Rare Gas Targets Research Articles

Thermomechanical design of a static gas target for electron accelerators

Gas targets are often used at accelerator facilities. A design of high-pressure gas cells that are suitable for hydrogen and helium isotopes at relatively high electron beam currents is presented. In particular, we consider rare gas targets, 3H2 and 3He. In the design, heat transfer and mechanical integrity of the target cell are emphasized. ANSYS 12 was used for the thermo-mechanical studies of the target cell. Since the ultimate goal in this study was to design a gas target for use at the Jefferson Laboratory (JLab), particular attention is given to the typical operating conditions found there. It is demonstrated that an aluminum alloy cell can meet the required design goals.

Charge-state-specific EUV spectra of Xe ions

Charge state specific extreme ultraviolet spectra from xenon ions have been recorded at Tokyo Metropolitan University. The Electron Cyclotron Resonance Source spectra were produced from charge exchange collisions between the xenon ions and rare gas target atoms. Atomic structure calculations were performed for Xe 16+ − Xe20+ using the Hartree-Fock with Configuration Interaction code of Cowan and showed that the spectra arise from 4p-4d and 4d-4f transitions. In addition it was necessary to allow for selective capture processes that occur in these slow collisions. The energies of the capture states involved in the single electron process are estimated using the classical over barrier model.

Stopping of protons – Improved accuracy of the UCA model

Recent theoretical developments in the unitary convolution approximation (UCA) for electronic energy losses of bare and screened ions are presented. Examples are given for proton beams and rare-gas targets. For gas targets there exists a sufficient amount of experimental data on charge exchange, for pinpointing the largely unknown stopping-power contribution of electron-capture processes at low and intermediate energies.

Configuration interaction in charge exchange spectra of tin and xenon

Charge-state-specific extreme ultraviolet spectra from both tin ions and xenon ions have been recorded at Tokyo Metropolitan University. The electron cyclotron resonance source spectra were produced from charge exchange collisions between the ions and rare gas target atoms. To identify unknown spectral lines of tin and xenon, atomic structure calculations were performed for Sn14+–Sn17+ and Xe16+–Xe20+ using the Hartree–Fock configuration interaction code of Cowan (1981 The Theory of Atomic Structure and Spectra (Berkeley, CA: University of California Press)). The energies of the capture states involved in the single-electron process that occurs in these slow collisions were estimated using the classical over-barrier model.

Momentum spectra of electrons rescattered from rare-gas targets following their extraction by one- and two-color femtosecond laser pulses

We have used velocity-map imaging to measure the three-dimensional momenta of electrons rescattered from Xe and Ar following the liberation of the electrons from these atoms by 45 fs, 800 nm intense laser pulses. Strong structure in the rescattering region is observed in both angle and energy, and is interpreted in terms of quantitative rescattering (QRS) theory. Momentum images have also been taken with two-color (800 nm $+$ 400 nm) pulses on Xe targets. A strong dependence of the spectra on the relative phase of the two colors is observed in the rescattering region. Interpretation of the phase dependence using both QRS theory and a full solution to the time-dependent Schr\odinger equation shows that the rescattered electrons provide a much more robust method for determining the relative phase of the two colors than do the direct electrons.

Multiphoton ionization of atoms with soft x-ray pulses

The new soft x-ray free-electron laser in Hamburg (FLASH) has opened the doorway to totally new experiments of materials research on nanometre and femtosecond scales. However, the mechanisms of photon–matter interaction are not well understood under the conditions of ultra-high photon intensities in conjunction with short wavelengths. In this context, we have quantitatively investigated nonlinear photoionization of rare gas atoms at FLASH by ion mass-to-charge spectroscopy and, thus, also the limits for the application of gas-ionization detectors for the characterization of x-ray lasers. By strong beam focusing, we have achieved irradiance levels beyond 1013 W cm−2 at about 40 eV photon energy and up to 1016 W cm−2 in the extreme ultraviolet at about 90 eV. Here, surprisingly high degrees of photoionization were observed on Xe atoms. By comparison with other rare gas targets, it emerged that the excitation of inner-shell resonances might play a significant role in the degree of atomic perturbation by the radiation field.

Letter: High-Energy Electron Transfer Dissociation Using a Tandem Time-of-Flight Mass Spectrometer with an Electrospray Ionization Ion Source

A tandem time-of-flight (TOF) mass spectrometer with an electrospray ionization (ESI) ion source (ESI-TOF/quadTOF) was enhanced to perform high-energy electron transfer dissociation (ETD) experiments. Specifically, the collision cell for rare gas targets of ESI-TOF/quadTOF was replaced with one for alkali metal targets. High-energy ETD spectra were obtained of doubly protonated polypeptides and a phosphopeptide using a Cs target. Peaks due to several c- and z-type fragment ions formed by the high-energy ETD process were clearly observed in all cases. These results demonstrate that high-energy ETD can be performed using ESI-TOF/quadTOF.

