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The time profiles of Cu, Sn, and Ni ions extracted from a hot-cavity resonant ionization laser ion source are investigated. The ions are produced in the ion source by three-photon resonant ionization with pulsed Ti:Sapphire lasers. Measurements show that the time spread of these ions generated within laser pulses of about 30 ns duration could be larger than 100 μs when the ions are extracted from the ion source. A one-dimensional ion-transport model using the Monte Carlo method is developed to simulate the time dependence of the ion pulses. The prediction of the model agrees reasonably well with the experimental data. To reproduce the observed ion time profiles, we find it necessary to postulate that ion-wall collisions are suppressed inside the ion source by an undetermined ion confinement mechanism and that a substantial fraction of the extracted ions are generated in the vapor-transfer tube rather than the hot cavity. Three-dimensional modeling will be necessary to understand the strong reduction in losses expected from ion-wall collisions which we interpret as evidence for confinement.

An all-permanent magnet electron cyclotron resonance (ECR) ion source is tuned to create a variety of intense molecular ion beams for basic research. Based on simultaneous injection of several gases with spectroscopic high purity or enriched isotope content (e.g., H 2 , D 2 , N 2 , O 2 or CO) and low power microwave heating, the ECR ion source produces diatomic molecular ion beams of H 2 + , D 2 + , HD + , HO + , DO + , NH + , ND + and more complex polyatomic molecular ions such as H 3 + , D 3 + , HD 2 + , H 2 O + , D 2 O + , H 3 O + , D 3 O + , and NH n + , ND n + with n =2,3,4 and possibly higher. Molecular ion beams have been produced with very high current intensities compared to other molecular beam sources. The recorded molecular ion beam spectra are discussed.

Dissociative recombination of the Zundel cation D(5)O(2)(+) almost exclusively produces D + 2 D(2)O with a maximum kinetic energy release of 5.1 eV. An imaging technique is used to investigate the distribution of the available reaction energy among these products. Analysis shows that as much as 4 eV can be stored internally by the molecular fragments, with a preference for producing highly excited molecular fragments, and that the deuteron shows a nonrandom distribution of kinetic energies. A possible mechanism and the implications for these observations are addressed.

An ISOLDE-type hot-cavity laser ion source based on high-repetition-rate Ti:Sapphire lasers has been set up at the Holifield radioactive ion beam facility. To assess the feasibility of the all-solid-state laser system for applications at advanced radioactive ion beam facilities, spectroscopy and performance tests have been conducted with this source. The results of recent studies on excitation schemes, source efficiency, beam emittance and ion time structure are presented.

We present recent measurements of absolute electron-impact ionization cross sections for Be-like C iii ,N iv, and O vformingLi-likeCiv,Nv,andOvi.Themeasurementsweretakenusingthecrossed-beamsapparatusatOakRidge National Laboratory. A gas cell beam attenuation method was used to independently measure the metastable fractions present in theion beams. The measuredionization cross sections were compared with calculations usingtheR-matrix with pseudostates and distorted-wave theoretical methods. Best agreement is found with the R-matrix with pseudostates cross sections results that account for the metastable fractions inferred from the gas attenuation measurements. We present a set of recommended rate coefficients for electron-impact single ionization from the ground state and metastable term of each ion. Subject headingg atomic data — atomic processes — methods: laboratory

In preparation of a laser ion source, we have investigated multi-step laser ionization via Rydberg and autoionizing states for atomic Ni and Ge using a mass separator with an ion beam energy of 20 keV. For both elements resonant three-step excitation schemes suitable for modern Ti:sapphire laser systems were developed. Rydberg series in the range of principal quantum numbers 20 n 80 were localized, assigned and quantum numbers were allocated to the individual resonances. Ionization potentials (IP) were extracted from fits of the individual series and quantum defects of individual levels were analysed for confirmation of series assignment. For Ni the ionization potential could be extracted with significantly increased precision compared to literature with a value of EIP (Ni) = 61 619.77(14) cm −1 . Also, at least one notable autoionizing state above the first IP was discovered for both elements, and the different ionization schemes via Rydberg or autoionizing states were compared with respect to line shape, ionization efficiency and selectivity.

