The ground-state dissociation (GSD) of ${\mathrm{H}}_{2}^{+}$ and ${\mathrm{D}}_{2}^{+}$ molecular ions induced by ionizing and electron-capture collisions of ${\mathrm{He}}^{+}$ projectiles with hydrogen and deuterium molecules was investigated at impact velocities of 0.25 and 0.5 a.u. Momentum distributions of slow ${\mathrm{H}}^{+}$ and ${\mathrm{D}}^{+}$ fragments were measured and compared with calculations based on GSD theory. At the higher velocity, and for both capture and ionization, these distributions are well accounted for by a model that attributes the momenta to the vibrational wave function of the parent ${\mathrm{H}}_{2}$ or ${\mathrm{D}}_{2}$ molecule. For the lower velocity, this model remains successful for capture but not for ionization. For the latter case we observe a large transverse-momentum component imparted to the vibrationally excited ${\mathrm{H}}_{2}^{+}$ or ${\mathrm{D}}_{2}^{+}$ system beyond that predicted by the model. This feature is interpreted as being caused by momentum transferred to the motion of the nuclei within the molecule's center-of-mass frame by way of a direct interaction between the projectile and one of the molecule's nuclei.

The process of single ionization has been studied in C6+-He collisions at the collision velocities of 1.17, 1.36, and 1.63 a.u. Ejected electrons were detected in coincidence with the recoil ions. Two components of the electron-momentum and the full-momentum vector of the recoil ion were measured. From these, two-dimensional momentum-space distributions of the continuum electrons were deduced, projected parallel or perpendicular to the collision plane, and for several values of the recoil transverse momentum. The distributions do not show the molecular-orbital-like patterns seen in similar collisions with singly and doubly charged projectiles. They do tend to lie preferentially along a ridge joining the target and projectile velocities, and show an increasing tendency to go in the direction of the projectile, both longitudinally and transversely, as the collision velocity is raised and the impact parameter lowered. The results are in agreement with classical trajectory Monte Carlo predictions for similar systems.

The COLTRIMS (COLd Target Recoil Ion Momentum Spectroscopy) approach to final-state momentum imaging is now being widely used in at least a dozen accelerator and synchrotron-radiation laboratories in the world and its use is growing rapidly. The technique combines fast imaging detectors with a supersonically cooled gas target to allow the charged particles from a collision, including both recoil ions and electrons, to be collected with extremely high efficiency and with fully measured vector momenta. It allows the investigation of correlations between ejected momentum fragments and in some cases the identification of collective modes of disintegration. When molecular targets are used, it allows the a posteriori determination of the alignment of the molecule at the time of the collision. We will discuss the use of this approach to study the single and double ionization of He and D-2 by the impact of photons and of charged particles over a wide range of velocities.

The ionization of H{sub 2}, leading to H{sub 2}{sup +} recoil ions in nondissociative states, and the ionization of He by incident He{sup +} ions were investigated in the 0.25-1.23-a.u. impact velocity range employing electron and target recoil-ion momentum-imaging techniques. Similarities as well as differences were observed in the electron velocity distributions from H{sub 2} and He targets. In both cases the data strongly suggest of the promotion of molecular orbitals formed between target and projectile within a rather well-defined projectile velocity window. In particular, the data support the promotion of the 2p{pi} molecular orbital populated via rotational coupling. Outside this molecular promotion window, in particular at lower velocities, mechanisms of a different kind appear to dominate the ionization process, and electron momentum distributions are very dissimilar for He and H{sub 2}. Reduced projectile scattering cross sections, derived from measured target recoil-ion transverse momentum distributions, support these conclusions and point to the coexistence, at certain impact velocities, of different ionization mechanisms. (c) 2000 The American Physical Society.

A review of some basic features of the COLTRIMS technique is presented. Examples of recent work on low-energy electron capture and ionization collisions and for high-velocity ionization from Kansas State University are given.

The technique of cold-target recoil-ion momentum spectroscopy is used to study the process of ionization in slow to intermediate-velocity collisions. Some evidence for the role of the quasi-molecular promotion in the ionization process is presented. The two collision systems He+–He and Ne+–Ne are chosen for illustration.

