Helium Target Research Articles

Photon and electron impact fragmentation of size selected sodium clusters

We investigate the fragmentation behaviour of size selected sodium clusters Na n , n≤5, for different ionization methods. The experimental technique, size selection by means of scattering the cluster beam from a helium target beam, enables us to find a unique correlation between the detected cluster ions and their neutral precursors. In the case of photoionization we use the light from a Xe-arc lamp in a broad spectral range down to a wavelength of about 220 nm, whereas for the pulsed, frequency doubled dye laser the wavelength is tuned to 265 and 290 nm. Electron impact ionization has been examined with an electron energy of 25 eV. The ionization by electron impact with the large excess energy results in a total fragmentation of the sodium trimer. In the case of photoionization the fragmentation probabilities depend sensitively on the excess energy.

The effect of collision energy, target gas and target gas purity on the high energy collision induced product ion spectrum of renin substrate

The product ion spectrum of renin substrate has been studied in detail using a tandem double focusing mass spectrometer with an array detector. The effect of collision energy, in centre of mass coordinates, in relation to the behaviour of the a n (a n + 1) and d n ions has been delineated. Contamination of a helium target gas by only 3% of a heavier gas (air) will significantly alter the spectrum and as little as 10% will completely mask the helium spectrum. The behaviour of the a n , (a n + 1) and d n , ions at 2 keV collision energy (laboratory coordinates) as a function of the ratio of helium to air in the gas cell is shown for the first time.

Positronium formation in positron-atom collisions at intermediate energies

The formation of ground-state positronium in collisions of positrons with hydrogen and helium atoms is considered within the distorted-wave Born approximation. In particular the authors analyse the eikonal and continuum-distorted-wave models in both their post and prior forms. Differential and total cross sections are calculated over a broad range of intermediate energies extending from 40 to 500 eV. The results confirm that the total cross section decreases rapidly with increasing energy, as predicted by the simple first Born approximation, and do not agree with experimental evidence of a slower decay. The computed total cross sections for helium targets agree with those of the close-coupling model for energies above 100 eV. Below 100 eV the authors find a large post-prior discrepancy. The calculated differential cross sections show evidence of a Thomas peak at high energies for the CDW models but not for the eikonal models.

Fragmentation Analysis of Size Selected Sodium Clusters

AbstractWe investigate the fragmentation behaviour of size selected sodium clusters Nan, n ≤ 4, for different ionization methods. The experimental technique, size selection by means of scattering the cluster beam from a helium target beam and subsequent time resolved measurement of the scattered particles, enables us to find a unique correlation between the detected cluster ions and their neutral precursors. In the case of photoionization we use the light from a Xe‐arc lamp in a broad spectral range down to a wavelength of about 220 nm, whereas for the pulsed, frequency doubled dye laser the wavelength is tuned to 290 nm. Electron impact ionization has been examined with an electron energy of 25 eV, which is far above the ionization threshold of all sodium clusters. The ionization by electron impact with the large excess energy results in a total fragmentation of the sodium trimer. In the case of photoionization the fragmentation probabilities do not follow always the energetically favoured pathways, which will be demonstrated for the sodium tetramer.

Theoretical study of electron-capture processes in the collision of the metastable N5+(1s2s) multicharged ion on a He target.

The potential-energy curves and the radial coupling matrix elements of the $^{3}\mathrm{\ensuremath{\Sigma}}^{+}$ and $^{3}\mathrm{\ensuremath{\Pi}}$ states involved in the collision of the metastable ${\mathrm{N}}^{5+}$(1s2s) $^{3}$S multicharged ion on a helium target have been calculated by means of an ab initio method with configuration interaction. The partial cross sections of capture on the 3l levels have been determined using a semiclassical method and compared with experiment. The transfer-excitation process has been considered.

Very low energy electron-H(2p) interaction studied in H-rare-gas collisions by translation electron spectroscopy

Detached electron energy spectral have been observed in the forward direction for collisions of H- ions with helium, neon and argon gas targets at 4 keV. Specifically, the channel corresponding to H(2p) excitation has been examined by recording the electron energy distributions in coincidence with Lyman- alpha photons. Near the 0' observation angle, electrons with very low energy in the projectile frame (1<Ep<100 meV) are observed that are kinematically shifted in the laboratory and which yield important information on the H(n=2)-e- interaction near threshold. The spectra reveal, in addition to the well known 1Po shape resonance of H-, a cusp typical of convoy electrons which is attributed to the long range dipole interaction between H(n=2) and an electron. In addition, weak structure near 4 meV above the H(2p) threshold could constitute further evidence in support of the existence of dipole oscillations.

Electron capture and target ionization in collisions of bare projectile ions incident on helium.

