Roman Laubert

Energy loss of atomic and molecular ion beams in thin foils

Abstract The energy loss of 0.3 to 1 MeV H + ions in thin (200–2000 A) carbon films has been measured for ions incident as independent accelerated hydrogen atomic ions (H + ) and for incidence as members of accelerated diatomic hydrogen molecular ions (H 2 + ). For the short traversal times encountered in these experiments, the molecular beam shows an energy loss per atom which is about 1.2 times that observed for an atomic beam of the same velocity. This ratio is less than the value of 1.5 which might be expected by simple equipartition of energy loss into close and distant collisions. Similar measurements have been carried out for O − and O 2 − beams with energies of 2.9 to 2.0 MeV per atom. As in the hydrogen case a ratio of ∼1.2 was observed for the energy loss of the molecular beam relative to the atomic beam for short transit times decreasing to ∼1.0 as the transit time increased. The increased width of the transmitted particle energy spectrum for the molecular beam can be attributed to Coulomb repulsion of the two ions resulting from electron loss by atoms of the diatomic molecule upon entry into the solid. Measurement of the energy width indicates a mean charge of +1 and +4±1 for the hydrogen and oxygen molecular ions respectively.

UNIFIED SPUTTERING THEORY.

Abstract A simple unified theory of sputtering describes the ablation of random targets by swift heavy particles in terms of the atomic processes triggered by the energy transfer from the impinging particles to the target atoms. A comprehensive sputtering yield curve is derived in an approximate manner, applicable to a wide range of particle-target combinations at particle energies large compared to the atomic displacement energy in the target. The properties and limitations of this yield curve are examined and compared with recent experimental data.

Binding effects in electronic excitations by heavy charged particles

The ratios of K-shell X-ray production cross sections by He+ and H+ ions of 25 to 50 keV per amu energy in Al and Mg are found to be influenced strongly by additional binding of the target electrons at the time of excitation.

Collisional Atomic Physics with Molecular Projectiles

Many collisional atomic physics processes are altered when solid targets are bombarded by molecular, compared to atomic, projectiles. We present energy and angular distribution measurements of exiting particles when 50-300 keV H+2 and HeH+ molecules are incident on thin (1.8-5 ¿g/cm2) solid carbon targets. We find that the ratio of molecular to atomic projectile energy loss is velocity and transit time dependent, and ranges from ~ 0.8 to 1.3. The energy distribution of the exiting charged, neutral, and molecular particles are compared and found to be consistent with the assumption that protons of a velocity ¿ v0 do not have bound states while moving in solids.

Z1 dependence of K-shell X-ray production by heavy charged particles

Abstract New measurements on Al at 0.1 – 0.9 MeV/amu give K-shell ionization cross section ratios σ( He ) 4σ( H ) > 1 , contrary to the plane wave Born approximation prediction of unity. The effect is similar to the Z31 effect found in stopping powers and may be caused by the same atomic processes.

Production of soft X-ray emitting slow multiply charged ions: Recoil ion spectroscopy

Abstract A study of Ne L-shell vacancy production by S ∼14+ , Cl ∼12+ ions has shown copious production of NeIINeVIII excited states with ∼10 −18 cm 2 cross sections and recoil velocities which may permit significantly higher resolution spectroscopy than is possible with beam-foil methods.

Propagation of swift atomic clusters through solids

Abstract Molecular ion yields and neutral and charged fractions are reported for 300 keV H 2 + , DH + , D 2 + , H 3 + , D 3 + , 3 HeD + , 4 HeH + , 3 He 2 + , 4 HeD + , 4 He 3 He + , and 4 He 2 + molecular ions bombarding carbon targets having area densities ranging between 5 and 40 μg/cm 2 . The molecular ion yields are consistent with the predictions of wake-riding theory.

Observation of intense low energy autoionization lines in the wings of the forward peak from fast ion-atom collisions

In the ejected electron spectra from collisions of C2+, O4+ and Si10,11,12+ with argon toms at energies of 2–4 MeV/amu, a series of intense lines has been observed, which originates from doubly excited autoionizing states of Belike projectiles. These lines correspond to electron energies between 2 and 20 eV in the projectile frame. For kinematic reasons these lines can only be detected near the forward direction.

Carbon Accumulation on Target Surfaces Irradiated by Protons

We show that the observed beam spot consists of accumulated carbon. The accumulation is the same on the front and back surface of a thin target, and is proportional to the bombardment time at current densities greater than ¿ 2¿A/cm2. In our energy range of investigation (100-300 keV), the accumulation rate is independent of energy, but does vary with the temperature of the target. For targets at ¿ 15°C, the accumulation rate for an oil diffusion pumped target chamber is ¿ 2¿g/cm2-hr or 2Å/min, which corresponds to a retention of one carbon atom per incident proton. Cooling a shield surrounding the target, or heating the target to 150°C, reduces this accumulation rate by an order of magnitude.

Studies of neonL-shell excitation by impact of highly ionized heavy ions

Passage of foil-excited 1.4 MeV/A S and 1.1 MeV/A Cl ions of neon charge state ∼ 12+ through neon gas targets at pressure ∼100 mTorr has been found to be accompanied by copious production of Ne II–VIII excited states. Comparable excitation cross-sections ∼10−18 cm2 are found for a large number of levels belonging to all of these charge states and corresponding to principal quantum numbersn=2, 3, 4. Vacancy distributions very similar to those found in beam-foil excitation of ∼1MeV neon beams are found. Because the Ne recoil velocities are small compared to the fast beam velocities characteristic of the beam-foil source, it is possible to reduce both Doppler shifts and spreads by 3–4 order of magnitude for equivalent collimation. It has also been found that there is an excitation cross-section change of a factor ∼5 for a corresponding projectile charge state change from 6+ to 12+, that efforts to classifyK x-ray satellite spectra byLshell vacancy labels (KL0,KL1,...) are probably inaccurate due to extensive population ofn≧3 spectator levels, that both the recoil ion and beam-foil spectra exhibit few lines withn≧4, and that for the allowed transitions studied here, collisional excited states quenching effects due to the ∼100mTorr target gas pressures used are negligible.