High-velocity Ions Research Articles

Electronically stimulated sputtering and luminescence from solid argon.

Electronic excitation of condensed-gas solids by MeV light ions results both in luminescence and in kinetic ejection of atoms and molecules from the surface of the solid. This work reveals that ejection of Ar atoms from electronically excited solid argon films arises from two nonradiative repulsive dissociation steps in the deexcitation decay sequence which contains the major radiative transition. A hole-diffusion model with nonradiative quenching at the metal substate on which the films are grown accounts for the dependence of the sputtering and luminescence on film thickness and gives an estimate of 230 A\r{} for the hole-diffusion length. The hole-diffusion model also explains the nearly linear dependence of the sputtering and luminescence yields on electronic stopping cross section for high-velocity incident ions. Intentionally added ${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2}$ impurities reduce the hole-diffusion length. These reductions provide values for effective reaction volumes, ${k}_{+}$${\ensuremath{\tau}}_{+}$, of 7\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}21}$ ${\mathrm{cm}}^{3}$ for ${\mathrm{O}}_{2}$ and 3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}21}$ ${\mathrm{cm}}^{3}$ for ${\mathrm{N}}_{2}$, where ${k}_{+}$ is the interaction rate constant of the holes with the impurity and ${\ensuremath{\tau}}_{+}$ is the lifetime of holes in pure films. The ${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2}$ impurities also quench nonmobile luminescent excimers, ${\mathrm{Ar}}_{2}^{\mathrm{*}}$, over large distances: 21 A\r{} for ${\mathrm{O}}_{2}$ and 11 A\r{} for ${\mathrm{N}}_{2}$. The magnitude of the sputtering yield characterizes the vacuum interface as primarily hole reflecting, but with a surface-hole-trapping rate about 6 times that of the bulk. Surface ${\mathrm{O}}_{2}$ impurity greatly enhances surface trapping of free holes. Sputtering of ${\mathrm{Ar}}_{2}^{\mathrm{*}}$ dimers is evident from the observation of 11.3-eV luminescence in the region in front of the argon films. The dimers are emitted with \ensuremath{\sim}0.8 eV of kinetic energy. This is consistent with the repulsive decay forming an energetic ${\mathrm{Ar}}^{\mathrm{*}}$, which forms ${\mathrm{Ar}}_{2}^{\mathrm{*}}$ upon exiting or, possibly, the ejection of an ${\mathrm{Ar}}_{2}^{\mathrm{*}}$ during trapping at the surface.

Differential cross sections for ionization of water vapor by high-velocity bare ions and electrons

A semiempirical model of single differential cross sections (SDCS) for ionization of water vapor by fast electrons and bare ions is presented. At low secondary-electron energy, the model is based on an asymptotic expansion of the first Born approximation with coefficients, that are independent of projectile properties, evaluated from experimental photoabsorption and proton-impact ionization data. As the secondary-electron energy increases, the model converges to a binary-encounter approximation. Comparisons with measured differential, total, and dissociative cross sections for ionization of water by fast electrons are used to test the model. For primary electrons with energy greater than about 500 eV, agreement with these data is generally within experimental uncertainty; however, some discrepancies of uncertain origin exist.

Some applications of collision spectroscopy of gas-phase ions

Three inelastic processes that can occur during collisions of high-velocity gas-phase ions with gas molecules are considered. These are: charge exchange (sometimes called electron transfer), in which, typically, one or more electrons from the gas are transferred to the ion; charge stripping, in which one or more free electrons leave the ion; and excitation, in which the ion and/or the gas become excited but neither changes its charge state. Deductions that can be made concerning the states populated in these processes and the critical energies involved from measurements of the translational energy loss (or gain) of the fast-moving product are outlined. Novel equipment designed to improve the attainable energy resolution in the third processes is described. For brevity, the discussion is restricted to consideration of positively charged ions.

Microscopic track structure of equal-let heavy ions

The spatial distributions of ionization and energy deposition produced by high-velocity heavy ions are crucial to an understanding of their radiation quality as exhibited eg., in track segment experiments of cell survival and chromosome aberrations of mammalian cells. The stopping power (or LET) of a high velocity ion is proportional to the ratio z 2 v 2 , apart from a slowly varying logarithmic factor. The maximum delta-ray energy that an ion can produce is proportional to v 2 (non-relativistically). Therefore, two HZE ions having the same LET, but in general differing z and v will have different maximum delta-ray energies and consequently will produce different spatial patterns of energy deposition along their paths. To begin to explore the implications of this fact for the microscopic dosimetry of heavy ions, we have calculated radial distributions in energy imparted and ionization for iron and neon ions of approximately equal LET in order to make a direct comparison of their delta-ray track structure. Monte Carlo techniques are used for the charged particle radiation transport simulation.