At ultrarelativistic energies, atomic collisions between heavy, highly-charged atoms produce extremely intense, rapidly varying electromagnetic fields that, in turn, give rise to very large ionization cross sections, diminished normal three-body electron capture cross sections, and copious lepton pair formation. Availability of beams of ultrarelativistic heavy ions at the CERN SPS enabled experimental studies of a variety of basic atomic collision phenomena in gases and solids, including radiative electron capture and electron capture from pair production, total energy loss, ionization and electron loss to continuum, and ion channeling. Results of some of these measurements and comparisons with theoretical treatments will be presented, as well as a number of remaining puzzles presented by the data.

The transmission of highly collimated multicharged ions through alumina nanocapillary arrays was studied using 200 MeV and 10- to 20-keV/q ion beams. The aligned nanopores each have a diameter and length of 100 nm and 60 μ, respectively. Emergent charge-state-selected angular distributions were measured at low energy. The distributions consist of sharp two-dimensional peaks that sit on a continuum distribution. Analysis of the results from target tilting experiments indicate that the observed structure can be explained simply by multiple collisions of the projectile with nanopore walls. We discuss these results and other effects produced by applying a fine wire mesh and Au thin films to the nanocapillary array.

A new 250 kV high-voltage platform has been installed at the ORNL multicharged ion research facility (MIRF) to extend the energy range of multicharged ions available for experimental investigations of their collisional interactions with electrons, atoms, molecules and solid surfaces. For the production of the multiply charged ions, a new all-permanent magnet electron cyclotron resonance (ECR) ion source, designed and fabricated at CEA/Grenoble, is being used. After a brief summary of the project background and the expanded research program made possible upon its completion, design details of the new platform, associated beam transport and control system are presented, together with information on the design and performance of the new ion source.

The paper reports an investigation of the dissociative recombination of at the heavy-ion storage ring CRYRING. The absolute cross-section has been measured as a function of centre-of-mass energy ...

Absolute cross sections for electron-impact dissociation of D13CO+ ions have been measured over a collision energy range from 4 to 100 eV with a crossed electron–ion beams method. Total experimental uncertainties are about 16% near the cross section peak. The role of resonant and direct dissociative excitation for energies below 21 eV is discussed in light of the energy levels and photo-dissociation cross sections of the HCO+ formyl cation predicted by ab initio multi-reference configuration interaction calculations.

In several recent dissociative recombination (DR) experiments, the observed DR products depend on the structure, bonding and charge center of the molecular ion. For examples, the dominant product channel observed in the DR of N2O+2, D+5, and D5O2+ suggests that the former two ions have the form NO+·NO, and D+3·D2, respectively, whilst the latter is known to have the form D2O·D+·D2O. We compare and contrast these observations by investigating the DR of one of the simplest such systems, Li+·H2. This system, a weakly bound cluster with the charge center located on the lithium atom, will provide us with an excellent opportunity for investigating the role played by the type of bonds and charge center in the DR process.

The scattering of multi-charged ions (MCI) from the [1 0 0] surface of a RbI single crystal has been studied using a 120° back-scattering technique in the keV energy regime. Scattered charge-state distributions were measured as a function of the polar incidence angle and the target azimuthal orientation. Preliminary results indicate that “quasi-binary” collisions associated with scattering from a Rb or I site can be clearly distinguished for each scattered final charge state. The charge-state distribution and the relative Rb/I binary collision yields for scattered ions are found to depend on both the beam incidence angle and the target azimuthal orientation. The preliminary RbI(1 0 0) results are discussed in the context of earlier experimental and theoretical studies of MCI scattering on a Au(1 1 0) surface.

Preliminary results are reported of projectile neutralization during 120° backscattering from RbI[1 0 0] of singly and multiply charged incident ions in the keV energy range. Scattered charge fractions are reported for 4.4 keV Ne8+ and 4.2 keV F+ normally incident on the ionic crystal. Collisions associated with scattering from a Rb or I site can be clearly distinguished for each scattered final charge state. Significant differences are observed in the intensities of the higher scattered charge states resulting from collisions with Rb and I sites. In contrast, at the target azimuth orientation of the present measurement, only minor differences in F− yield are observed for hard scattering from the two lattice sites.

