Synopsis We present experimental cross section for electron impact single and double ionization of He(1s2s 3S) and for ionization of He−(1s2s2p 4P). The experiment has required the development of a novel source producing a fast, intense beam of He(1s2s 3S) with high purity, based upon the photodetachment of He−. The results for single ionization of He(1s2s 3S) solve a long-lasting discrepancy between theory and experiment, while the results for double ionization are the first determination of the cross section for these processes.

We present a combined experimental and theoretical study of the fragmentation of HDO2+ molecular ions produced by electron-impact ionization of HDO+ in the collision energy range 20-2500 eV. Experimental absolute partial inclusive cross sections for the production of OD+, OH+, and O+ are reported and compared successfully to theoretical predictions. Ab initio methods are used to calculate the electron-impact ionization cross sections of the cationic ground state and first excited state leading to the first seven dicationic states. Dissociation probabilities of each channel are obtained by performing classical molecular dynamics on fitted dicationic potential energy surfaces. The predictive character of the theoretical modeling allows us to estimate that the nonmeasured dissociation channel giving a neutral oxygen atom contributes to 30% of the total ionization cross section. The isotopic ratio OD+/OH+ deduced from the experiment is (3.1 +/- 0.2) on average, constant in the 30-2500 eV energy range. The calculated isotopic ratio is found to be strongly dependent on the vibrational excitation of the target. Good agreement with the experimental value is obtained for a vibrational excitation corresponding to a temperature of about 2500 K, which is compatible with typical characteristics of electron cyclotron resonance (ECR) ion sources.

Electron transmission through insulating Al2O3 nanocapillaries of different diameters (40 and 270 nm) and 15 mu m length has been investigated for low-energy electrons (2-120 eV). The total intensity of transmitted current weakly depends on the incident electron energy and tilt angle defined with respect to the capillary axis. On the other hand, the intensity of elastically transmitted electrons significantly varies with the alteration of electron energy and tilt angle. In addition, we measured an energy distribution of electrons transmitted both in the straightforward direction and at large tilt angle. The measured spectra show that inelastic processes dominate and, in particular, a large amount of low-energy electrons. These low-energy electrons can be either inelastically scattered projectiles or secondary electrons emitted within the capillaries. Furthermore, a change of the tilt angle appears to influence significantly only the intensity of the elastic transmission. The present results suggest a more complex nature of low-energy electron transport through insulating nanocapillaries than proposed for positive ions. Copyright (C) EPLA, 2009

Electron transmission through insulating Al2O3 nanocapillaries (diameter 140 nm and aspect ratio 110) has been investigated, for low incident energies from 2 to 120 eV. An energy dependence of the transmission function has been investigated both with and without an energy analysis of the electrons and differences are discussed. Significant intensities of transmitted electrons (without the energy analysis) were observed even at the lowest electron energies.

Our developments of the time-resolved laser-induced fluorescence (TR-LIF) detection system for biomolecules are presented. This system is based on the tunable (320 nm to 475 nm) Nd:YAG laser pulses used to excite various biomolecules. The detection part is the Streak System for Fluorescence Lifetime Spectroscopy (Hamamatsu, Japan). The system consists of a C4334-01 streakscope, as a detector, DG 535 digital pulse/delay generator, C5094-S Spectrograph and HPD-TA System, as a temporal analyzer. The TR-LIF spectrometer is designed primarily to study the temperature and pressure effects on fluorescence behavior of biomolecules upon excitation with a single nanosecond pulse. The design of this system has capability to combine laser-induced breakdown (LIB) with fluorescence, as well to study optodynamic behavior of fluorescence biomolecules.

A joint experimental and theoretical study of near-threshold electron-impact excitation of the 3(3)S and 3(1)S states in helium is reported. A high-resolution electron spectrometer is used to study integral cross sections in the energy region of the n = 3 - 5 negative-ion resonances. Photons are detected from the decay of the two states and the observed intensities are normalized to theoretical predictions using a new B-spline R-matrix (close-coupling) method that allows for non-orthogonal orbitals to improve the target description. Remarkable agreement between experiment and theory is demonstrated in both the overall energy dependence of the cross section and the fine details of a wealth of resonance structures. A detailed list of the resonances in the individual partial waves, along with their widths, is presented.

A joint experimental and theoretical study of near-threshold electron-impact excitation of the 33S and 31S states in helium is reported. A high-resolution electron spectrometer is used to study integral cross sections in the energy region of the n = 3 − 5 negative-ion resonances. Photons are detected from the decay of the two states and the observed intensities are normalized to theoretical predictions using a new B-spline R-matrix (close-coupling) method that allows for non-orthogonal orbitals to improve the target description. Remarkable agreement between experiment and theory is demonstrated in both the overall energy dependence of the cross section and the fine details of a wealth of resonance structures. A detailed list of the resonances in the individual partial waves, along with their widths, is presented.

The method of modulating an atom beam profile by an immaterial magnetic mask generated in a Stern–Gerlach interferometer is recalled. A special magnetic configuration aimed at producing a single central bright interference fringe (atomic spot) was used. The effects of velocity spread, source coherence and source size on the limiting spot size at large values of the magnetic gradient are discussed. The observation of such small sizes requires a high spatial resolution of the position-sensitive detector. A new electron optical device is described, which images the secondary electron source generated by the impact of the atomic beam on a metallic electrode (detection in real time). Magnifications as high as 65 are accessible, leading to a better than 100 nm resolution of the atomic beam profile when a position-sensitive detector of a few µm resolution is used. Geometric and chromatic aberrations are discussed and, according to simulations, they do not significantly deteriorate the resolution.

