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Physics with fast molecular-ion beams. Proceedings of workshop held at Argonne National Laboratory, August 20-21, 1979. [Workshop]

Description: The Workshop on Physics with Fast Molecular-Ion Beams was held in the Physics Division, Argonne National Laboratory on August 20 and 21, 1979. The meeting brought together representatives from several groups studying the interactions of fast (MeV) molecular-ion beams with matter. By keeping the Workshop program sharply focussed on current work related to the interactions of fast molecular ions, it was made possible for the participants to engage in vigorous and detailed discussions concerning such specialized topics as molecular-ion dissociation and transmission, wake effects, ionic charge states, cluster stopping powers, beam-foil spectroscopy, electron-emissions studies with molecular-ion beams, and molecular-ion structure determinations.
Date: January 1, 1979
Creator: Gemmell, D.S. (ed.)
Partner: UNT Libraries Government Documents Department

Coulomb-explosion technique for determining geometrical structures of molecular ions

Description: Traditional experimental techniques (e.g. studies on photon absorption or emission) for determining the sterochemical structures of neutral molecules are extremeley difficult to apply to molecular ions because of problems in obtaining a sufficient spatial density of the ions to be studied. Recent high-resolution measurements on the energy and angle distributions of the fragments produced when fast (MeV) molecular-ion beams from an electrostatic accelerator dissociate (Coulomb explode) in thin foils and in gases, offer promising possibilities for deducing the sterochemical structures of the molecular ions constituting the incident beams. Bond lengths have been determined in this way for several diatomic projectiles (H/sub 2//sup +/, HeH/sup +/, CH/sup +/, NH/sup +/, OH/sup +/, N/sub 2//sup +/, O/sub 2//sup +/, etc.) with an accuracy of approx. 0.01 A. H/sub 3//sup +/ has been demonstrated (for the first time) to be equilateral triangular and the interproton distance measured. Measurements on single fragments from CO/sub 2//sup +/, N/sub 2/O/sup +/, C/sub 3/H/sub 3//sup +/, and CH/sub n//sup +/ have revealed the gross structures of the projectiles. An apparatus has recently been constructed at Argonne to permit precise measurements on fragments in coincidence. The apparatus has been tested on a known structure (OH/sub 2//sup +/). The O-H bond length was found to be 1.0 +- 0.04 A and the H-O-H bond angle was measured as 110 --- 2/sup 0/. These values are in excellent agreement with those found in optical experiments (0.999 A and 110.5/sup 0/). This Coulomb explosion technique can be expected to be refined in accuracy and to be extended to a wide range of molecular ions whose structures are inaccessible by other means.
Date: January 1, 1981
Creator: Gemmell, D.S.
Partner: UNT Libraries Government Documents Department

Using an electrostatic accelerator to determine the stereochemical structures of molecular ions

Description: Recent high-resolution measurements on the energy and angle distributions of the fragments produced when fast (MeV) molecular-ion beams from an electrostatic accelerator dissociate (Coulomb explode) in thin foils and in gases, offer promising possibilities for deducing the stereochemical structures of the molecular ions constituting the incident beams. Bond lengths have been determined in this way for several diatomic projectiles (H/sub 2//sup +/, HeH/sup +/, CH/sup +/, NH/sup +/, OH/sup +/, N/sub 2//sup +/, O/sub 2//sup +/, etc.) with an accuracy of approx. 0.01 A. H/sub 3//sup +/ has been demonstrated (for the first time) to be equilateral triangular and the interproton distance measured. Measurements on single fragments from CO/sub 2//sup +/, N/sub 2/O/sup +/, C/sub 3/H/sub 3//sup +/, and CH/sub n//sup +/ have revealed the gross structures of the projectiles. An apparatus has recently been constructed at Argonne to permit precise measurements on fragments in coincidence. The apparatus has been tested on a known structure (OH/sub 2//sup +/). The O-H bond length was found to be 1.0 +- 0.04 A and the H-O-H bond angle was measured as 110 +- 2/sup 0/. These values are in excellent agreement with those found in optical experiments (0.999 A and 110.5/sup 0/). This Coulomb explosion technique can be expected to be refined in accuracy and to be extended to a wide range of molecular ions whose structures are inaccessible by other means.
Date: January 1, 1980
Creator: Gemmell, D.S.
Partner: UNT Libraries Government Documents Department