We report on measurements of absolute scattered projectile charge fractions for Ar11+ ions with incident energies in the range 3–30 keV, that have been 120° back-scattered from CsI(1 0 0) in quasi-binary collisions. Use of a time-of-flight technique that incorporates a biased drift region permitted full separation of all scattered charged states, including neutrals. In contrast to our Ar11+ results for Au(1 1 0), the scattered neutral fraction is smaller, and relatively independent of incident projectile energy over the entire investigated range. In addition, we have measured, at a fixed energy of ∼5 keV, scattered charged state distributions as function of incident charge states in the range 1+ to 13+. In a separate measurement utilizing electrostatic instead of TOF analysis of the scattered charge states, we attempted to evaluate the effect of surface charging on energy loss of low energy scattered projectiles by absolute measurements of the scattered 1+ energies of incident Ar11+ ions incident on CsI(1 0 0) at energies down to 10 eV/q. Apart from small deviations from the elastic binary collision energy loss expected for large angle scattering, ascribable to the image charge interaction, no measurable effect due to surface charging was found down to the lowest investigated energies.

In this paper, we report results regarding how LiH2+ fragments as a result of a low-energy collision with an electron (dissociative recombination), a reaction that contains only elements and partic ...

The time delays in fission induced by bombardment of W with 180 MeV $^{32}\mathrm{S}$, $240\text{\ensuremath{-}}255$ MeV $^{48}\mathrm{Ti}$, $330\text{\ensuremath{-}}375$ MeV $^{58}\mathrm{Ni}$, and 390 MeV $^{74}\mathrm{Ge}$ have been measured by observation of crystal blocking. Nearly all results are consistent with exponential decay with lifetimes of order ${10}^{\ensuremath{-}18}$ s which depend weakly on the atomic number of the composite nucleus. This is inconsistent with the Bohr-Wheeler model of fission from a compound nucleus in statistical equilibrium at each stage in a neutron evaporation cascade and supports a picture of strongly damped quasifission. A simple diffusion model with one-body dissipation reproduces roughly the observed time scale and the exponential decay. It suggests that the outer fission barrier could play a significant role in the observed, very slow decays.

We have measured total nuclear disintegration cross sections for ${}^{208}\mathrm{Pb}$ ions at 33 TeV $(160\ensuremath{-}\mathrm{GeV}A)$ colliding with C, Si, Cu, Sn, and Pb. Using well established theory, we calculate the nuclear electromagnetic, electron electromagnetic, and the hadronic contributions and find that their sum underestimates the measured cross sections. An additive correction term linear in target ${Z}_{T}$ (i.e., ${120Z}_{T}\mathrm{mb}$) is necessary to bring agreement between theory and experiment. The source of this additional term is unknown.

Free positrons from electron-positron pairs produced by 33-TeV Pb{sup 82+} ions in Coulomb collisions with targets of carbon [(CH){sub x}], aluminum, palladium, and gold have been measured to determine cross sections and momentum distributions. Upper limits have been established for contributions from multiple pair formation. Comparison with similar data taken for 6.4-TeV S{sup 16+} ions shows that cross sections scale as the product of the squares of the projectile and target nuclear charges. Positron momentum distributions for S{sup 16+} and Pb{sup 82+} ions on all targets are observed to be similar, but indicate a tendency for higher-energy positron emission for the Pb{sup 82+} projectiles. {copyright} {ital 1997} {ital The American Physical Society}

We have used beams of 6.4 TeV S ions and 33.2 TeV Pb ions obtained from the CERN-SPS facility to study collision processes of heavy ions at ultrarelativistic energies. Measurements have been made of electron-positron pair production, and capture of an electron produced from the negative continuum onto bound states of the moving ion. Here we report on some aspects of these results. In a new formulation for the theory of electronic stopping power of ions at ultrarelativistic energies/Lindhard and Sorensen (LS) find that nuclear size effects should act to reduce the momentum transfer to electrons and hence the stopping power. Using the 33.2-Pb ions, we measured d E d x in C, Si, Cu, Sn and Pb targets. The LS theory was confirmed.