Absolute cross sections for the processes of single capture, transfer ionization, and single and double ionization were measured for ${\mathrm{C}}^{6+}$, ${\mathrm{N}}^{7+}$, ${\mathrm{O}}^{8+}$, and ${\mathrm{F}}^{9+}$ projectiles incident on a helium target in the projectile velocity range of 0.25--2.0 MeV/amu. The cross sections were determined by measuring projectile-ion final charge states in coincidence with target recoil-ion final charge states. The single-capture and transfer-ionization cross sections were corrected for ionization from impurity charge states in the beam and for double-collision processes in the target gas cell. These corrections, which are necessary for the relative as well as the absolute cross sections, were found to be as large as 50% of the cross section. The measured cross sections are compared to the multistate, coupled-channel calculations of Shingal and Lin using an independent-electron model.

Spectroscopy of Highly Charged Ions Using Complementary Techniques

The far uv spectroscopy of highly ionised oxygen and argon has been studied with particular attention paid to core-excited metastable states. Emission from foil-excited MeV beams is compared with that following single- and double-electron capture into excited states during low-energy charge-exchange collisions of highly charged ions in helium and hydrogen targets. The use of complementary techniques aids in line identification through the high charge-state selectivity of the charge-exchange process, the use of lifetime measurements, and consideration of the different excitation and cascade processes involved, as well as systematics along isoelectronic sequences, and model-potential and multiconfigurational Dirac-Fock (MCDF) calculations. A number of new lines are reported.

A note on Δn ≠ 0 Stark transitions in hydrogenlike atoms

AbstractIn a gaseous helium or hydrogen target slow muons or antiprotons are captured into orbits with a high principal quantum number (n = 15 to 50) to form (μ− α)+ ions, (pα)+ ions, or (pp) atoms respectively. In the subsequent deexcitation process Stark mixing of the intermediary states plays an important role. The successful Mainz Cascade Model assumed Δn = 0 for the Stark transitions, although formally no such selection rule exists. This note examines the reasons why Δn ≠ 0 Stark transitions play only a negligible role in the deexcitation cascade.

Laser polarized muonic helium.

We have developed laser techniques to polarize targets of muonic helium and plan to use these targets to measure the spin dependence of nuclear muon capture.

Differential and total cross sections for ionization of helium and hydrogen by electrons.

A comprehensive semiempirical model for singly and doubly differential and total ionization cross sections for electron production by electron impact is presented. The model is discussed in relation to several theoretically required constraints. Available experimental data for hydrogen and helium targets are examined, and recommended values of the data are given for all ranges of primary and secondary energies and angles of ejection.

Single electron loss and single electron capture for 0.6-2.2 MeV He+ colliding with rare gases

The authors have measured the cross sections for single electron loss and single electron capture for 0.6-2.2 MeV He+ colliding with He, Ne and Ar using the growth rate method and a molecular beam target. The results are compared with other available experimental data. For single electron loss theoretical comparisons are also made with the quantal first Born approximation and some semiclassical calculations. For single electron capture, comparisons are made with the modified classical calculations based on the binary encounter model. To date there seems to be no one theory which accurately describes the data. The theory of Kaneko (1985, 1986) is clearly promising for the neon target at energies greater than 0.15 MeV amu-1 but this could be fortuitous since the theory does not similarly reproduce the data for helium and argon targets. The theory of Kumar and Roy (1979) reproduces the energy dependence of the capture cross sections in the range 0.1-1.0 MeV amu-1 but the absolute values of the cross sections are subject to a scaling of the effective charge of the He. Unfortunately no single value of this effective charge applies to all three target gases.

Electron emission in H 0 atom collisions: a coincidence study of the angular dependence

Differential electron emission occurring as the result of fast hydrogen atom impact on helium and argon targets has been studied using standard noncoincidence and emitted-electron-iom'zed-projectile coincidence techniques. Impact energies were 0.5 and 1 MeV; electron emission was measured between approximately 20 and 2000 eV for selected laboratory emission angles ranging from 0° to 180°. The data demonstrates the importance of simultaneous target-projectile ionisation as previously observed for energetic He + impact. The experimental data for the helium target, when compared to PWBA calculations using hydrogenic wave functions, indicate good agreement with theory for projectile ionisation and, indirectly, reasonably good agreement for target ionisation. Simultaneous target-projectile ionisation events were not included in the model. The argon data are compared with more sophisticated calculations for electron loss. These comparisons indicate the importance of second order effects at large emission angles.