Target technology development for the research of high energy density physics and inertial fusion at the RFNC–VNIIEF

AbstractResults in some directions of the target technology for research on high energy density and laser fusion at the Russian Federal Nuclear Centre–All-Russia Research Institute of Experimental Physics for the last three years are presented. The results of development of optical and X-ray methods of characterization and manufacturing techniques of targets for studying the equation-of-state at high pressures and the condensed rare gas targets for the influence of pulse-repeated laser irradiation are given.

VUV emission spectra in charge-exchange collisions of multiply charged Sn ions with rare gases

We have measured the VUV (vacuum ultraviolet) emission spectra following charge transfer collisions of Snq+(q = 3-6) with neutral rare gas targets at collision energies of 50–100 keV. To understand the emission lines in observed spectra, we have executed the GRASP92 and RATIP programs, which provide both emission wavelengths and transition probabilities between the excited states of the Sn5+ ions.

Identification of 4d–5p transitions in the spectra of Sn XV–Sn XIX recorded from collisions between Sn ions and He

Charge-state-specific extreme ultraviolet (EUV) spectra produced from charge exchange collisions between tin ions in charge states from Sn15+ to Sn18+ and rare gas target atoms using an ECR (electron cyclotron resonance) source have been recorded at Tokyo Metropolitan University. At low target gas pressures, the spectra were produced by single collision and capture events, and in each case they were dominated by intense emission near 13 nm. In the spectra of Sn XV–Sn XIX this emission has been shown to arise from 4s24pm−14d–4s24pm−14f + 4s24pm−24d2 transitions. However, in addition, a distinct group of lines resulting from 4s24pm−14d–4s24pm−15p (2 ⩽ m ⩽ 6) transitions centred near 11.5 nm in Sn XV was seen to shift to shorter wavelength with increasing ionic charge, and the strongest lines were identified by comparison with atomic structure calculations. These identifications are the subject of the present work.

Complementary spectroscopy of tin ions using ion and electron beams

Extreme ultra-violet (EUV) emission spectra of multiply charged tin ions were measured in the wavelength range of 10-22 nm following charge exchange collisions of Snq+(q = 15-21) ions with rare gas targets at 20 keV/q or the electron impact excitation of tin ions with electron energy of 312-613 eV. In charge exchange collisions, we observed both the resonance lines and the emission lines corresponding to the transitions between the excited states. On the other hand, we observed mainly the resonance lines in the electron impact experiments. We can distinguish the resonance lines from other emission lines in the charge exchange spectrum by comparison with the emission lines in the electron impact spectrum. The comparison of the complementary experimental results can be regarded as a new method of spectroscopy of multiply charged ions.

Energy gain measurements of Xe9+ ions in collisions with He and Ar

To clarify a collision dynamics of multiply charged Xe ions with rare gas target, we have measured energy gain spectra of Xe9+ in collisions with He and Ar at 200 q eV and have compared with the theoretical energy gain peak calculated by the classical over barrier model. Using energy levels of excited Xe8+(nl) ions referred to the NIST atomic spectra database, we have determined capture states of Xe9+ ions after single electron transfer.

Vacuum ultraviolet spectra in charge transfer collisions of multiply charged Sn ions

We measured the vacuum ultraviolet (VUV) emission spectra following charge transfer collisions of Snq+ (q = 3-6) with neutral rare gas targets at collision energies of 50-100 keV. We identified some of emission lines from reported energy levels. Several lines were observed in collisions of different charge states of ions. This finding means that the multiple-electron capture processes can take place effectively in our experiment. Significant target dependences were observed in the spectra with the projectile of Sn4+ and Sn6+.

Storage of negative carbon ions in an electrostatic ring

Negative carbon atomic ions and cluster ions were stored in an electrostatic ion storage ring. The lifetime of the metastable atomic ion is heavily affected by the blackbody radiation from the surrounding environment. By introducing rare gas target into the ring, collision-induced detachment cross sections of the atomic ion C- were obtained. For cluster ions Cn- (n = 2 ∼ 8), decay of metastable ions with sub-millisecond to millisecond lifetime was commonly observed.

電荷逆転質量分析法の基礎 Ｉ：歴史的展望と４種の電荷逆転の違い

The most valuable technique for the structure analysis in mass spectrometry, tandem mass spectrometry (MS/MS), involves the excitation of mass-selected species followed by their dissociation into products, the mass-to-charge ratios of which are also measured. Collisionally Activated Dissociation (CAD) has been used as a most common technique to dissociate the mass-selected precursor ions. Charge inversion mass spectrometry is a special branch of MS/MS method, in which the electric charge of the precursor ions is opposite to that of the secondary product ions. Charge inversion mass spectrometry is classified into four types depending on the electric charge and time scales of collisions. Electric charges, which are either positive ions or negative ions, provide the characteristic difference between reactants and products. Either successive single electron transfer in two collisions or double electron transfer in one collision depend time scales of collisions. Charge inversion mass spectrometry using collisions with gaseous targets in the keV collision energy range have provided insights into the structures and reactions of ions and neutral molecules. The characteristics of these four types of charge inversion experiments are presented in terms of the reaction endothermicities, the cross sections and their dependence on the target species. In the case of rare gas or simple molecular targets, double electron transfer in one collision is effective to form positive ions from negative ions, while in the case of alkali metal targets successive single electron transfers in two collisions is effective to form negative ions from positive ions. While CAD is due to dissociation of activated ions with broad internal energy distributions, the charge inversion processes using alkali metal target is due to dissociation of energy-selected neutral species with narrow internal energy distributions.