Recent atomic physics studies involving ultrarelativistic Pb ions required solid target positioners, scintillators, and a sophisticated data acquisition and control system placed in a remote location at the CERN Super Proton Synchrotron near Geneva, Switzerland. The apparatus, installed in a high-radiation zone underground, had to (i) function for months, (ii) automatically respond to failures such as power outages and particle-induced computer upsets, and (iii) communicate with the outside world via a telephone line. The heart of the apparatus developed was an Apple Macintosh-based CAMAC system that answered the telephone and interpreted and executed remote control commands that (i) sensed and set targets, (ii) controlled voltages and discriminator levels for scintillators, (iii) modified data acquisition hardware logic, (iv) reported control information, and (v) automatically synchronized data acquisition to the CERN spill cycle via a modem signal and transmitted experimental data to a remote computer. No problems were experienced using intercontinental telephone connections at 1200 baud. Our successful “virtual laboratory” approach that uses off-the-shelf electronics is generally adaptable to more conventional bench-type experiments.

Recent availability of relativistic and ultrarelativistic beams of heavy ions has permitted the first controlled studies of atomic collisions at energies sufficient to measure effects of several new basic phenomena. These include measurements substantiating recently predicted finite nuclear size effects resulting in a reduction in the total electronic energy loss of heavy ions in matter, and measurements of Coulomb collisions in which electrons are excited from the Dirac negative energy continuum. Measurements of total energy loss, free electron-positron pair production, and electron capture from pair production have been recently performed using 33-TeV Pb82+ ions from the CERN SPS accelerator in Geneva. Results of these studies are presented, along with comparisons with relevant theory.

We present a study into the collision-induced dissociation (possibly including electron stripping) of O-3(+) and N-3(+) with rest gas molecules (predominantly H-2) in the heavy-ion storage ring CRY ...

Channeled ions experience a restoring force from adjacent atomic rows or planes that govern their trajectories and hence limit their interactions to those with valence or conduction electrons. Trajectory information can be gained from emergence patterns and studies of radiative electron capture (REC) can give information on the electron momentum distributions and densities in the channel. With this knowledge, one can study ion-electron collisions in dense media; in particular, dielectronic excitation and recombination. The states of the energetic channeled ions can be influenced by the crystal field and by the wake field that follows them. They can also be perturbed by the periodic electromagnetic force with a frequency depending on their velocity and the periodicity of the lattice. When the frequency of the perturbation v caused by passing along a given crystal direction with atomic spacing d at velocity v reaches a resonance, v res = ( v d )K , where K = 1, 2, 3, …, resonant coherent excitation (RCE) can occur to non-degenerate eigenstates of the ion. Ions in these excited states can then be ionized by the channel electrons or, if not ionized, can escape from the crystal and radiate. RCE has been studied in both axial and planar channeling by varying the ion velocity or, in the case of planar channeling, by varying the apparent interatomic spacing by changing the angle with respect to an axis. In both cases, information can be gained concerning the states of channeled ions and the nature of the crystal channel.

The ratio {ital R}({Delta}{ital E}) of double to single ionization of He by fast proton impact has been measured as a function of energy transfer ({Delta}{ital E}). While {ital R}({Delta}{ital E}) is observed to be nearly independent of proton energy (1{endash}6MeV) within experimental error, it decreases with increasing energy transfer, from 2{percent} at {Delta}{ital E}=1 keV to below 1{percent} at {Delta}{ital E}=10 keV. Further comparisons of these ratios with those obtained from photoization and Compton scattering are made. {copyright} {ital 1996 The American Physical Society.}

Interest in astrophysical sources of charge exchange X-rays has been growing steadily since the discovery of X-ray emission from the comet Hyakutake with ROSAT in 1996. Since then, charge exchange has been observed between solar wind ions and neutrals in the geocorona and in the atmospheres of Mars and Jupiter. Charge exchange with interstellar neutrals within the heliosphere between solar wind ions and neutral hydrogen and helium from the interstellar medium is now acknowledged as contributing a considerable (although currently unknown) fraction of the soft X-ray background. We make a brief survey of the heliospheric, Galactic, and extragalactic systems in which charge exchange has been observed or is predicted to take place. Experiments measuring velocity dependent cross-section and line ratios for Lyman-series lines and He-like triplets are needed to check current theoretical models of charge exchange emission and aid interpretation of observations. We point out a number of systems that are of astrophy...

Hydrogenic ions penetrating a crystal are perturbed by the frequency with which they pass atoms lying in rows or ordered planes. This frequency, when resonant, can cause excitation of the ion with subsequent collisional ionization or radiative relaxation following escape from the crystal. The various possible m states being non-degenerate in the channel are separable. The associated alignment is observed in both the separate ionization probabilities and X-ray anisotropy.