Far-UV spectroscopy of highly ionised argon using complementary techniques

Far-UV spectroscopy of highly ionised argon has been studied with particular attention paid to core-excited metastable states. Excitation by foil-excited MeV beams is compared with single- and double- electron capture into excited states during low-energy charge exchange of highly ionised argon ions in helium and hydrogen targets. Complementary techniques are used for a) line identification by the high charge-state selectivity of the charge-exchange process, b) lifetime measurements, c) considerations concerning different excitation and cascade processes involved, d) systematics along isoelectronic sequences, and e) model-potential and MCDF calculations. A number of new lines are reported.

Collision‐Induced decomposition of peptides. An investigation into the effect of collision gas pressure on translational energy losses

AbstractThe dependence on the gas pressure of translational energy losses, ΔE, suffered by incident ions in collision with helium target gas atoms was investigated for the [M + Na]+ ion of valinomycin. ΔE was measured from the fragment ion peak positions in the mass‐analysed ion kinetic energy (MIKE) spectra. The variation of ΔE with target gas pressure was found to be different for different fragment ions. The implications of the results for the number of collisions which may occur under normal collision‐induced decomposition conditions (30–40% transmission) are discussed.

Charge transfer and excitation in proton-helium collisions

Using the semiclassical impact parameter method, the authors have computed cross sections for excitation of He by H+ and for one-electron charge transfer, the collision energy being in the range from 10-235 keV. A basis of travelling atomic orbitals was used, comprising 11 states on the helium target and 29 states on the projectile. Results are presented both including and neglecting the effects of two-electron exchange arising from the Pauli exclusion principle. Comparison is made with measurements of the density matrix elements relating to capture into the n=2 and n=3 states of hydrogen and with measurements of the cross sections for excitation of the 1s2s 1S and 1s2p 1P levels of helium. Comparison is also made with relevant calculations, including recent computations based upon a one-electron model.

Track-depth resolved luminescence of 5 MeV/amu N-ions injected into near-liquid and liquid helium

A track scope was developed using an imaging quartz fiber and a position-sensitive photon counter. Using this scope, specific luminescence, d L/d x, and luminescence efficiency, d L/d E, were measured as functions of range and stopping power for 5 MeV/amu N ions incident on dense gas and liquid helium targets. Also, measurements were done while varying the density of helium present. The peak positions of d L/d x, which shift to shorter ranges with increased density, coincide when the tentative excitation density is plotted as a function of range or d E/d x. An extra peak in d L/d E observed near the track termination cannot be explained by the high density effect nor by the usual scintillation theory. It suggests the importance of extra processes, such as direct excitation and charge exchange.

Cross sections for deexcitation of He(2 3S, 2 1S and 2 1P) by SiH 4 and GeH 4

The cross sections for the deexcitation of He(2 3S), He(2 1S), and He(2 1P) by SiH 4 and GeH 4 have been measured and compared with those for the deexcitation by CH 4. The major deexcitation processes are ascribed to Penning ionization. The cross sections obtained for He (2 3S) are more than two hundred times as large as the total optical-emission cross sections determined recently by another group. The magnitudes of the cross-section values obtained, which are dependent on both electronic states of excited helium atoms and target molecules, are discussed in terms of the energy-transfer mechanism or the interaction responsible for deexcitation transition.

Double photoionization of helium: Analysis of photoelectrons with respect to energies and angles.

The basic sixfold differential cross section ${\mathit{d}}^{6}$${\mathrm{\ensuremath{\sigma}}}^{2+}$/${\mathit{d}}^{3}$k${\mathit{d}}^{3}$k' (SDCS) of the double photoionization process is derived within a ``wave-function approach'' (WFA) using a highly correlated ground-state wave function and a double-continuum final state approximated by a symmetrized product of one-electron Coulomb waves with k and k' asymptotic momenta. Relevant integrations of the SDCS are shown to provide a complete description of the (\ensuremath{\gamma},2e) process. The theory is illustrated by calculations for helium targets: total cross sections, kinetic energy spectra, and angular plots of photoelectrons are presented. Considering the total cross section, it is found first that the agreement between the length and velocity result is worse than expected from the previous studies. In addition, the present model provides kinetic energy distributions of photoelectrons having the symmetry required by the Pauli principle. This is in contrast with earlier formulations of the WFA. For the SDCS, calculations have been done with final states where Coulomb waves were provided with either fixed charge (Z=2: unscreened nucleus) or variable effective charges. In the former case, it is found that the event where electrons escape with the same energy, along the polarization direction, has a significant probability. This deficiency is removed by use of angle-dependent effective charges since final-state correlation is partly accounted for in this way.

Single and double electron capture by C4+ ions colliding with helium target

Differential cross sections for single and double electron capture by C4+ ions on helium target are measured at energies of 6 and 9.6 keV. These data are interpreted using a simple four-state model. Concerning the double capture, the theoretical analysis shows that a simultaneous two electron transition induced by electron-exchange interaction provides a possible alternative to transitions purely induced by electron-electron interaction usually considered.