Single target ionization was investigated in the asymmetric collision systems He+-Ne and Ne+-He at projectile velocities of 0.25, 0.35 and 0.5 au employing electron and recoil momentum imaging techniques. At the two lower velocities the recoil transverse momentum distributions were found to be donut shaped. In addition, a concentric double-donut was observed in the He+-Ne system at 0.25 au. At 0.35 and 0.5 au the electron distributions are characterized by a pronounced asymmetry with respect to the beam axis, an asymmetry which is almost completely reversed by exchanging projectile and target. This latter finding is interpreted as confirmation of the molecular nature of the ionization process at these velocities.

Target ionization in collisions of singly charged Ne+ ions with Ne has been investigated at projectile velocities from 0.25 to 0.55 a.u. using electron and recoil momentum imaging techniques. The momentum distributions of the ejected electrons were found to carry a distinct signature strongly suggesting that ionization is taking place by successive promotions through molecular orbitals. The observed recoil transverse momentum distributions are donut-shaped, indicating that single ionization is confined to a well-defined impact-parameter window. [S1050-2947(98)50311-3].

Cold-target recoil-ion-momentum spectroscopy (COLTRIMS) has been used to study single electron capture from He by ${\mathrm{Ar}}^{8+}$ ions at projectile velocities between 0.2 and 1.0 a.u. Populations of $3d$ through $7l$ states on the final ion are resolved, and angular distributions are presented for separated major final channels. As the projectile velocity is raised, the reaction window is observed to spread. Contrary to expectations based on a Landau-Zener picture of the process, higher $n$ and $l$ become favored with higher $v.$ The results are in excellent agreement with coupled-channel calculations.

Single and double electron capture from He targets by Ar16+ ions have been studied at projectile velocities from 0.3 to 1.5 a.u. Cold-target recoil-ion momentum spectroscopy was used to record the energy gain and scattering angle simultaneously. For single capture, the reaction window is found to spread in width approximately as the square root of the projectile velocity and to shift slightly toward smaller energy-gain values as the velocity increases. The angular distributions center at the half Coulomb angle over most of the velocity range covered, but differ in shape from multichannel Landau-Zener model results. For double capture, transfer ionization dominates and feeds primarily n-symmetric states, where n is the principal quantum number. True double capture feeds mainly n-asymmetric states. The angular distributions for double capture lie outside the half Coulomb angle, indicating the importance of two-step processes in populating doubly excited states. [S1050-2947(98)05610-8].

A systematic search was performed for the manifestation of quantum interference effects in the shape and angular distribution of the binary-encounter electron peak in collisions of partially stripped, or structured, heavy ions with noble gases and molecular hydrogen. The ionic species investigated were Cu5+,19+, I7+,23+, Au11+,29+ and U13+, all at the same nominal velocity equivalent to 0.6 MeV amu-1. Experimental double-differential cross sections for secondary electron emission in the binary encounter energy region are compared with a simple model based on the elastic scattering of quasi-free target electrons in the projectile field as well as with results of impulse approximation (IA) calculations. While these calculations provide a good qualitative overall description of the observed quantum effects for noble gas targets, this is not the case for H2 targets. An attempt was made to incorporate target molecular structure into the impulse approximation code by allowing the binary electron amplitudes from each of the hydrogen atoms, assumed to constitute the H2 molecule, to interfere. This approach, while demonstrating the strong influence of molecular orientation upon the intermediate energy region of the cross sections, did not meet with success, thereby indicating the necessity to consider the final-state interaction of the binary electron with the two protons of the residual H2+ or H22+ target.

We have measured the energy distributions of the H+ and the H- ions resulting from the collisional dissociation H-3+ --> 2H+ + H- in argon at a projectile energy of 2700 keV. The fragment ions were separated by an analyzing magnet and detected by silicon surface barrier detectors, with the H+ and the H- ions being detected in coincidence. The energy distributions of the fragments in the projectile center-of-mass frame were extracted from their transverse spatial distributions observed at a position 150 cm downstream from the collision chamber. These distributions were used subsequently to determine the total kinetic energy of the fragments and the correlation angle theta(12) between the H+ momenta. A comparison is carried out with recently published data obtained in the low-velocity regime in helium.