The projectile energy loss for 7.5--25-MeV C[sup 6+,5+] and F[sup 6+] ions was measured for single collisions with He, Ne, Ar, and Kr as a function of the recoil-ion charge state and the projectile scattering. This energy loss was measured for collisions in which the projectile captured an electron and for those involving just direct ionization. We investigated and found a large average energy transfer (100--250 eV/electron) to the continuum electrons. A strong increase of the scattering angle with recoil-ion charge state was observed for both capture and direct ionization. The results imply that, for smaller impact parameters, higher recoil-ion charge states are produced and that higher energy losses are obtained. We observed a weak target-[ital Z] dependence of the energy loss. The results are compared with [ital n]-body classical-trajectory Monte Carlo calculations by Olson, semiclassical-approximation calculations by Schuch [ital et] [ital al]. [Nucl. Instrum. Methods Phys. Res. Sect. B 42, 566 (1989)], and the energy-deposition model.

Time delays in fission induced by bombardment of Mo with 170- and 180-MeV ${}^{32}$S, 225- and 240-MeV ${}^{48}$Ti, and 300-MeV ${}^{58}$Ni have been measured by observation of crystal blocking of fission fragments. In contrast to earlier measurements with a W target, the results are consistent with fission of a compound nucleus in competition with mainly neutron emission. Most of the fissions happen on a time scale much shorter than attoseconds but there is a significant component of fission with much longer lifetimes. The measurements are reproduced with a standard statistical model, including a Kramers correction to fission widths from the viscosity of hot nuclear matter. These new results support the interpretation of our earlier measurements with a W target, which indicate that there is a transition in heavy-ion-induced fission at large atomic number and mass, from multichance fission in the standard Bohr-Wheeler picture to fission without formation of a compound nucleus. The process is slowed down by nuclear viscosity, with measured delays of order attoseconds.

Absolute cross sections for electron-impact dissociation of N${}_{2}$D${}^{+}$ producing N${}_{2}^{+}$, ND${}^{+}$, and N${}^{+}$ ion fragments were measured in the 5- to 100-eV range using a crossed electron-ion beams technique. In the 5- to 20-eV region, in which dissociative excitation (DE) is the principal contributing mechanism, N${}_{2}^{+}$ production dominates. The N${}_{2}^{+}$ + D dissociation channel shows a large resonant-like structure in the DE cross section, as observed previously in electron impact dissociation of triatomic dihydride species [M. Fogle, E. M. Bahati, M. E. Bannister, S. H. M. Deng, C. R. Vane, R. D. Thomas, and V. Zhaunerchyk, Phys. Rev. A 82, 042720 (2010)]. In the dissociative ionization (DI) region, 20- to 100-eV, N${}_{2}^{+}$, ND${}^{+}$, and N${}^{+}$ ion fragment production are comparable. The observance of the ND${}^{+}$ and N${}^{+}$ ion fragments indicate breaking of the $\mathrm{N}\phantom{\rule{-0.16em}{0ex}}\ensuremath{-}\phantom{\rule{-0.16em}{0ex}}\mathrm{N}$ bond along certain dissociation channels.

We have observed selection rules for the resonant excitation of swift hydrogenlike ions ([ital Z]=6,7) axially channeled in the [l angle]111[r angle] direction of a diamond lattice (silicon). The resonant excitation arises from a coherent periodic perturbation by the atoms in the bounding crystal rows. We show that the selection rules are the consequence of destructive and constructive interference in the quantum mechanical transition amplitude for excitation.

Absolute cross sections for electron-impact dissociation of O3+ ions yielding O+ and O2+ fragment ions have been measured using a crossed electron-ion beams method for energies from about 3 eV to 100 eV. While the O2+ channel dominates the dissociation cross section over the measured energy range, a strong enhancement is observed in the O+ channel at low energy.

Absolute cross sections for electron-impact dissociation of XH{sub 2}{sup +} (X= B, C, N, O, F) producing XH{sup +} and X{sup +} ion fragments were measured in the 3- to 100-eV range using a crossed-electron-ion beams technique. Dissociative excitation of BD{sub 2}{sup +} and CH{sub 2}{sup +} producing B{sup +} and C{sup +}, respectively, show a propensity toward a two-body dissociation while the remaining species all tend to show a three-body dissociation dynamic. The BD{sup +} and CH{sup +} dissociative excitation channels show a large resonant-type contribution to the cross sections at {approx}10 eV. For the X{sup +} fragment ion production cross sections, a clear dependence on the threshold energy, as it relates to the rate of rise in the cross section above threshold, is observed.