Determination of molecular-ion structures through the use of accelerated beams

Description: In this talk we report on recent research on molecular-ion structures using fast molecular-ion beams provided by Argonne's 5-MV Dynamitron accelerator. The method has become known as the ''Coulomb-explosion'' technique. When molecular-ion projectiles travelling at velocities of a few percent of the velocity of light strike a foil, the electrons that bind the molecular projectiles are almost always totally stripped off within the first few Angstroms of penetration into the solid target. This leaves a cluster of bare (or nearly bare) nuclei which separate rapidly as a result of their mutual Coulomb repulsion. This violent dissociation process in which the initial electrostatic potential energy is converted into kinetic energy of relative motion in the center-of-mass, has been termed a ''Coulomb explosion.'' 4 refs., 2 figs.
Date: January 1, 1987
Creator: Gemmell, D.S.
Partner: UNT Libraries Government Documents Department

Nuclear transitions induced by synchrotron x-rays.

Description: We discuss two rare but interesting processes by which synchrotron x-rays with energies up to about 100 keV may be used to induce nuclear transitions. In the NEET (Nuclear Excitation by Electronic Transition) process, an intense x-ray beam is employed to make vacancies, e.g. K-holes, in the atoms of a specific nuclear isotope. When a vacancy is filled by an electronic transition from a higher atomic level, there is some probability that instead of the usual x-ray or Auger emission, the nucleus of the atom itself will be excited. This is then followed by a nuclear decay exhibiting characteristic gamma-rays or other types of radiation, with time delays typical of the nuclear states involved. The probability for NEET increases when the energies of the atomic and the nuclear transitions become close. We address some theoretical aspects of the process and describe experimental efforts to observe it in {sup 189}Os and {sup 197}Au. The second process to be discussed is the possibility of ''triggering'' the decay of a nuclear isomer by irradiation with an x-ray beam. We focus on the case of the 31-year, 2.4-MeV, 16+ isomer of {sup 178}Hf. There has been speculation that if one could isolate gram quantities, say, of this isomer and then have the capability to accelerate its decay in a controlled way, one would have a powerful triggerable source of enormous energy. This could be used to generate explosions, for rapid irradiations, or for more general energy-storage applications, depending on the rate of energy release. We describe attempts to observe this process.
Date: June 5, 2002
Creator: Gemmell, D. S.
Partner: UNT Libraries Government Documents Department

Collision induced fragmentation of fast molecular ions in solids and gases. [Review, wake effects, excited states]

Description: A brief review is given of recent high resolution measurements on fragments arising from the collision-induced dissociation of fast (MeV) molecular ions. For solid targets, strong wake effects are observed. For gaseous targets, excited electronic states of the projectile ions play an important role. Measurements of this type provide useful information on the charge states of fast ions traversing matter. The experimental techniques show promise as a unique method for determining the geometrical structures of the molecular-ion projectiles. 41 references.
Date: January 1, 1979
Creator: Gemmell, D S
Partner: UNT Libraries Government Documents Department

Interactions of fast atomic and molecular ions with matter

Description: Brief discussions are presented of research performed in the following areas: microwave field ionization of fast Rydberg atoms; electric field ionization of foil-excited Rydberg states of fast heavy ions; post-foil interaction in foil-induced molecular dissociation; and angular distributions of foil-excited ions bearing inner shell vacancies. 19 references. (GHT)
Date: January 1, 1984
Creator: Kanter, E.P.; Gemmell, D.S. & Vager, Z.
Partner: UNT Libraries Government Documents Department

Dissociation of fast molecular-ion beams in thin foils. [1 MeV/nucleon]

Description: Distributions in angle and in energy were measured for the break-up fragments produced when fast (approximately MeV/nucleon) beams of molecular ions (H/sub 2//sup +/, HeH/sup +/, OH/sup +/, etc.) bombard thin foil targets. Studies were made using both amorphous and crystalline targets. The results display some dramatic departures from expectations based on a simple Coulomb explosion picture in which a cluster of nuclei stripped of their binding electrons move apart under the influence of their mutual Coulomb repulsion. An explanation for these differences is offered in terms of an interaction between the projectiles and polarization oscillations induced in the target by the passage of the clusters. A theoretical model is outlined which not only accounts well for the total electronic stopping power of the solid (due to the braking effect of the polarization charges) but also gives good agreement with the experimental results on dissociation of fast molecular-ion beams. Data obtained for the heavier molecules show interesting possibilities for determining effective charges inside solids.
Date: January 1, 1976
Creator: Gemmell, D. S.; Vager, Z. & Zabransky, B. J.
Partner: UNT Libraries Government Documents Department

