Ionization Of Rare Gas Atoms Research Articles

Femtosecond envelope of the high-harmonic emission from ablation plasmas

We characterize the temporal profile of the high-order harmonic emission from ablation plasma plumes using cross-correlations with the infrared (IR) laser beam provided by two-photon harmonic+IR ionization of rare gas atoms. We study both non-resonant plasmas (lead, gold and chrome) and resonant plasmas (indium and tin), i.e. plasmas presenting in the singly charged ions a strong radiative transition coinciding with a harmonic order. The cross-correlation traces are found to be very similar for all harmonic orders and all plasma targets. The recovered harmonic pulse durations are very similar to the driving laser, with a tendency towards being shorter, demonstrating that the emission is a directly laser-driven process even in the case of resonant harmonics. This provides a valuable input for theories describing resonant-harmonic emission and opens the perspective of a very high flux tabletop XUV source for applications.

Low-energy structure of above-threshold-ionization electron spectra: Role of the Coulomb threshold effect

Recent experimental observations of above-threshold ionization of rare gas atoms and diatomic molecules by midinfrared laser fields [C. I. Blaga et al., Nat. Phys. 5, 335 (2009); W. Quan et al., Phys. Rev. Lett. 103, 093001 (2009)] revealed a prominent maximum in the electron energy spectrum very close to the ionization threshold which is not reproduced by widely used Keldysh-Faisal-Reiss theories. We have performed fully ab initio theoretical analysis and precision calculations to explore the quantum origin of the low-energy structure (LES) observed in the experiments. Our study shows that an important role in shaping of LES is played by the effect of Coulomb attraction in the final electron state and the Coulomb threshold effect.

Electron impact double ionization of rare gases: A kinematical analysis of the non-first order effects at intermediate impact energy

The (e,3-1e) four-fold differential cross sections (4DCS) are measured for the electron impact double ionization of rare gas atoms (He, Ne, Ar) in coplanar asymmetric geometry for a wide range of ejected electron energies (5 to 144 eV) and at an incident energy of about 600 – 700 eV. Though we only show here a sample of the results, all experimental angular distributions of the 4DCS are characterized by large angular shifts of the forward and backward lobes with respect to the momentum transfer direction or its opposite, respectively. This validates our previously published results [Lahmam-Bennani et al 2002 J. Phys. B: At. Mol. Opt. Phys. 35 L59] which were questioned by Götz et al [2003 J. Phys. B: At. Mol. Opt. Phys. 36 L77]. A qualitative, kinematical analysis is given which allows relating these shifts and the observed structures (or sub-lobes) in the cross section distributions to the second order, 'two-step 2' double ionization mechanism, which is shown to predominate over the first-order 'shake-off' and 'two-step 1' mechanisms under the present kinematics.

Enhancements of rescattered electron yields in above-threshold ionization of molecules

In above-threshold ionization of rare-gas atoms, photoelectron spectra recorded in the ${10}^{13}$--${10}^{14}$ W cm${}^{\ensuremath{-}2}$ range exhibit enhancements in the rescattering plateaus that do not have a unified theoretical interpretation yet. Here an experimental search for such enhancements is reported in simple molecules with ionization potentials near those of rare-gas atoms such as ${\mathrm{H}}_{2}$ and ${\mathrm{N}}_{2}$ for argon and ${\mathrm{O}}_{2}$ for xenon, and in other molecules such as CO${}_{2}$ and ${\mathrm{N}}_{2}$O. Only ${\mathrm{H}}_{2}$ exhibits the enhancements previously observed in atoms. The ${\mathrm{H}}_{2}$ particularity is interpreted in terms of its simpler ion structure and associated ionization paths compared with other molecules.