H-2 molecules were ionized by Ti:sapphire (45 fs, 800 nm) and Nd-doped yttrium aluminum garnet lasers (6 ns, 1064 nm). The relative populations of the vibrational levels of the H-2(+) ions were determined and found to be concentrated in the lowest vibrational levels. Tunneling ionization calculations with exact field-modified potential curves reproduce the experimental results. The reason for the departure from conventional Franck-Condon-like distributions is the rapid variation of the ionization rate with internuclear distance.

Absolute total cross sections have been measured for electron impact dissociative excitation and dissociative ionization of H-2(+) and D-2(+) in the energy range 5-3000 eV. The vibrational population of the primary H-2(+) beam has been analysed by dissociative charge exchange on a potassium target, and is in good agreement with the measurements of von Busch and Dunn (1972 Phys. Rev. A 5 1726). Kinetic energy release (KER) distributions have been extracted from momentum analysis of the released protons and deuterons at selected impact energies. A model calculation has been performed to interpret the different spectra. Below 100 eV, the distributions exhibit a sharp peak in the range 0-1 eV that is attributed to the dissociative excitation of high vibrational levels to the 2psigma(u) repulsive state in the vicinity of their outer turning point. This observation is consistent with the measured vibrational population extending up to upsilon = 13, as confirmed by the appearance threshold of the dissociative ionization (DI) channel. The KER distributions exhibit a second contribution peaking between 1 and 5 eV, resulting from the admixture of the (1ssigma(g) --> 2psigma(u)), (1ssigma(g) --> 2ppi(u)) and (1ssigma(g) --> 2ssigma(g)) electronic transitions. A distinctive hump is also present around 9 eV, that coincides both with the maximum of the DI contribution, and with the high-energy shoulder of the 2ppi(u) and 2ssigma(g) contributions. The present measurements are in qualitative agreement with the previous results of Caudano and Delfosse, and are fairly well reproduced by our first-order model.

We have studied the vibrational population of H-2(+) after photoionization of H-2 with a femtosecond laser. A first series of measurements was performed at the fundamental wavelength (800 nm) with both linear and circular polarization and for various intensities of the laser beam. The population obtained in each case differs from the classical Franck-Condon distribution by being shifted towards smaller vibrational numbers. In addition, we used the second harmonic of the laser (400 am) with linear polarization. In this case, it appears that we witness a transition from the tunneling to the multiphoton regime. At low intensity, almost only v' = 0 is observed.

The absolute cross section for associative ionization in H(1s) + H(3s) and D(1s) + D(3s) collisions has been measured, in the energy range 0.006-3.6 eV, in a merged beam apparatus. The 3s state is populated by exciting metastable atoms, with CW laser radiation, in a static field. For both isotopes, the cross section exhibits an E-1 behaviour at low energy, and a faster decrease above the ionization threshold of the 3s state. In the intermediate energy region, isotope-dependent oscillations of the cross section are observed, which we attribute to interferences between several reaction pathways.

The iodine molecule has been investigated by a threshold electron spectrometer in the incident electron energy range from 1.00 eV to 3.5eV. From the threshold electron spectrum contributions for excitation of three states known from optical spectrometry have been subtracted. The remaining signal has been interpreted by four curves similar in shape to those in absorption optical spectroscopy into antibonding energy states. The detected maxima have been attributed to the following valence excited states of the iodine molecule: 3?2u(2u), 3?0u-(0u-), 3?2g(2g), and 3?1g(1g), respectively. With the knowledge of the energy ranges within which the signal from a particular state appears, the predicted values of equilibrium distances of iodine atom nuclei and estimated values of vibrational transition frequency potential energy curves have been constructed, too.

We describe a new experiment dealing with the multiphoton ionization of metastable atomic hydrogen H(2s). The ion yield has been measured in a relative way but it provides absolute photoionization rates. In the present contribution, the wavelength is tuned in the vicinity of the 2s-3p transition and the laser intensity is in the 10(7)-5 x 10(10) W cm(-2) range. Experimental photoionization rates and resonance width are found to be in agreement with theoretical predictions.

Absolute total cross sections have been measured for the reaction H (2s)+H(2s) to H2++e- in the collision energy range 0.004-0.65 eV. Single beam measurements yield a cross section of 2.1*10-15 cm2 at an average collision energy of 4.1 meV. Relative measurements performed with merged beams indicate, together with the absolute measurements, an overall E-1 energy dependence.

New absolute cross section measurement for the reaction H- + e --> H+ + 3e- is reported for interaction energies from threshold to 2 keV by using the animated crossed beam method. The present results confirm the disagreement between the previous results of this laboratory and the results of Peart et al.

A low-energy crossed-beam electron spectrometer is used to investigate excitation of ammonia. Spectra obtained at low residual energies and large scattering angles in both constant residual energy mode and energy loss mode show excitation of two triplet states of ammonia, labelled a and b. A triplet b state is observed for the first time in the present work, while for the a state several lower, previously unobserved vibrational levels are detected. The threshold electron spectrum obtained by detecting zero residual energy electrons reveals a strong influence of threshold resonance in the excitation of he lowest Rydberg a state. Also a very large threshold peak in the excitation of the v=1 level of the symmetric stretch mode (v1) of the ground electronic state suggests a resonant process at very low energies.

The first absolute cross section measurements for single and double electron impact ionization of sodium-like Ar7+ are reported. The animated crossed beams method has been employed in the energy range from threshold to 3000 eV. The measured cross sections for single ionization are higher than the theoretical and semi-empirical predictions by about 20-50%. This discrepancy has been associated with the contribution of the indirect ionization processes. The double ionization cross section is only 1% of the single one.