence of gamma rays on the crystals. Single crystals, blocks and crystals with depth of interaction encoding can be specified, as can the method of backprojection (planar, or 3D), so that the detector geometry can be optimized.