A Superconducting Half-Wave Resonator for Accelerating High Velocity Ions

A computer model for the design of superconducting resonators has been developed which includes feedback effects due to non-linear superconducting surface resistance. These non-linearites may be due to either magnetic field or temperature. We have utilized this model to optimize the design of accelerating resonators for heavy ions. A half-wave structure of this design has been fabricated and is being evaluated with respect to the parameters of the model. Preliminary experimental results agree with the model to within a few percent. The half-wave design is particularly well suited to high frequency, high ß resonators because of its low magnetic field, or to low frequency, very low ß resonators because of its good mechanical stability.

Shock-heated gas in the I Per OB association

In response to a suggestion by Phillips & Gondhalekar (1981) that hotgas is receding from the I Per OB association with a velocity of ∼30 km s−1, we have examined the IUE high-dispersion spectra of stars in the association for blueshifted lines of interstellar CIV, SiIV and AlIII. As a result, the high-velocity ions are now known to be present in front of at least five stars. Our analysis of the radial velocities of the ions and of the column density ratio N(CIV)/N(SiIV) in the different sight-lines leads to a conclusion similar to that of Phillips & Gondhalekar, namely that the motion of the highly ionized gas is due to a supernova. We believe, however, that the expansion of the HI shell in front of I Per is unrelated and that it is probably due to a recombining HII region within which the supernova event has occurred.

Secondary electron emission from the entrance and exit surfaces of thin carbon foils under fast ion bombardment

The total secondary electron emission (SEE) yield from the entrance and exit surfaces of thin carbon foils under fast ion ( 16O, 19F, 35Cl) bombardment has been measured as a function of the ion energy and the ion beam current intensity. Using a retarding field, the energy distribution of secondary electrons integrated over almost all angles of emission in the backward and forward directions has also been measured. It is found that total forward emission is larger than backward emission by factors of up to 2.5, 2.7 and 3.4 for 16O +3, 19F +3 and 35Cl +5, respectively. It is suggested that the enhancement of forward SEE may be partly due to effects from the instantaneous charge state of the heavy ion beam in the solid in addition to the binary collisions of the projectile with individual electrons in the target. It is also shown that the total SEE yield from the entrance and exit surfaces of the target foils decreases with ion beam current intensity; this may be a beam-induced temperature effect. The total SEE yield in both the forward and backward directions is less sensitive to surface conditions for high velocity ions than for low velocity ions and the total yield from both surfaces of the foils is proportional to the ion stopping power in the target, where the constant of proportionality depends on the properties of material.

A review of recent high energy ion-atom collision physics

Over the last two years there have been several experiments with high velocity ion beams directed towards the precision measurement of K X-ray transition energies in one- and two-electron ions. One of the purposes of these measurements has been to determine with some precision the 1s lamb shift in one-electron ions. To date measurements have been performed in the Z range from 17 up to 36. All of the experiments have used the same technique of measuring the total absolute energy of the 2p to 1s transition of the one-electron ion. Over this range of Z, the 2p 1 2 -1s 1 2 transition energy varies from ∼ 2958.5 eV to ∼ 13429 eV, and the predicted 1s Lamb shift varies from 0.9383(6) eV to 11.858(11) eV. The main parameter that has been varied in these experiments has been the means of excitation used to produce the one-electron ion. These methods so far have included beam-foil excitation of high velocity beams, gas phase excitation of an accel—decel beam, direct ionization of a gas target atom (recoil ion) by a highly charged high-velocity beam and excitation in a hot tokamak plasma. The results of these measurements will be discussed and compared with QED calculations. The limitations of the experiments and possible future improvements will be discussed. The potential of producing a beam of low energy, bare and one-electron high Z recoil ions with high velocity heavy-ion beams will also be discussed.

An experimental investigation of double ionisation of helium atoms in collisions with fast, fully stripped ions

Experimental cross sections for single and double ionisation of helium by 0.13-15 MeV AMU-1 ions of charge 1-8 are presented. Where necessary, coincidence techniques were applied to distinguish between pure ionisation and ionisation involving electron capture. For high ion velocities, double ionisation takes place either through a shake-off or through a two-step process. The perturbative shake-off mechanism dominates only for the lowest ion charges and projectile energies above 10 MeV AMU-1. A reliable semi-empirical expression which determines the ratio between the double and single ionisation cross sections over a wide region of ion charge and velocity is given. The difference between double ionisation by equi-velocity electrons and positive ions is discussed. It is concluded that no reliable theoretical description of double ionisation exists for combinations of ion charge and ion velocity which give cross sections close to the maximum values.

