Charge State Of Incident Ions Research Articles

Bombardment of CO Ice by Cosmic Rays. I. Experimental Insights into the Microphysics of Molecule Destruction and Sputtering

We present a dedicated experimental study of microscopic mechanisms controlling radiolysis and sputtering of astrophysical ices upon bombardment by cosmic-ray ions. Such ions are slowed down owing to inelastic collisions with bound electrons, resulting in ionization and excitation of ice molecules. In experiments on CO ice irradiation, we show that the relative contribution of these two mechanisms of energy loss to molecule destruction and sputtering can be probed by selecting ion energies near the peak of the electronic stopping power. We have observed a significant asymmetry, in both the destruction cross section and the sputtering yield, for pairs of ion energies corresponding to the same values of the stopping power on either side of the peak. This implies that the stopping power does not solely control these processes, as usually assumed in the literature. Our results suggest that electronic excitations represent a significantly more efficient channel for radiolysis and, likely, for sputtering of CO ice. We also show that the charge state of incident ions and the rate for CO+ production in the ice have a negligible effect on these processes.

Visible light emission from surface of nickel bombarded by slow Xeq+ (4 q 20) ion

Bombarded by slow highly charged ion (SHCI), particles including ions and atoms of metal are excited and ejected from the sample. Optical emission can be observed for the radiative de-excitation of some excited atomic particles. The important information about particle ejection and incident ion neutralization, as well as the nature, the kinetic energy, and the number of the sputtered excited particles can be obtained by studying the optical emission process. The optical emission from the the collisions between slow (V~0.38 VBohr) highly charged Xeq+ (4 q 20) ions and high purity Ni (99.995%) surface is studied. The experiment is carried out at the 320 kV for multi-discipline research with HCIs in the Institute of Modern Physics, Chinese Academy of Sciences. The spectral lines are analyzed by using an Sp-2558 spectrometer equipped with a pattern of 1200 groves/mm blazed at 500 nm and an R955 photomultiplier tube at the exit slit. The target beam current corresponding to the dwell time is recorded, which can be translated into the incident ion current. Based on the formula of Y=N/(t/Ceq), the spectral line intensity is normalized. The normalized spectrum can be obtained from the interaction of 0.38VBohr Xe20+ ions with Ni surface in a wavelength range of 400-510 nm. The species at excited state can be identified by comparing the wavelengths of spectral lines with those in the standard spectroscopic table. Most of the observed spectral lines are identified as being from the electron transitions of Ni I 3d9(2D)4p-3d9(2D5/2)4d, Ni I 3d8(3F)4s4p(3P)-3d84s(4F)5s and Ni Ⅱ 3p63d9-3p63d8(3P)4s, as well as Xe I 5p5(2P3/2)6s-5p5(2P3/2)8p, Xe Ⅱ 5p4(3P2)6p-5p4(3P2)6d and Xe Ⅲ 5s25p3(2D)6s-5s25p3(2D)6p. Compared with the single charged ion, some neutralized incident ions yield Xe I, Xe Ⅱ, Xe Ⅲ spectral lines. The photon yields of spectral lines, such as Xe Ⅱ 410.419, Xe Ⅲ 430.444, Xe Ⅱ 434.200, Xe Ⅱ 486.254, Ni I 498.245, Ni I 501.697, Ni I 503.502, Ni I 505.061 and Ni I 508.293 nm, are presented each as a function of charge state of incident ion. The results show that the photon yield increases with the increase of the charge state, which is consistent with the potential energy of the incident ion. The potential energy is the driving force for photon emission of excited Ni atom. The neutralization of Xeq+ is in good agreement with that indicated by the classical over-the-barrier model.

Slow ions 84Kr15+, 17+ bombardment on GaAs

We have investigated surface morphology and visible light emission from slow ions Kr15+, 17+ colliding with GaAs (100). The surface disorder of GaAs films mainly depends on the charge state of incident ions. The two spectral lines of target atom Ga belong to transitions of GaⅠ 4p 2P1/2o5s 2S1/2 at 403.2 nm and 4p 2P3/2o5s 2S1/2 at 417.0 nm. Light emissions of target species depend on the energy of the incident ions deposited on the target surface atoms. During the neutralization process, the four spectral lines of Kr+ respectively can be attributed to the transitions of Kr Ⅱ 4d 4F7/25p 2D5/2o at 410.0 nm, 5s 2P3/25p 4S3/2o at 430.4 nm, 5p 4D3/2o4d 2D3/2 at 434.0 nm and Kr Ⅱ 4d 4D1/25p 2S1/2o at 486.0 nm. They are induced by cascade de-excitation after many electrons of the conductions band of the solid surface captured in highly excited states of the incident ion. Intensities of these six spectral lines from incident ions Kr17+ are obviously larger than Kr15+'s.