Direct Test of Laser Tunneling with Electron Momentum Imaging

Tunneling is often used to describe multiphoton ionization of rare gas atoms in infrared fields. We test the tunneling approximation and its nonadiabatic extension by measuring the unperturbed momentum distribution along the κ direction of a circularly polarized light pulse. We find substantial, but not total, agreement between our results and the predictions of the model. As predicted, the κ direction momentum distribution is Gaussian and its width increases with the square root of electric field strength. However, the width is 15% too large and we find no evidence of nonadiabatic effects as we approach the expected limits of the approximation.

Multiple ionization of rare gas atoms by intense free electron laser radiation at 51 and 61 nm

We have studied multiple ionization of rare gas atoms (Ne, Ar, Kr, Xe) by intense 51 nm and 61 nm radiation delivered by the EUV-FEL at SPring-8 Compact SASE Source (SCSS) Test Accelerator. Charge states as high as Ar7+ and Kr8+ have been observed. We present intensity dependent yields of multiply charged ions providing benchmark data for theory, and discuss the role of direct, sequential and resonant ionization channels.

R-matrix with pseudo-states calculations for electron-impact excitation and ionization of rare gas atoms

R-matrix with pseudo-state calculations have been carried out for electron-impact ionization of the ground 2p6 and excited 2p53s, 2p54s, 2p55s states of neutral Neon. In addition, R-matrix calculations for electron impact excitation along the Argon isonuclear sequence are reported.

Coincidence technique using synchrotron radiation for triple photoionization: Results on rare gas atoms

Final-state trication spectra and electron distributions produced by soft x-ray single-photon triple ionization of rare gas atoms have been obtained by a multiple-coincidence technique using storage-ring synchrotron radiation. The technique uses electron time of flight with ion detection to overcome the problem of high repetition rates in single-bunch operation. A correction needed to the triple-ionization energy of Kr currently listed in standard tables is confirmed, and the method's ability to examine the three-electron distributions, characterizing the ionization mechanisms and post-collision interactions, is illustrated.

Angular differential cross sections for multiple ionization of rare gas atoms by protons with inclusion of Auger-like processes

Based on the independent particle model, we consider single and multiple ionization of neon and argon atoms induced by fast proton impact. Multiple ionization at high projectile energies (EP ⪆ 1 MeV/amu) is dominated by Auger-like processes and can be described appropriately by our statistical model introduced recently. In the present work we use several scattering models to extract angular differential cross sections. With our results a long-standing discrepancy between different experimental data sets is resolved.

Spectroscopy of rare gas clusters using VUV light from a free-electron-laser

We performed time-of-flight (TOF) spectroscopy on a jet of rare gas clusters using light pulses at λ FEL = 32 nm produced by the FLASH VUV freeelectron-laser (Hamburg, Germany) with a maximum peak power density of about 3 × 10 13 W/cm 2 at the sample. Energy deposition on the clusters was found to be strongly dependent on lambda and much more efficient in the VUV than in the IR and visible spectral range. We observed multiple ionization of rare gas atoms coming from clusters; the latter fragment completely upon absorption of a single pulse. We have also measured high quality photoelectron spectra. Small angle soft X-ray scattering (SAXS) from a single light pulse ( λ FEL = 32 nm), was recorded with high signal to noise ratio for very large ( N atoms 3 × 10 6) Argon clusters.

Multiple Ionization of Rare Gas Atoms Irradiated with Intense VUV Radiation

The interaction of intense vacuum-ultraviolet radiation from a free-electron laser with rare gas atoms is investigated. The ionization products of xenon and argon atomic beams are analyzed with time-of-flight mass spectroscopy. At 98 nm wavelength and approximately 10(13) W/cm(2) multiple charged ions up to Xe6+ (Ar4+) are detected. From the intensity dependence of multiple charged ion yields the mechanisms of multiphoton processes were derived. In the range of approximately 10(12)-10(13) W/cm(2) the ionization is attributed to sequential multiphoton processes. The production of multiple charged ions saturates at 5-30 times lower power densities than at 193 and 564 nm wavelength, respectively.