We investigate the use of energetic electron beams for high-resolution radiography of flaws embedded in thick solid objects. A bright, monoenergetic electron beam (with energy >100 MeV) was generated by the process of laser-wakefield acceleration through the interaction of 50-TW, 30-fs laser pulses with a supersonic helium jet. The high energy, low divergence, and small source size of these beams make them ideal for high-resolution radiographic studies of cracks or voids embedded in dense materials that are placed at a large distance from the source. We report radiographic imaging of steel with submillimeter resolution.

The time spreads of Mn ions produced by three-photon resonant ionization in a hot-cavity laser ion source are measured. A one-dimensional ion-transport model is developed to simulate the observed ion time structures. Assuming ions are generated with a Maxwellian velocity distribution and are guided by an axial electric field, the predictions of the model agree reasonably well with the experimental data and suggest that the ions are radially confined in the ion source and a substantial fraction of the ions in the transport tube are extracted.

Angular and momentum distributions have been measured for positrons from electron-positron pairs created in peripheral collisions of 6.4-TeV sulfur ions with fixed targets of Al, Pd, and Au. The data are compared with results of several theoretical treatments. Measured differential cross sections peak at low momentum ({le}1 MeV/{ital c}), extend significantly to much higher momenta ({gt}17 MeV/{ital c}), and concentrate sharply in the forward direction, along the ion-beam axis. Positron yields scale as the square of the target nuclear charge as predicted by theory.

X-rays generated by fully stripped titanium ions channeled along the 〈100〉 axis of a thin gold single crystal have been measured in coincidence with charge-changed ions as a function of ion energy loss. X-rays from the process of radiative electron capture are used to derive mean target electron densities for three values of mean energy loss. These densities are compared with densities calculated from a free-electron gas model for stopping. The results are used in a model calculation for Lyman series X-ray emission yields which includes effects of strong competition from secondary electron impact excitation and ionization of excited-state hydrogenic ions formed by electron capture in the crystal.

Using a crossed electron-ion beams method, we measured absolute cross sections for electron-impact dissociation of the CD3+ molecular ions producing CD2+ fragment ions and CH3+ ions yielding CH+ and C+ fragment ions over a collision energy range from a few eV up to 100 eV. The total experimental uncertainties are about 12% at the maximum of the curves of cross sections (peak of the cross section, for the CH+ channel). The obtained results suggest important roles played by pre-dissociation of bound states in the production of both the CH+ and C+ fragment ions. Good agreement is found with other results reported for the CH+ fragment, but some differences are found for the CD2+ and C+.

A resonant ionization laser ion source based on all‐solid‐state, tunable Ti:Sapphire lasers is being developed for the production of pure radioactive ion beams. It consists of a hot‐cavity ion source and three pulsed Ti:Sapphire lasers operating at a 10 kHz pulse repetition rate. Spectroscopic studies are being conducted to develop ionization schemes that lead to ionizing an excited atom through an auto‐ionization or a Rydberg state for numerous elements of interest. Three‐photon resonant ionization of 12 elements has been recently demonstrated. The overall efficiency of the laser ion source measured for some of these elements ranges from 1 to 40%. The results indicate that Ti:Sapphire lasers could be well suited for laser ion source applications. The time structures of the ions produced by the pulsed lasers are investigated. The information may help to improve the laser ion source performance.

Dielectronic excitation of H-like S, Ca and Ti is shown to occur in the dense electron gas of a crystal channel. Cross sections for collisional ionization of the short lived excited states can then be determined. Ionic excitation can also be achieved by resonant coherent excitation in which case specific m states can be excited for further study.