The use of a quadrupole mass filter and a large electric sector to study dissociations of high-velocity ions

Wnen an excited, polyatomic ion dissociates, part of its energy in excess of the critical energy is given to the fragments in the form of translational energy T. For ions having high velocities, as in most sectored mass spectrometers, T is considerably amplified in the laboratory frame of reference. Fragment-ion peaks are broadened which allows their shapes, and thus the distribution of translational energies released, to be determined accurately. The broadness of the peaks leads, however, to low resolution in the fragmention spectra. The spectrometer described in this paper overcomes this difficulty. Two of its main components are a large electric sector and a quadrupole mass filter; these are scanned in various ways under computer control. Fragment-ion spectra are measured with unit-mass resolution and also each of the dissociation reactions of a chosen parent-ion species can be studied to obtain information on the translational energies released.

Differential cross sections for ionization of methane, ammonia, and water vapor by high velocity ions

Cross sections, differential in the energy of secondary electrons, for ionization of methane, ammonia, and water vapor by high energy protons are presented. The results are based on a model that uses photoabsorption and ion impact ionization data to evaluate the coefficients in Bethe’s asymptotic cross section for inelastic scattering of high velocity ions. Model cross sections are compared with previously published data and new data on ionization of methane and water vapor by 3.0 and 4.2 MeV protons. The simple, analytic model should be very useful in transport calculations where differential ionization cross sections over a broad range of primary and secondary energies are needed.

High-precision spectroscopic study of heliumlike iron

The x-ray spectrum emitted by high-velocity heliumlike iron ions has been studied with a crystal spectrometer. The absolute energies of the $n=2\ensuremath{\rightarrow}n=1$ lines have been measured with a precision of 40 ppm. A very good agreement has been found between our experimental values and a very accurate multiconfiguration Dirac-Fock calculation. The precision of the measurement and the calculation of the energies are such that for the first time the magnetic correlation energy (spin-spin) as well as the screening of quantum-electrodynamic effects can now be appreciated. The contamination of the considered lines by the so-called dielectronic recombination satellites has been studied in great detail by varying the nature and the thickness of the targets.

Charge stripping of ethane ions

Values for the vertical ionization energies of ions from ethane have been determined by charge stripping high-velocity ions with nitrogen collision gas and measuring the value of Q min, the minimum translational energy loss. The ionization energy IE (m + → m 2+) can be measured directly for processes in which the product m 2+ ions are sufficiently stable to reach the ion collector. This is the case for C 2H 5 2+·, C 2H 4 2+, C 2H 3 2+·, C 2H 2 2+ and C 2H 2+·. Ions C 2H 6 2+ are, however, unstable and fragment to give C 2H 5 2+· ions among the products. The translational energy of these product ions can be used to determine IE (C 2H 6 +· → C 2H 6 2+). Measurements were also obtained on all the singly-charged ions from pentadeuteroethane. In this case, the ion kinetic energy spectrum of C 2HD 5 +· showed a peak due to the process C 2HD 5 +· → (C 2HD 5 2+) → C 2HD 3 2+. The position of this peak enabled a value for IE (C 2HD 5 +· → C 2HD 5 2+) to be calculated. The values obtained from the pentadeutero compound confirmed those from ethane.

Differential cross sections for ionization of helium, neon, and argon by high-velocity ions

A model based on Bethe's theory for inelastic scattering of fast charged particles is used to extrapolate single differential cross sections (SDCS) for ionization of helium, neon, and argon by protons with energy between 0.3 and 1.5 MeV to higher ion energies. Results predicted by the model show good agreement with measured total and SDCS and with ab initio calculations using the first Born approximation.

Resolution dependence and asymmetry of electron capture to the continuum spectra

Velocity distributions of electrons ejected into the forward direction have been measured with proton beams of energies comprised of between 40 and 240 keV interacting with a helium-gas target. An electron spectrometer was used, which permitted us to study the shape of the resulting peaks as a function of the instrumental angular acceptance. These data are analyzed by introducing a general parametric expression for the scattering amplitude of the ionization process. Strong skewness of the electron cusps towards lower electron velocities are observed, which appear to be independent of the large variation of peak widths as determined by different angular acceptances. It is found that this skewness is important up to the closest neighborhood of the peak top and that it leads to the inclusion of an asymmetric singular behavior of the cross section. Partial peak widths at half maximum, measured towards each side of the peak top, and also the total peak width depend linearly on the transverse experimental acceptance, as given by the product of the ion velocity times the half-angle of the electron acceptance cone. The cusp skewness, defined by the ratio of the partial widths, tends to decrease at the higher ion velocities.