Kinetics of multiple ionization of rare-gas atoms in a circularly polarized laser field

The multielectron tunneling ionization probabilities have been calculated for Ar and Kr atoms in circularly polarized laser fields with the pulse durations of 50 fs and 5 fs. The channels of sequential, nonsequential ionization, and ionization with ionic core excitation (inelastic tunneling) have been taken into account. The calculated results demonstrate that many-body effects in tunneling, connected mostly with ionic core excitation, are very important for creation of ${\mathrm{Ar}}^{4+},$ ${\mathrm{Ar}}^{5+},$ ${\mathrm{Kr}}^{4+},$ and ${\mathrm{Kr}}^{5+}$ ions.

Atomic dynamics in single and multi-photon double ionization: An experimental comparison

We have used a multi-particle imaging technique (COLTRIMS) to observe the double ionization of rare gas atoms by multi-photon absorption of 800 nm (1.5 eV) femto-second laser pulses and by single photon absorption (synchrotron radiation). Both processes are mediated by electron correlation. We discuss similarities and differences in the three-body final state momentum distributions.

On possibility of direct electroionizing conversion of nuclear energy to electricity

The objective of the present article is theoretical investigation of possibility of direct conversion of the nuclear energy to electricity without all intermediate stages: the thermal and the mechanical. The principle of this method (so-called ‘electroionizing’) is ionization of rare gas atoms with fission fragments and separation of generated positive and negative ions with the external electric field. Under certain conditions, when so-called the Debye’s screening of charged particles within the produced plasma takes place, the recombination among these particles is insignificant. It is discovered that because of the absence of Carno’s limit on the efficiency of conversion, the performance coefficient may be 40–50% irrespective of temperature. The conditions of serviceability of the present method are definited.

New Developments in Atom Probe Techniques and Potential Applications to Materials Science

The emergence of the field ion microscope, invented by E.W. Müller in 1951, allowed, for the first time, direct observation of metallic materials in real space on an atomic scale. Field ionization of rare gas atoms near the surface of the material allowed production of an atomic resolution image of the material; field evaporation of surface atoms allowed observation into the depth of the sample.Field ion microscopy (FIM) has produced numerous impressive results, one of them being three-dimensional reconstruction of defect cascades produced by heavy ion irradiation in metals. FIM techniques have also contributed in a spectacular way to the knowledge of the crystallographic structure of grain boundaries. Surface diffusion as well as surface reconstruction phenomena are applications where, again, FIM gave surprisingly detailed information. This overview of applications of FIM in materials science emphasizes recent work. Detailed information and descriptions of earlier work can be found elsewhere.During the development of FIM techniques, the basic shortcomings of the instrument for investigating metallic alloys were rapidly recognized. In Cu3Pd alloys, for instance, Cu atoms are darkly imaged and therefore appear as vacancies similar to Co atoms in PtCo and Pt3Co. It is thus impossible to distinguish vacancies from solute atoms. This clearly limited the quantitative use of FIM to study short-rangeorder in dilute alloys or to investigate long-range order (Figure 1). Similarly, in two-phase materials, precipitates often give rise to quite visible contrast. For instance, in nickel-based alloys, aluminum-enriched precipitates appear in bright contrast. It is therefore possible to find the size and also the number density of particles present in the material. However, FIM does not provide any composition data.

1-W cw Zn ion laser

We have obtained 1.2 W of cw laser power on the 4911.6- and 4924.0-Å transitions of Zn II by exciting a He-Zn gas mixture with a dc glow discharge electron beam. In addition, 0.25-W output power has been obtained on the 6149.9-Å line of Hg+ using the same excitation scheme. The combination of electron beam ionization of rare gas atoms and subsequent charge transfer excitation to metal ion levels is shown to have the potential of significantly increasing the efficiency of ion lasers. cw multiwatt visible and ultraviolet ion lasers operating at efficiencies >10−3 appear feasible using this excitation scheme.