Generation of Strong Optical Field in Soft X-Ray Region by Using High-Order Harmonics

For further development of a variety of applications, one of the most important issues is the improvement of output harmonic energy and conversion efficiency. We review our work on the energy scaling of high-order harmonic generation based on phase-matching using a loosely focused beam. Our harmonic energy scaling method can be universally applied to harmonic generation in the neutral rare gas target. In addition, we demonstrate a new concept for spatial separation between the high-energy pump and harmonic beam to increase the available soft X-ray energy for the applications. This method is very simple and highly useful for not only high-order harmonic generation but also longitudinally pumped X-ray lasers. By combining the high-energy high harmonic source and new spatial separation method, we successfully demonstrate focusing of the soft X-ray beam with a peak intensity of 1/spl times/10/sup 14/ W/cm/sup 2/, which is to our knowledge the highest ever reported in the soft X-ray region.

Experimental observation of soft X-rays and suprathermal ions from rare-gas cryogenic solid targets

We investigated characteristics of the soft X-ray emission in the wavelength range of 6–16 nm using various rare-gas cryogenic solid targets irradiated with a nanosecond laser pulse. Suprathermal ions having a maximum velocity of the order of 107 cm/s were detected for three rare-gas targets (Ar, Kr, and Xe). The spatial distributions of the soft X-rays and the suprathermal ions were measured for Xe cryogenic solid targets and found to vary as cos2θ and cos5θ, respectively, where θ is the angle to the target normal. By integrating the observed cos2θ spatial distribution, the conversion efficiency of the soft X-ray emission in the wavelength range from 6 to 16 nm was evaluated to be 0.2%/pulse.

Collisional orientation of the benzene molecular plane in supersonic seeded expansions, probed by infrared polarized laser absorption spectroscopy and by molecular beam scattering

The large number of elastic and inelastic collisions which take place during supersonic gaseous expansions produce not only acceleration and internal cooling of molecules, but also their alignment or orientation. The collisional alignment of the rotational angular momentum, corresponding to the orientation of the benzene molecular plane, in supersonic seeded expansions with lighter carrier gases is demonstrated via two complementary experiments: one interrogating benzene via polarized laser light IR absorption the other one probing its orientation via molecular beam scattering on rare gas targets. Typical seeding gases are helium, neon and their mixtures, and molecular hydrogen. Total stagnation pressures are of the order of ∼1 bar and ∼0.1 mm nozzle. A propensity is demonstrated for benzene molecules in seeded molecular beams to fly with the molecular plane preferentially oriented parallel to the molecular beam propagation direction. The analysis of the experimental results has been carried out using a phenomenological model which provides the fraction of molecules traveling in such a “frisbee” mode. A frisbee propensity function is defined and found to range between 0.71 and 0.85, corresponding to IR and to scattering experiments, respectively. These values are significantly higher than the value 2/3, expected for a random distribution of molecular plane orientations. The trend in the measured values is discussed in terms of different angular cones of the supersonic expansion sampled in the two experiments and evidence is provided that the orientation is higher the narrower is the sampled cone aperture.

Orientation of benzene in supersonic expansions, probed by IR-laser absorption and by molecular beam scattering.

This work represents the first experimental demonstration that planar molecules tend to travel as a "frisbee" when a gaseous mixture with lighter carriers expands into a vacuum, the orientation being due to collisions. The molecule is benzene, the prototype of aromatic chemistry. The demonstration is via two complementary experiments: interrogating benzene by IR-laser light and controlling its orientation by selective scattering on rare gas targets. The results cast new light on the microscopic mechanisms of collisional alignment and suggest a useful way to produce intense beams of aligned molecules, permitting studies of steric effects in gas-phase processes and in surface catalysis.

Fragmentation of fast positive and negative C $\mathsf{_{60}}$ ions in collisions with rare gas atoms

The fragmentation of anions and cations resulting from 50 keV collisions with rare gas targets is studied. Positive ion fragment patterns are recorded, and dramatic changes in these patterns are observed as a function of target atom number. The fragment pattern dependence on the target atom size is investigated within a simple model, normally used for stopping power calculations. Fair agreement is obtained between calculated and experimental spectra. From these comparisons we conclude that the range of the screened atomic potentials, as e.g., the Thomas-Fermi potential, is an essential parameter in the collisional induced fragmentation process.