Interactions of fast molecular-ion beams with matter

Description: Topics covered include: contribution of field-ionized Rydberg atoms to convoy electron spectra; microwave field ionization of fast Rydberg atoms; coherent Stark states in foil-excited fast Rydberg atoms; and equilibration lengths of K-Vacancy production in solids. (GHT)
Date: January 1, 1983
Creator: Gemmell, D.S.; Kanter, E.P. & Schneider, D.
Partner: UNT Libraries Government Documents Department

Ion-source dependence of the distributions of internuclear separations in 2-MeV HeH/sup +/ beams

Description: Experiments involving the use of MeV molecular-ion beams have yielded new information on atomic collisions in solids. A central part of the analyses of such experiments is a knowledge of the distribution of internuclear separations contained in the incident beam. In an attempt to determine how these distributions depend on ion-source gas conditions, we have studied foil-induced dissociations of H/sub 2//sup +/, H/sub 3//sup +/, HeH/sup +/, and OH/sub 2//sup +/ ions. Although changes of ion-source gas compositions and pressure were found to have no measurable influence on the vibrational state populations of the beams reaching our target, for HeH/sup +/ we found that beams produced in our rf source were vibrationally hotter than beams produced in a duoplasmatron. This was also seen in studies of neutral fragments and transmitted molecules.
Date: January 1, 1981
Creator: Kanter, E.P.; Gemmell, D.S.; Plesser, I. & Vager, Z.
Partner: UNT Libraries Government Documents Department

Atomic physics with hard X-rays from high brilliance synchrotron light sources

Description: A century after the discovery of x rays, the experimental capability for studying atomic structure and dynamics with hard, bright synchrotron radiation is increasing remarkably. Tempting opportunities arise for experiments on many-body effects, aspects of fundamental photon-atom interaction processes, and relativistic and quantum-electrodynamic phenomena. Some of these possibilities are surveyed in general terms.
Date: August 1, 1996
Creator: Southworth, S. & Gemmell, D.
Partner: UNT Libraries Government Documents Department

Some new effects seen in the passage of swift ion clusters through solids

Description: Distributions in angle and in energy have been measured for ions transmitted through thin solid targets bombarded by various molecular-ion beams. The results, which differ markedly from those expected on the basis only of a simple ''Coulomb explosion,'' indicate the existence of an additional force tending to align the internuclear vectors of each cluster with the beam direction. An explanation is suggested in terms of a potential attributable to the plasma oscillations generated by the particles as they traverse the solid. Calculations are presented to substantiate this idea. (auth)
Date: January 1, 1975
Creator: Gemmell, D.S.; Remillieux, J.; Poizat, J.C.; Gaillard, M.J.; Holland, R.E. & Vager, Z.
Partner: UNT Libraries Government Documents Department

The (178m2)Hf Controversy

Description: Since its discovery in the 1960's the {sup 178m2}Hf isomer has garnered high attention from both the basic and applied communities in nuclear science. It's combination of high spin (16+), long half life (31 yrs), and high excitation energy (2.446 MeV) offer unique possibilities as an energy storage medium. Interest in the isomer was rekindled beginning in 1999 when a series of publications began to appear from a group (referred to here as the ''Texas collaboration'') primarily based at the University of Texas, Dallas [1]. They reported observations that some of the stored energy could be released (''triggered'') when the isomer was exposed to a fluence of photons in the energy range {approx}10 to {approx}60 keV. The implications of this observation are profound. Even though the claimed cross section for the process was {approx}7 orders of magnitude greater than would be predicted from the known systematics of photon absorption by nuclei in this mass range [2], such a highly efficient method for triggering the isomeric deexcitation immediately suggested applications utilizing the explosive or the controlled gradual energy release from a very compact source. The prospect of such applications has focused considerable interest on realizing the promise that is implicit in the reported observations. However, two experiments performed by a group from ANL/LANL/LLNL at the Advanced Photon Source at Argonne (the ''APS collaboration'') reported negative results for the observation of any photon-triggered deexcitation of the {sup 178m2}Hf isomer [3]. This has led to a continued controversy, where both sides have adamantly defended their observations. At this point an outsider has difficulty determining whether there is indeed a triggering effect that should be pursued energetically with substantial resources, or whether the phenomenon consists of overly optimistic interpretation of data.
Date: July 24, 2003
Creator: Becker, J A; Gemmell, D S; Schiffer, J P & Wilhelmy, J B
Partner: UNT Libraries Government Documents Department

