Cross sections for electron capture by protons have been measured at energies from 440 keV to 13.8 MeV in N 2 and in Ar; to 5.41 MeV in He and to 2.45 MeV in H 2. Electron-loss cross sections in the same gases at 1.027 and 2.44 MeV are also presented. The measurements were carried out by analysis of particle-beam composition after exit from a gas target of known composition and thickness. The beam was separated magnetically, and the charged and neutral corn-ponents were detected by a Faraday cup and a scintillator, respectively. The energy range of our results overlaps that of Barnett and Reynolds between 440 keY and I MeV. Our electron-capture cross (cid:9) sections in this region agree within the experimental uncertainty for H 2 and He, but are larger by up to 50% in N2 and Ar.

Measurements have been made of the total back-scattered D/sup -/ yields from Cs, Rb, K, Na and Li surfaces bombarded with D/sub 2//sup +/ and D/sub 3//sup +/ in the energy range 0.05 to 3.5 keV/nucleon. All measurements were made at a background pressure less than 10/sup -9/ Torr and the alkali-metal surfaces were evaporated onto a substrate in situ to assure uncontaminated surfaces. For each target, the D/sup -/ yield is at a maximum (as high as 12% per incident deuteron for Cs) between 150 and 300 eV/nucleon, and at any measured energy, the D/sup -/ yield decreases from Cs to Li in the order given above.

Dielectronic recombination1 (DR) and the analogous process resonant transfer and excitation2,3 (RTE) have been the subjects of many experiments over the past decade. Most of these experiments have struggled with the fact that the electron velocity distributions or Compton profiles were so broad that details of individual resonances could not be measured. Other than the one case of an RTE measurement4 of U90+ on H2 it has only been in the last couple of years with the development of EBITs and storage rings and electron coolers that isolated resonances could be measured. Therefore, this experiment was designed to exploit the fact that when an ion travels down a crystal axis it has been shown that the ion loses little energy because it interacts very weakly with the atoms in the crystal and in fact the primary collisions involve only valence electrons of the atoms lining a channel. Datz, et a1.5 have reported a large probability that ions travelling down a crystal axis will maintain the initial charge state even though the crystal is many times thicker than the equilibrium thickness in amorphous targets. It has been shown by Datz et al.6 that channeled ions can be used to measure resonant coherent excitation by observing the stripping of the excited ions in the beam because of the increased size of the excited ion compared to the size of the crystal channel. Also Andriamonje et al.7 and Datz et al.8 have measured radiative electron capture9 (REC) from channeled ions and in the former case have shown a strong dependence of the REC width on the crystal orientation.

Resonant transfer excitation (RTE), which occurs when electron transfer (capture) is accompanied by projectile excitation in a single-collision event, has been investigated for relativistic uranium projectiles colliding with carbon foils and molecular hydrogen. The RTE process is mediated by the electron-electron interaction, and is analogous to dielectronic recombination (DR). As such, the present results provide a test of relativistic DR theory and, for the H2 target, show that RTE can contribute significantly to total single-electron capture in very high velocity collisions.

Radiative Auger emission (RAE) from lithiumlike20Ca17+ projectiles excited in collisions with He has been measured. The intensity of RAE photons relative to Kα x-ray emission is enhanced by a factor of 10 – 17 compared with theoretical calculations for ions with few electron vacancies. The enhancement of RAE for Ca17+ is consistent with the results reported previously for lithiumlike16S13+ and23V20+ and indicates a systematic dependence on Z. Both the enhancement and the relative RAE transition rate increase with Z.

Resonant transfer and excitation (RTE), resulting from simultaneous electron capture and K—shell excitation in a single collision, has been measured for 97 to 150 MeV/u U90+ ions in molecular hydrogen. This measurement, the first for a very heavy projectile, provides a test of relativistic dielectronic—recombination (DR) theory.

Resonant transfer and excitation (RTE), resulting from simultaneous electron capture and K-shell excitation in a single collision, has been measured for 97--150 MeV/u ${\mathrm{U}}^{90+}$ ions in hydrogen. Distinct maxima, attributed to RTE contributions from the formation of intermediate excited states, were observed superimposed on a monotonically decreasing background due to single-electron capture. This measurement, the first for a very heavy projectile, provides a test of relativistic dielectronic-recombination theory.

Charge-changing processes have been investigated for 267--320-MeV ${\mathrm{Ti}}^{19+}$ and ${\mathrm{Ti}}^{20+}$ ions channeled in a thin gold crystal. High-resolution measurements of the energy losses suffered by the charge-changed ions show that electron capture by the well-channeled particles is significantly enhanced when the projectile velocity corresponds to ejected KLL Auger-electron energies from doubly excited states in the rest frame of the ion. The measurements of dielectronic capture exhibit exceedingly narrow resonance widths and reveal a shift of the resonances toward higher energies.