Formation of monolayer graphene on a basal HOPG surface irradiated with Xe ions

Highly charged Xeq+ (q=5, 21, 26) ions with kinetic energy of 450keV were extracted from electron cyclotron resonance ion source. Highly oriented pyrolytic graphite (HOPG) basal surfaces were impacted by the Xeq+ ions. The samples were analyzed with Raman spectroscopy. There appeared a tiny peak in Raman spectrum at position of 2643cm−1 for the impact of Xe5+ ion, which suggested the monolayer graphene was formed on the HOPG surface. The intensity of the peak depended on the charge state of incident ions.

Nanoscale Track Diameter and Hydrogen Yield: Dependence upon Charge State of Incident Ion on Polystyrene

The study of radiation damage of high- molecular weight substances due to MeV ion interactions is of interest for engineering and scientific applications. In the present study polystyrene (PS) was irradiated with 107Ag ions of three different charge states (q) 11+, 14+ and 25+ and of 130 MeV energy. The emission of hydrogen from PS was monitored as a function of the incident ion fluence. The experimental results showed that the hydrogen depletion per incident ion from PS varies as qn, where n was found to be 2.1 as compared to the value 2.7 to 3.0 reported in the literature. Radii of the nanometric damaged zones or ion tracks formed were analyzed from the slope of the hydrogen depletion versus ion fluence curves as a function of charge state of incident ion. These have values between 3.2 - 6.8 nm. These radii were found to depend upon the charge state of the incident ion and vary as qm, where m has the value 0.9.

Role of Ion-Surface Interaction at the Entry Surface on the Energy Loss of Highly Charged Slow Ions in Solids

Evidence is obtained from the data of an earlier measurement that the effect of ion-surface interaction on the stopping power of highly charged slow ions is not at all tiny rather remarkably large, even it supersedes the bulk stopping power. The stopping power due to the surface interactions is directly proportional to the charge state of incident ions.

SPM observation of slow highly charged ion induced nanodots on highly orientated pyrolytic graphite

We have observed nanodots on a highly orientated pyrolytic graphite (HOPG) surface produced by highly charged ion impacts using a scanning tunneling microscope. Previous measurements have con.rmed the dominant role of the potential energy or the incident ion charge state on the size and height of the observed nanodots. The present results extend these previous measurements to much lower kinetic energy. It appears that there is no observable influence on the lateral size of the nanodots due to the incident ion kinetic energy down to approximately 200 eV. In contrast some slight reduction in the nanodot height was observed as the kinetic energy was reduced.

Dependence of hydrogen released on the charge state of incident ions

The behavior of polymers under heavy ion bombardment is of great interest. In the present study, hydrogen released from polypropylene (PP) and polyethylene terephthalate (PET) was investigated as a function of charge state (11+, 14+, and 25+) for 130 MeV 107Ag ions. It was found that hydrogen released from the polymers varies as α q n , where n was found to be 2.98 and 1.94 for PP and PET, respectively, when compared with the value of ∼3.0 reported in the literature for different polymers and ion combinations. Radii of the damaged zones or ion track formed were deduced from the slope of the hydrogen released versus ion fluence curves. This radius was also found to depend upon the charge state of the incident ion. It varies as β q m , where m is 1.25 and 0.741 for PP and PET, respectively.

Electron emission induced by hydrogen atoms and protons incident on aluminium: effect of the projectile charge state

It is well known that the charge state of incident ions has an influence on the stopping power as well as on the electron emission characteristics. We have studied here electron emission for protons and hydrogen atoms incident on polycrystalline Al targets for impact velocities up to 2 v 0 ( v 0 is the Bohr velocity). Besides the direct excitation of target electrons by the incident projectile, we have considered different charge changing processes that give rise to additional electron excitation, hence contributing to electron emission. These processes are also responsible for the charge state dynamics of the moving projectile. We compare our results with previous results and in particular with the predictions of the simple model proposed by Ghosh and Khare [Phys. Rev. 125 (1962) 1254].

