Search Results (12 total)

We present a combined experimental and theoretical study of fragmentation of small C_n clusters (n=5,7,9) produced in charge transfer collisions of fast (v=2.6  a.u.) singly charged C_n⁺ clusters with He. Branching ratios for all possible fragmentation channels have been measured. Comparison with microcanonical Metropolis Monte Carlo simulations based on quantum chemistry calculations allows us to determine the energy distribution of the excited clusters just after the collision.

A Comment on the Letter by F. Gobet et al., Phys. Rev. Lett. 89, 183403 (2002). The authors of the Letter offer a Reply.

Using high resolution x-ray spectroscopy, we have measured projectile electron single and multiple cross sections when a two-electron Ar¹⁶⁺ ion collides with neutral target atoms. For a fixed impact velocity (v_p  =  23  a.u.), but using various targets from He to Xe, a range from the perturbative regime to the strong interaction regime has been investigated. Double excitation cross sections are found to be well reproduced by an independent electron model. First measurements of capture-ionization cross sections are also reported and show the importance of this often-neglected process.

Clear experimental evidence of plasma stripping enhancement for 1.5 MeV/u chlorine ions is contrasted with its cold gas homologue at the same density (1×10¹⁹ e^-/cm^-2). The plasma is created by an electrical discharge in hydrogen and diagnosed with optical spectroscopic methods and beam probe analysis. The velocity dependence for the charge distributions is modeled both in cold gas and in plasma experiments. The calculations are found to be in good agreement with the hypothesis of a strongly reduced capture rate in the plasma case. Those results document quantitatively the basic mechanisms underlying the enhanced ion projectile charge in a plasma target. They also add credence to currently advocated scenarios for heavy-ion-driven fusion.

A simple model employing the independent-electron approximation was developed to investigate orientation effects on the cross sections for multielectron ionization of diatomic molecules in fast-heavy-ion collisions. The model was used to compare K- and L-shell ionization cross sections for O₂ molecules with internuclear axes oriented parallel and perpendicular to the beam.

Data on the production and dissociation of CO^Q+ molecular ions (where Q=1 through 7) obtained by ion-ion coincidence time-of-flight measurements were analyzed to determine production yields, dissociation fractions, and branching ratios. A detailed comparison of the dissociation fractions for CO⁺ and CO²⁺ for several collision systems in the same perturbative regime revealed them to be quite similar, whereas the dissociation fraction for CO⁺ produced by valence-electron photoionization is a factor of 1.8 to 3.6 larger. The results for Q≥2 indicated a preference for dissociation channels leading to symmetric or nearly symmetric charge division. An enhancement of the total ionization yields for Q>4 was observed, and it suggests that electron transfer followed by LMM Auger decay plays an important role in determining the final charges of the dissociation products.

Cross sections for K x-ray production in solid targets (Z=13, 22, 26, 29, 32, 40, 42, 46, and 50) by 30-MeV/amu beams of H, N, Ne, and Ar were measured. The cross sections were determined by recording the spectra of K x rays with a Si(Li) detector in coincidence with beam particles detected in a microchannel plate assembly. The K-shell-ionization cross sections deduced from these data agree quite well with the predictions of the perturbed-stationary-state theory with energy loss, Coulomb deflection, and relativistic corrections. Detailed analyses of the projectile and target Z dependences of the cross sections were performed. Also, the relative intensities of K x rays from double K-shell ionization of the higher-Z targets are presented for N, Ne, and Ar projectiles.

Transient molecular ions of CO^q+ (where q=2–7) were produced in single collisions of 97-MeV Ar¹⁴⁺ projectiles with neutral CO molecules. The resulting dissociation products were identified by coincidence time-of-flight spectroscopy in which the time of flight of the first ion to reach the detector and the time difference between the first ion and its partner were recorded event by event. An iterative matrix-transformation procedure was employed to convert the time-difference spectra for the prominent dissociation channels into total-kinetic-energy distributions. Analysis of the total-kinetic-energy distributions and comparisons with the available data for CO²⁺ and CO³⁺ from synchrotron radiation experiments led to the conclusion that ionization by Ar-ion impact populates states having considerably higher excitation energies than those accessed by photoionization.

The first comparison of core-state populations of initially bare Kr³⁶⁺ ions emerging from gaseous and solid targets is presented. Another new important feature of this experiment lies in the fact that for such fast heavy ions, the single-collision condition is fulfilled even in solid targets. Our results can be explained by a wake-field–induced Stark mixing of the substates in a solid. Further coherence angles and field strength measurement are discussed.

Relative intensities and polarization rates of n¹P→1¹S heliumlike Lyman x rays emitted in collisions of 4-keV/amu Ne⁹⁺ ions with H₂ have been measured. Our experimental results are discussed in connection with molecular calculations for the near-system F⁹⁺+H. High polarization rates (∼60%) are measured for direct x rays, indicating a strong predominance of m=0 substates in final n¹P states, in agreement with the theory. This shows the existence of a strong rotational coupling in the molecular frame. The difference of polarization observed for the 2P→1S and 3P→1S x rays may originate from a higher anisotropy of the low-angular-momentum states with respect to the high ones, as predicted by the theory.

A variational scattering amplitude derived in the impact-parameter formalism is used to investigate the electronic excitation of highly charged ions by ion impact at intermediate velocities. This treatment predicts that the cross sections tend to finite limits when the charge of the projectile increases. This feature is illustrated by the good agreement between our theoretical and experimental results for the excitation of Fe²⁴⁺ by He, N, and Ar. The excitation of H by H⁺ is studied as a check of our theory.

In this Letter we report the first observation of inelastic x-ray scattering from an atom in which both Raman and Compton processes have been observed simultaneously. It has been observed that x-ray Raman and Compton scattering in the vicinity of a deep atomic level exhibit different behavior as a function of incident photon energy.