Collisions in which a fast highly charged ion passes within the orbit of K electrons of a target gas atom are selected by emission of a K x‐ray from the projectile or target. Measurement of the projectile charge state after the collision, in coincidence with the K x‐ray, allows measurement of the charge‐transfer probability during these close collisions. When the projectile velocity is approximately the same as that of target electrons, a large number of electrons can be transferred to the projectile in a single collision. The electron‐capture probability is found to be a linear function of the number of vacancies in the projectile L shell for 47‐MeV calcium ions in an Ar target.

The predicted charge-state dependence of dielectronic-recombination cross sections involving \ensuremath{\Delta}n>=1 L-shell excitation has been tested by measurements of resonant transfer and excitation (RTE) for $_{41}\mathrm{Nb}^{\mathrm{q}+}$+${\mathrm{H}}_{2}$ (q=28 to 32) collisions at 3.7 and 4.0 MeV/u. The measured RTE cross sections increase significantly with increasing charge state and thus disagree substantially with theoretical calculations that predict essentially equal cross sections for ${\mathrm{Nb}}^{29+}$, ${\mathrm{Nb}}^{30+}$, and ${\mathrm{Nb}}^{31+}$. This disagreement is contrary to the good agreement previously found between theory and experiment for K-shell RTE.

Collisions in which a fast highly charged ion passes within the orbit of inner-shell electrons of a target gas atom are selected by emission of an X-ray from the projectile or target. Measurement of the projectile charge state after the collision, in coincidence with the X-ray, allows measurement of the charge-transfer probability during these close collisions. When the projectile velocity is approximately the same as that of the target electrons, a large number of electrons can be transferred to the projectile in a single collision. Results for 47–160 MeV Ca17+ + Ar show that the relative electron-capture probability in a close encounter decreases with increasing projectile velocity.

Measurements have been made of projectile X-ray spectra coincident with single electron loss in collisions of 3.5-9.0 MeV amu-1 23Vq+(q=19, 20, 21) ions with He targets under single collision conditions. Nonmonoenergetic X-rays observed in the coincidence spectra for V20+ (lithium-like) projectiles are attributed to the radiative Auger effect (RAE). The intensity of RAE photons relative to the characteristic K X-ray yield is more than an order of magnitude larger than expected from theoretical calculations and from earlier measurements for atomic targets.

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Coincidence measurements of charge transfer and simultaneous projectile electron excitation provide insight into correlated two-electron processes in energetic ion-atom collisions. Projectile excitation and electron capture can occur simultaneously in a collision of a highly charged ion with a target atom; this process is called resonant transfer and excitation (RTE). The intermediate excited state which is thus formed can subsequently decay by photon emission or by Auger-electron emission. Results are shown for RTE in both the K-shell of Ca ions and the L-shell of Nb ions, for simultaneous projectile electron loss and excitation, and for the effect of RTE on electron capture.

Observations of resonant electron transfer and L-shell excitation (RTE) using coincidence techniques are reported for 41Nb31+ and 57La40+ ions colliding with H2. The measured RTE cross sections are about ten times larger than those previously found for RTE involving the K shell and constitute as much as half of the total L alpha beta X-ray production. Overall agreement with theory is reasonable but discrepancies similar to those previously reported for K-shell RTE exist. The results demonstrate the importance of electron correlation in the production of L vacancies for projectile energies where RTE is possible.

X-ray emission associated with projectile charge-changing events in fast ion-atom collisions can be used to isolate and investigate excitation, ionization and charge transfer, as well as combinations of these processes. The major emphasis to date has been on the study of two-electron processes. Several such processes have been studied in detail. Resonant and nonresonant transfer and excitation (RTE and NTE) occur when electron capture and projectile excitation, with stabilization by X-ray emission, take place together in a single collision. Loss and excitation (LE) occurs when a projectile electron is removed and the ion is excited in a single collision. The cross sections for loss and excitation have been measured for a wide range of highly stripped projectiles over a broad energy range. The results shed light on projectile K X-ray production processes in heavy-ion collisions. The use of ion-X-ray coincidence techniques in exploring these and other collision processes will be discussed.

Recent experimental studies of resonant transfer and excitation (RTE) in ion-atom collisions are reviewed. In the RTE process correlated electron capture and projectile excitation occur together in a single encounter with a target atom. Measurements of Caq+ + H2 (q= 10–19) from 100 to 370 MeV establish the projectile charge-state dependence of K-shell RTE and provide a detailed test of the theory. Structure due to RTE is observed in the energy dependence of the total electron-capture cross sections for this collision system. A comparison of the Ca17++H2 data with previous results for Ca17++He demonstrates the effect of the target-electron momentum distribution on the RTE process. Studies of 230–610 MeV Nb31+ +H2 provide information about RTE involving projectile L-shell excitation. All the measurements are in reasonable agreement with theoretical calculations.