Secondary Electron/Reflected Particle Coincidence Studies during Slow Highly Charged Ion-Surface Interactions

We have measured the secondary electron emission statistics (ES) for 5 keV Nq+ (q = 1–4) ions incident at 10° on polycrystalline aluminium, in coincidence with specularly reflected N0. In this arrangement the kinetic contribution to secondary electron emission is minimised. The experimental data shows that the coincident electron yield, γ, increases linearly with incident ion charge state. The kinetic emission contribution has also been determined from this data. The ES due to 2 and 4 keV He2+ impact on polycrystalline aluminium in coincidence with specularly reflected He+ and He0 have also been determined. The process He2+ → He0 yields a larger γ value than the process He2+ → He+.

Charge transfer and memory loss in keV oxygen-ion scattering from Cu(001)

Oxygen ion and neutral atom yields were measured for ${\mathrm{O}}^{+}$ and ${\mathrm{O}}^{\ensuremath{-}}$ ions incident on a clean Cu(001) surface for ion energies from 0.4 to 5 keV. Scattering geometries were chosen that simplify the selection of single collisions between the incident ions and surface atoms and consequently also simplify the identification of the charge transfer mechanisms occurring in the O-Cu system. It was found that the ratio of ${\mathrm{O}}^{+}$ to ${\mathrm{O}}^{\ensuremath{-}}$ ions is independent of the incident ion charge state, demonstrating that memory loss occurs over the entire range of energies and scattering geometries investigated. From general arguments based on the energies of the electronic states of the O-Cu system, we attribute the formation of scattered ${\mathrm{O}}^{\ensuremath{-}}$ ions and neutral O atoms to a resonant charge transfer process. Evidence is presented that the formation of ${\mathrm{O}}^{+}$ ions is due to a hard collision between the incident ions and top layer surface atoms. Similar measurements were made for low energy $(<~4 \mathrm{keV}) {\mathrm{C}}^{+}$ ions incident on Cu(001), over the same range of scattering geometries. The behavior of the single scattering peaks in the ${\mathrm{C}}^{\ensuremath{-}}$ spectra is also consistent with a resonant charge transfer process. No ${\mathrm{C}}^{+}$ ions were observed in the scattered flux for all energies and scattering geometries investigated.

Effect of the pre-equilibrium charge state on the Li + emission yield

The secondary emission yield of Li + from a LiF target bombarded with 2.0 MeV nitrogen ions was measured as functions of the incident ion charge state and of the target temperature. The results were discussed considering current models (Bitensky et al., Nucl. Instr. and Meth. B 72 (1992) 380; da Silveira and Matos, Surf. Sci. 326 (1995) 370) and the effects of the contaminant layer adsorbed on the target surface. It was found that the Li + emission yield shows a dependence on the ion pre-equilibrium charge state in contrast with the H + emission yield, which is known to be strongly dependent on the incident charge state of the projectile.

Charge-state dependence of image-charge acceleration of convoy electrons in fast, grazing collisions of carbon ions with a silicon (100) surface.

Electron emission distributions, triply differential in emission energy and angles, are presented for collisions of singly through quadruply charged C ions impinging on a Si(100) surface, with ion energies between 3.6 and 6.0 MeV and grazing angles of incidence between 0.1{degree} and 1.0{degree}, respectively. The electron emission spectra measured under these conditions confirm previous reports of a shift of the convoy electron peak toward higher energy due to surface wake or image acceleration. Detailed angular distributions demonstrate a shift of convoy emission towards larger angles with respect to the surface plane, which is further evidence of the acceleration of emitted electrons by the surface wake potential. In addition, we observe the peak energy shift to depend upon the incident ion charge state for the higher projectile velocity ({approximately}4.5 a.u.). This dependence may be understood as a consequence of incomplete projectile charge equilibration at the time of convoy electron production. The charge-state dependence is not observed at the lower collision velocity ({approximately}3.5 a.u.). {copyright} {ital 1996 The American Physical Society.}

Light emission from a NaCl target bombarded by Ar ions and the mechanism for the beam energy dependence of the light yield from Ag and Na atoms

Light emitted by Na atoms following the bombardment of a NaCl target with Ar ions has been studied. Light yields that include contributions from both displaced surface and sputtered atoms have been obtained as a function of incident ion beam kinetic energy and with different Ar ion charge states. At a bombarding kinetic energy of 48 keV, we find that the light yield increases as the incident ion charge state increases. The light yield was found to increase and then slowly decrease as the ion beam kinetic energy changed from 8 to 99 keV. Sputtering calculations have been carried out for both NaCl and Ag targets to investigate possible mechanisms for explaining the observed light yield in these two system s as a function of the incident beam kinetic energy.