Dissociative recombination of BH$_2^+$: the dominance of two-body break-up and an understanding of the fragmentation

The dissociative recombination of the ozone cation has been studied at the heavy-ion storage ring CRYRING. The total cross section and branching fractions have been measured. The cross section from {approx}0eV to 0.2 eV follows a nearly E{sup -1} dependence, which was theoretically predicted to be a characteristic of the direct dissociative recombination mechanism. The thermal rate coefficient has been deduced from the cross section to be 7.37x10{sup -7}(T/300){sup -0.55}cm{sup 3}s{sup -1}. The branching fraction analysis carried out at {approx}0eV interaction energy has shown a strong propensity (94%) to dissociate through the three-body channel. Due to the overwhelming dominance of this channel it has been investigated in more detail. Of the six energetically available three-body pathways only three are significantly populated, such that the production of O(S1) is highly unfavored and all atomic oxygen fragments are predominantly formed in P3 and D1 states. Analysis of the breakup geometries has been performed by means of the Dalitz plot. It is observed that the molecules dissociating through the O(P3)+O(P3)+O(P3) and O(P3)+O(P3)+O(D1) channels have an open linear geometry where the cleavage of two valence bonds occurs preferentially in unison, while the O(P3)+O(D1)+O(D1) breakup might proceed partly through a sequential mechanism.

Autoionization of He atoms following double excitation by electrons, protons, C{sup {ital Q}+} ({ital Q}=4--6), and F{sup {ital Q}+} ({ital Q}=7--9) ions has been studied. The electron-emission yields from the doubly excited 2{ital s}{sup 2}({sup 1}{ital S}), 2{ital s}2{ital p}({sup 1}{ital P}), and 2{ital p}{sup 2}({sup 1}{ital P}) states were measured at the reduced projectile energy of 1.5 MeV/nucleon for observation angles between 10{degree} and 60{degree}. The results indicate excitation to the 2{ital s}{sup 2}({sup 1}{ital S}) and 2{ital p}{sup 2}({sup 1}{ital D}) states increases as approximately {ital Q}{sup 3}, while excitation to the 2{ital s}2{ital p}({sup 1}{ital P}) state varies as approximately {ital Q}{sup 2}, where {ital Q} is the charge of the projectile. These charge dependences are significantly less than the {ital Q}{sup 4} dependence expected in the independent-electron model, suggesting the interaction between the two target electrons is important in creating the doubly excited states.

We report cross sections for ionization of He coincident with electron loss from He, Li, C, O, and Ne pro jectiles. For He, Li, C, and O pro jectiles, the cross sections were measured directly, while the Ne cross sections were obtained by transforming results for He pro jectiles colliding with Ne. We find that, at energies of about 100-500 keV/u, neutral pro jectiles can ionize a He target almost as effectively as a charged pro jectile. The contribution to ionization due to electron-electron interactions is found to scale with the number of available pro jectile electrons. Comparing ionization by the bound electrons on projectiles to ionization by free electrons, we find the cross sections for ionization by bound electrons are systematically smaller than those for free electrons.

The time delay in fission induced by bombardment of W with 180 MeV $^{32}\mathrm{S}$, 240--255 MeV $^{48}\mathrm{Ti}$, and 315--375 MeV $^{58}\mathrm{Ni}$ has been measured by observation of crystal blocking. There is a clear narrowing and a small increase in the minimum yield of the angular dips for fission compared with scaled dips for elastically scattered ions. This is interpreted as a fission delay of about 2 as, only weakly dependent on energy and atomic number. The delay is longer by 1 to 2 orders of magnitude than obtained from standard interpretations of measurements of prescission neutrons and giant-dipole-resonance gamma rays and from calculations of the nuclear dynamics in heavy-ion reactions.

We report the first observation of almost exclusive three-body breakup in the dissociative recombination of a covalent triatomic molecular ion O3+. The three-body channel, constituting about 94% of the total reactivity, has been investigated in detail. The atomic fragments are formed in only the first two electronic states, 3P and 1D, while formation in the 1S state has not been observed. The breakup predominantly proceeds through dissociative states with linear geometry.

Absolute cross sections for electron-impact dissociation of CH2+ producing CH+ and C+ fragment ions were measured in the 3-100 eV range using a crossed electron-ion beams technique with total uncertainties of about 11% near the cross section peak. The cross sections are nearly identical for energies above 15 eV, but they are dramatically different at lower energies. The CH+ channel exhibits a strong peak rising from an observed threshold of about 6 eV; the C+ channel is relatively flat down to the lowest measured energy. Ionization cross sections for the CH2+ ion are also presented.