Experimental confirmation of the Jahn-Teller distortion of CH/sub 4//sup +/

Description: Measured energy and angular distributions are reported for H/sup +/ and C/sup n+/ (n = 2,3,4) fragments resulting from the collisional dissociation of 200-keV/amu CH/sub m//sup +/ (m = 0 to 4) in thin carbon targets. From the systematic trends of these Coulomb explosion spectra, qualitative information can be obtained on the structures of these species. In particular, the series displays a narrowing of the carbon angular and energy widths as protons are symmetrically added around a central carbon atom and provide a focusing effect. Because of the Jahn-Teller distortion, the carbon width in CH/sub 4//sup +/ is dramatically increased. 8 references.
Date: January 1, 1979
Creator: Gemmell, D S; Kanter, E P & Pietsch, W J
Partner: UNT Libraries Government Documents Department

UV-laser photofragmentation of a 2-MeV H/sub 2//sup +/ beam

Description: A 2-Mev beam of H/sub 2//sup +/ molecular ions was excited by 193-nm photons from an ArF laser. The H/sup +/ photofragments were energy-analyzed by an electrostatic analyzer and the potential energy released in the H/sub 2//sup +/ rest frame was determined. The released energy corresponds to absorption from low-lying vibrational levels of the ls sigma/sub g/ ground state to the 2p sigma/sub u/ state of H/sub 2//sup +/.
Date: January 1, 1981
Creator: Cue, N.; Edwards, A.K.; Gemmell, D.S.; Kanter, E.P. & Kutina, R.
Partner: UNT Libraries Government Documents Department

Physics with fast molecular ions

Description: Argonne's 5-MV Dynamitron accelerator is currently being used to study the interactions of fast (MeV) atomic and molecular ions with matter. The present emphasis of the work is upon the problem of determining the stereochemical structures of molecular-ion projectiles. The technique is based upon the ''Coulomb explosion'' of these projectiles when they strike a thin-foil target. By making careful determinations of the momenta of all of the fragments emitted from these Coulomb explosions, one can infer the original configuration of the molecular constituents in the projectile and, in favorable cases, their vibrational motions. This technique is particularly useful for those cases where the more traditional spectroscopic approaches cannot be applied. During the past year, new ideas and developments in the Coulomb-explosion technique have come to fruition. Radically new multiparticle detectors have been developed and implemented. Extremely thin film targets have been produced which reduce the ''blurring'' effect of multiple scattering, and a low-excitation ion source has been installed. These developments have resulted in direct measurements of the complete stereostructures of several small polyatomic molecular ions. In this paper we describe the experimental techniques, discuss some of the difficulties, and present some recent results. 19 refs., 5 figs.
Date: January 1, 1987
Creator: Gemmell, D.S.; Faibis, A.; Kanter, E.P.; Vager, Z. & Zabransky, B.J.
Partner: UNT Libraries Government Documents Department

Determination of molecular-ion structures through studies of the collisionally-induced dissociation of fast (MeV) molecular ions

Description: High-resolution studies of the angle and energy distributions of fragments detected in coincidence from the foil- and gas-induced dissociation of fast (MeV) polyatomic molecular ions are described. The aim of the measurements is to explore the possibilities that these new techniques offer for determining the stereochemistry of molecular ions.
Date: January 1, 1979
Creator: Gemmell, D S; Cooney, P J & Kanter, E P
Partner: UNT Libraries Government Documents Department

Recent work with fast molecular-ion beams at Argonne National Laboratory

Description: Research in these areas during 1979 is summarized: (a) studies of molecular-ion dissociation in gaseous targets, (b) developing an understanding of the origins of central peaks and of the two phenomena of the transmission of fast molecular ions through thin foil targets and of the production of neutral fragments from collision-induced dissociation of fast molecular projectiles, (c) studies exploring the extent to which high-resolution measurements on dissociation fragments can be used to determine the stereochemical structures of the molecular ions in the incident beam, (d) extensive modifications to the beam-line and apparatus at the 4-MV Dynamitron so as to permit a wide variety of coincidence measurements on fragments from collision-induced molecular-ion dissociation.
Date: January 1, 1979
Creator: Cooney, P.J.; Gemmell, D.S.; Groeneveld, K.O.; Kanter, E.P.; Pietsch, W.J.; Vager, Z. et al.
Partner: UNT Libraries Government Documents Department

Precise tests of x-ray scattering theories in the Compton regime.