Review lecture: Unimolecular and collision-induced reactions of high velocity ions

It is possible to produce beams of high velocity ions, either positively or negatively charged and with a translational energy homogeneous to within 1 part in 104. The currents carried by such beams are typically of the order 10-9A. By using a magnetic field, it is possible to select ions of a particular mass: charge ratio from such beams and to study their fragmentation behaviour in field-free space. Fragmentation may occur unimolecularly or may be induced to occur by causing the ion beam to interact with a collision gas, or in other ways. The fragmentation products of such reactions that also travel with high velocity can be analysed by using a magnetic field (momentum analysis) or an electric field (translational energy analysis). The method provides information concerning the structures of ions and the kinetics, energetics and mechanisms of the reactions; the pattern of translational energy release during the fragmentation provides information on the partitioning of the available energy between internal and translational modes. By selecting reactions that occur over a limited range of rate constants, ions containing a narrow range of internal energies may be selected for study. In collision-induced reactions, part of the translational energy of the ion is converted into electronic excitation. Ions in excited electronic states can also be produced by photo-irradiation of the beam. Ions excited in these ways can be used to give further information concerning energy partitioning during fragmentation. Reactions involving a change in the sign of the ionic charge, or in the number of charges carried, can be induced to occur by interaction with a collision gas. They provide information on ionization energies and electron affinities and on long-lived electronically excited states.

Ion emission from micrometeorite impacts on atmosphereless planets

Conclusions have been drawn on the generation of intensive high-velocity ions and atoms fluxes on the surfaces of the Moon, Mercury and the atmosphereless satellites of Jupiter by micrometeorite impacts. About 30% of each flux is ejected at small angles and detained by planetary reliefs, forming surface-active layer of alkalines. The combined erosional effect of that layer, the thermocycling and the radiational tractks have been studied. The ion fluxes leaving Jupiter's atmosphereless satellites may well be one of the main sources of the short UV-emission in their vicinities.

Thermospheric wind measurements in the polar region

Ion and neutral winds at an altitude of about 225 km in the polar cap F-region have been determined from the motion of barium and strontium tracers released from sounding rockets launched in 1971 and 1972. The neutral wind measured at 0800 local time, 67° north geographic latitude, and just equatorward of the polar cusp, was 94 m/s directed about 51° west of geographic north. Motion of a second cloud deployed farther north during the same flight indicated that the velocity of an air cell which has passed through the polar cusp was affected very little by the local perturbation. An analysis of the motion of the equatorward cloud using a local approximation and including ion drag and Coriolis terms is reasonably consistent with models based on satellite drag and recent in situ measurements of the thermospheric temperature distribution. Due to the low ion density in the winter ionosphere the wind was geostrophic and had a large poleward component even at 0800 where the pressure gradient was southwest. These data, taken together with the ion drag due to enhanced density from particle precipitation in the cusp and anti-sunward ion flow across the polar cap, suggest that the thermospheric winds blow strongly from day to night across the winter polar cap. Winds measured near midnight during two events in August were directed south geographically and had magnitudes of 254 m/s and 333 m/s. These velocities are more than a factor of two higher than predicted from thermospheric pressure gradients driven solely by solar thermal effects and could be due to distortion of the thermospheric temperature distribution by heating in the auroral oval. Another possibility is that high ion velocities in the polar cap, coupled with enhanced plasma density due to particle precipitation in the polar cap, put the neutral air in motion at higher latitudes. The inertia in this motion could then keep the flow anti-sunward even when the ion density has decayed sufficiently that ions no longer can strongly effect the neutrals. The analysis suggests that inertial terms as well as ion drag and non-solar heating are important in any attempt to construct a detailed model of high latitude thermospheric winds.

Large transient magnetic fields at high ion velocities in polarized iron

Nuclear spin precessions due to the transient magnetic field in polarized Fe have been measured as a function of the initial velocity of28Si ions in the first-excited nuclear state. The transient field was found to increase linearly with the ion velocityv in the regionv/c=0.006–0.049. This is in contrast to the Lindhard and Winther model, which requires an inverse proportionality with ion velocity. Reanalysis of an earlier measurement on30Si(21+) with the linear velocity dependence yields a reduced value for theg-factor ofg=0.37±0.12. Other available velocity-dependent data for22Ne,56Fe and196Pt are consistent with a linear velocity dependence and suggest in addition a linear dependence on the nuclear charge Z of the moving ion. The increase of the transient field with recoil velocity can be explained semi-quantitatively by the capture of polarized Fe electrons into 1s and 2s vacancies in the moving ion. The velocity-dependent data and other discrepancies from the Lindhard and Winther model for16N,18O and very recently, for12C are also discussed in terms of the proposed microscopic model.

Can projectiles traversing solids bind electrons at all velocities?

PAC measurements on the 4.43 MeV 12C(2 +) state on recoil in magnetized iron at velocities υ ion = 0.058 c, 0.071 c yield integral nuclear precession angles of Φ = −0.18 (30), +0.17 (39) mrad respectively. Evidently, the high electron-spin-polarization transfer to the innermost carbon orbits implied by a previous measurement (υ ion = 0.03 c, Φ = +0.85 (14) mrad) is suppressed at higher ion velocities.