Relative double differential cross sections for electron emission in collisions of Iq+ and Xeq+ projectile ions incidence on H2 and He targets are presented for observation angles of 0° to 60° with respect to the beam. The measurements were performed over a wide range of projectile velocities (0.6 to 3.6 MeV/amu) and charge states (q=7 to 23). As the velocity or charge state of the projectile are decreased, drastic changes in the behavior of the binary encounter peak can be observed. These effects can be attributed to diffraction of the target electrons by the non‐Coulomb potential of the clothed projectile ion. A theoretical treatment of the double differential cross sections has been developed where the binary encounter electrons are treated within the impulse approximation as the elastic scattering of quasi‐free target electrons by the screened potential of the projectile ion.

We report on the measurements of double differential cross sections, at zero degree, for the production of secondary electrons from the collision of 0.6 MeV amu(-1) Cu5+, I7+, Au11+, U13+, Cu19+, I23+ and Au29+ projectiles with H-2 and He. The shapes ad positions of the observed binary encounter electron peaks are compared with continuum distorted wave-eikonal initial state (CDW-EIS), impulse approximation and classical trajectory Monte Carlo calculations, as well as with predictions From an adiabatic resonant tunnelling model recently proposed by Fainstein and co-workers. The experimentally observed binary peak positions were found to be essentially determined by the projectile's ionic charge q and to be independent of the projectile's nuclear charge Z(P). Although the experimental conditions correspond to the strong perturbation regime, i.e. 1.0 less than or equal to q/V-P less than or equal to 5.9 (V-P being the projectile velocity), the CDW-EIS approach accounts well for the position and shape of the binary peak.

Double differential cross sections are reported for the production of binary encounter electrons in collisions of 0.6 MeV amu-1 I23+ and 1.4, 2.4, and 3.6 MeV amu-1 Xe21+ projectiles incident on He and Ar targets. Electron energy spectra were measured between 0-degrees and 45-degrees in the case of the two lower projectile energies, and between 17.5-degrees and 60-degrees for the two higher projectile energies. The data are compared with quantum mechanical impulse approximation and classical trajectory Monte Carlo calculations. While the quantum model calculation predicts a rapid disappearance of diffraction effects in the binary encounter peak with increasing projectile energy, these remain visible in the experimental results up to the highest energy measured. The necessity of including multiple target ionization involving inner shell electrons in the theoretical description of the collision process is demonstrated by the classical trajectory Monte Carlo calculation, which accounts well for the shape of the 2.4 and 3.6 MeV amu-1 cross sections, except at angles where diffraction effects are manifest. Systematic shifts of the binary encounter peak position towards lower energies with increasing emission angle were observed for all projectile energies.

A systematic investigation of binary encounter electron production from collisions of partially stripped ions with hydrogen and noble gases is currently under way. We present experimental double-differential binary encounter electron cross sections as well as model calculations for 0.6 MeV amu−1 Cu19+, I23+ and Au29+ ions colliding with argon. The results show how, with increasing the screening of the projectile nuclear charge, quantum diffraction effects manifest themselves in the angular behaviour and shape of the binary encounter peak.

The differential cross sections for Compton scattering of 662-keV photons from K electrons of tin and lead were measured at scattering angles between 15\ifmmode^\circ\else\textdegree\fi{} and 150\ifmmode^\circ\else\textdegree\fi{}. Both cross sections were found to be similar and to exhibit the qualitative features predicted by relativistic calculations. Additional experiments were carried out with two thick silver and platinum targets, and the influence of second-order scattering processes on the values of the cross sections was estimated.

The feasibility of using x-ray fluorescence techniques to determine the concentrations of silver, bromine, and iodine in silver halogenide holographic films is studied. A small apparatus employing a 30-mCi (57)Co radioactive source and a germanium photon detector is described. AgBr films were exposed to different amounts of light and subjected to several types of chemical processing, and their silver and halogen contents were investigated. In addition, the relationship between the concentration of absorbing silver in the emulsion and its optical density was determined.