X-ray emission from Bi 50+ to Bi 71+ ions incident on a gold surface

Bi ions with charge states q = 50 to 71 from the Livermore National Laboratory's EBIT were accelerated to energies of q × (7 keV) and directed to a gold target. Using a Si(Li) detector, two families of X-rays were observed, one of these in the range of 750–2000 eV and the other between 2500 and 6000 eV. Both families are found to shift to higher energies with increasing incident ion charge state. The lower energy group which dominates the spectra for lower incident charge states is attributed to the filling of vacancies in the N shell while the higher energy group, which predominates for the higher incident charge states, is attributed to the filling of M-shell vacancies. M X-rays are observed even for those incident charge states for which M vacancies are not expected. A similar result observed for U ions incident upon a Be target was attributed by Schuch and coworkers to a dielectronic excitation mechanism.

Light emission from a Ag target bombarded by multiply-charged Ar ions

Light emitted by Ag atoms following the bombardment of a Ag target with Ar ions has been studied. Light yields that include contributions from both displaced surface and sputtered atoms have been obtained as a function of incident ion beam kinetic energy and with different Ar ion charge states. No change in the light yield was observed as the incident ion charge state was varied. The light yield was found to increase by nearly a factor of 10 as the ion beam kinetic energy changed from 8 to 100 keV.

Gain characteristics of a microchannel plate and a channel-electron multiplier for low energy multiply charged ions

Pulse height distribution are measured when multiply charged ions of S, Ne, F, O, N and C are incident on a microchannel plate of chevron type (MCP) and a channel electron multiplier (CEM) in the velocity range of 3–4.5 × 10 5 m/s. The pulse height distributions obtained with the CEM shows no significant dependence of the gain on the energy, mass and charge state of incident ions. On the other hand, for the case of the MCP, an increase of the gain is observed with an increase of the charge state of incident ions irrespectively of ionic species. Such behavior is discussed in connection with secondary electron emission yield at the input and electron multiplication mechanism iin the channels.

Charge state-dependent effects in electronic sputtering by fast ions

Abstract Reported yields of ejecta sputtered from dielectric and organic materials by MeV/amu O and F ions show a strong dependence on the incident ion charge state. These observations are consistent with a (dE/dx)2-dependence of sputtering yield, provided that proper account is taken of the charge-state equilibration process as the ion beam enters the solid. From a knowledge of this process, an effective interaction depth for the coupling of electronic excitation to prompt sputtering is inferred to be ≲ 6 nm for the case of 1.24 MeV/amu O incident on glycylclycine. A somewhat smaller depth is inferred for uranium sputtered by 1 MeV/amu F incident on UF4.

Interpretation of High-Q Scattering Experiments

A theoretical model has been developed for the calculation of the states of ionization of target atoms after impact by an energetic high charge state ion. The calculational method is based upon using the three-body classical-trajectory Monte Carlo approach combined with the independent electron model to determine the target ionization cross sections. Calculated cross sections for rare gas targets are in qualitative agreement with experimental values for incident ion charge states of q = +4 to +54 and energies in the 1 to 5 MeV/amu range. The calculations indicate that the gross ionization cross sections measured using the condenser-plate approach can be scaled to one reduced curve for each target species by plotting cross section divided by incident ion charge state versus energy per nucleon divided by incident ion charge state.

Projectile Auger spectra of gas- and foil-excited oxygen ions at MeV energies

High resolution projectile Auger spectra of singly and doubly core-excited oxygen ions are studied as a function of incident beam energy (2--18 MeV), exciter gas (helium, neon, argon, and krypton) or foil (carbon), incident charge state (1+--4+), and delay time. Auger electron production from excited oxygen states varies considerably for collisions with different beam energies and exciter gases. A striking similarity in both the linewidths and relative Auger line intensities is observed for single collision gas and multiple collision foil excitation. The incident ion charge state does not appreciably affect the excitation cross sections and the charge state of the outgoing projectile. In order to assign the measured Auger peaks numerous excitation and Auger transition energies for singly and doubly core-excited states are calculated using semiempirical and ab initio methods. Calculated transition energies are compared with experimental line structures. Possible excitation and deexcitation mechanisms are suggested and discussed within a quasimolecular single-particle model.