Description: The authors report two experiments intended to test the accuracy of state-of-the-art theoretical predictions for x-ray scattering from low-Z atoms. The first one deals with the differential x-ray scattering cross sections in Ne and He from 11-22 keV and the Ne Compton-to-Rayleigh scattering ratio in this energy range. It was found that, in order to be consistent with the experimental results, an accurate description at low Z must include nonlocal exchange, electron correlation, and dynamic effects. The second experiment concerns the ratio of helium double-to-single ionization for Compton scattering in the 8-28 keV energy range where published experimental and theoretical results so far fail to give a consistent picture. The progress of the experiment and the data analysis is reported.
Date: January 15, 1999
Creator: Dunford, R. W.; Gemmell, D. S.; Kanter, E. P.; Krassig, B.; Southworth, S. H. & Young, L.
Partner: UNT Libraries Government Documents Department

Ion charge-state production and photoionization near the K edge in argon and potassium

Description: We have measured the time-of-flight charge distributions of ions of argon and potassium following x-ray absorption at energies near their respective K edges. We confirm previously observed enhancements of the higher charge states at energies up to 100 eV below the K edge in argon. The measurements confirm recent calculations suggesting excitation of a virtual 1s state in this energy range.
Date: October 1, 1994
Creator: Berry, H. G.; Azuma, Y.; Cowan, P. L.; Gemmell, D. S.; LeBrun, T. & Amusia, M. Y.
Partner: UNT Libraries Government Documents Department

Neutral hydrogen from the foil-induced dissociation of /sup 4/HeH/sup +/, /sup 3/HeH/sup +/, and H/sub 2//sup +/. [170 to 1800 keV/amu]

Description: High resolution energy spectra and angular distributions of H/sup 0/ from the dissociation of /sup 4/HeH/sup +/, /sup 3/HeH/sup +/, and H/sub 2//sup +/ in thin carbon foils are presented for incident ion energies ranging from 170 to 1800 keV/amu. For the range of dwell times inside the target foils studied (about 1 to 10 fsec), the dominant mechanism for such dissociation is the Coulomb explosion of the ions inside the target followed by electron capture near the rear surface of the foil. At the shortest of these dwell times, the yield of H/sup 0/ for incident ions oriented perpendicular to the beam relative to that for ions aligned with the beam is a factor of two greater than at longer dwell times. These data indicate that, for ion fragments which emerge from the rear surface of the target within a few angstroms of one another, those in the perpendicular orientation have a greater electron capture probability than those emerging in other orientations. 9 references.
Date: January 1, 1979
Creator: Cooney, P J; Gemmell, D S; Kanter, E P; Pietsch, W J & Zabransky, B J
Partner: UNT Libraries Government Documents Department

Measurement of the distributions of internuclear separations in 3. 0-MeV H/sub 2//sup +/ and 3. 63-MeV HeH/sup +/ beams

Description: Angular distributions of charged dissociation fragments are measured for 3.0-MeV H/sub 2//sup +/ and 3.63-MeV HeH/sup +/ ions incident on approx. 160 A carbon targets. By using the reflection method for a pure Coulomb potential, there are unfolded from these data the distributions of internuclear separations for each molecular-ion species prior to dissociation. These results are insensitive to ion-source conditions. For H/sub 2//sup +/ this distribution, while approx. 2 times wider than a pure ground vibrational state population, is markedly different from the Franck-Condon distribution that has been previously assumed by other authors with similar rf and duo-plasmatron ion sources. For HeH/sup +/ the distribution is slightly broader (approx. 1.5 times) than that expected for a pure ground state population. From the data, the initial vibrational state population in the incident beam can be extracted. 11 references.
Date: January 1, 1979
Creator: Kanter, E P; Cooney, P J; Gemmell, D S; Vager, Z; Pietsch, W J & Zabransky, B J
Partner: UNT Libraries Government Documents Department