Carlos O. Reinhold

Chemical sputtering from amorphous carbon under bombardment by deuterium atoms and molecules

We perform classical molecular dynamics simulations of the chemical sputtering of deuterated amorphous carbon surfaces by D and D2, at energies of 7.5–30 eV D−1. Particular attention is paid to the preparation of the target surfaces for varying impact projectile fluence, energy and species, to the vibrational state of D2 projectiles, as well as to the variation in sputtering yields with target surface and impact projectile. The methane and acetylene sputtering yields per deuteron, obtained with atomic and molecular projectiles, agree quantitatively with recent experimental values. We study the distribution of sputtered species, as well as their kinetic energy and angular spectra.

The classical limit of ionization in fast ion-atom collisions

The authors study the classical-quantum correspondence for ionization in three-body fast ion-atom collisions. The existence of a classical limit of the quantum mechanical transition probabilities as a function of the momentum transferred to the electron during the collision, the impact parameter, the energy and angle of the emitted electron, and the initial quantum level of the target is investigated. A well behaved classical limit is shown to exist for large momentum transfers whereas ionization by small momentum transfers or at large impact parameters is shown to be classically suppressed. It is found that ionization at small impact parameters does not possess a well-defined classical limit. Modifications of classical trajectory Monte Carlo (CTMC) methods to account for non-classical ionization are suggested. Applications related to recent data for electron ejection into large angles are presented.

Engineering atomic Rydberg states with pulsed electric fields

Atoms in high-lying Rydberg states with large values of the principal quantum number n, n {ge} 300, form a valuable laboratory in which to explore the control and manipulation of quantum states of mesoscopic size using carefully tailored sequences of short electric field pulses whose characteristic times (duration and/or rise/fall times) are less than the classical electron orbital period. Atoms react to such pulse sequences very differently than to short laser or microwave pulses providing the foundation for a number of new approaches to engineering atomic wavefunctions. The remarkable level of control that can be achieved is illustrated with reference to the generation of localized wavepackets in Bohr-like near-circular orbits, and the production of non-dispersive wavepackets under periodic driving and their transport to targeted regions of phase space. The testing of these control schemes, together with their reversibility, through the creation of electric dipole echoes in Stark wavepackets, is also described. New protocols continue to be developed that will allow even tighter control with the promise of new insights into quantum-classical correspondence, information storage in mesoscopic systems, physics in the ultra-fast ultra-intense regime and nonlinear dynamics in driven systems.

Chemical sputtering by impact of excited molecules

We study chemical sputtering of deuterated amorphous carbon surfaces by D atoms, vibrationally excited D2, and dissociating D3 molecules, in a range of impact energies, 7.5–30 eV/D. We analyze the role of the internal state, i.e. the vibrationally excited and dissociating states of the neutral molecules resulting from above-surface neutralization of impacting molecular ions in typical sputtering experiments. The sputtering yields are shown to considerably increase with the internal vibrational energy at the lowest impact energies. By comparison of calculated and measured yields we draw conclusions regarding the possible mechanisms for neutralization.

IONIZATION OF HYDROGEN AND HYDROGENIC IONS BY ANTIPROTONS

Motivated by earlier theoretical studies which utilized simplified models and by a very recent experiment regarding antiproton-impact of hydrogen, the authors present a description of ionization of hydrogen and hydrogenic ions based on very large scale numerical solutions of the time-dependent Schroedinger equation in three spatial dimensions and on analysis of the topology of the electronic eigenenergy surfaces in the plane of complex internuclear distance. It is illustrated how ionization of atomic hydrogen and hydrogenic ions by antiprotons is quite different from that for impact by positively charged particles at low energies. Most significantly, for hydrogen targets, the quasi-molecular electronic eigenenergies approach close to and ultimately merge with the continuum at small distances, leading to a plateau of the low energy ionization cross section.

Fast neutralization of highly charged ions in grazing incidence collisions with surfaces

We present first results of simulations for the neutralization and relaxation of multiply charged Oq+ ions in grazing incidence collisions with a gold surface. The simulation treats the direct quasi-resonant charge transfer into the L shell of the projectile within the over-the-barrier model. The speed of neutralization is significantly enhanced by the upward shift of energy levels at the surface due to dynamical screening. We find complete relaxation, neutralization, and negative ion formations within the available interaction time of a few times 10−14 s in agreement with experiment.

Ionization of Rydberg atoms by half-cycle pulses

We study the ionization of Rydberg atoms by half-cycle electromagnetic pulses as a function of the duration of the pulses covering a broad range of time intervals connecting the adiabatic and sudden regimes. The threshold fields for ionization lie on a universal classical scaling invariant curve. We present quantum mechanical results which agree well with the universal curve. The n-2 threshold field scaling found experimentally is a linear approximation to the universal curve by the tangent in the region where the pulse duration is of the order of the orbital period of the atom. We predict the existence of an ultrashort pulse limit in which the threshold field scales as n-1. Effects due to the non-hydrogenic core are negligible.

Energy and angle spectra of sputtered particles for low-energy deuterium impact of deuterated amorphous carbon

We study the translational, vibrational, and rotational energy spectra of atoms and molecules reflected or sputtered from deuterated amorphous carbon surfaces by impact of low-energy (1–30 eV) deuterium atoms. Both the rovibrational and translational energies of sputtered deuterium molecules are found to be close to 1 eV over the whole impact energy range, with approximate equipartition between rotational and vibrational modes, particularly at the higher impact energies. Sputtered carbon-containing molecules are vibrationally energetic, with rovibrational energies in the range of 1.5–2.5 eV; translational and rotational motions are less energetic, close to 0.5 eV, but hotter, with more energy per degree of freedom. The energy distributions of ejected molecules confirm the partial thermalization of the impact cascade. We also study the angular spectrum of the velocity of the outgoing particles as well as their angular momentum. While the velocity vectors are described well by a cosine distribution, a prefe...

Low energy chemical sputtering of ATJ graphite by atomic and molecular deuterium ions

We present experimental chemical sputtering results for D+, D2+ and D3+ ions incident on ATJ graphite in the energy range 5–60 eV D−1, and compare them with simulations for deuterated amorphous carbon impacted by neutral D, D2 and D3. The measured methane yields/D for the different species compared at the same energy/D diverge below about 60 eV D−1, the incident triatomic molecular ions leading to the largest yields/D, and the atomic ions to the smallest, reaching a factor of two difference at 10 eV/D. The measured yields/D are in reasonable agreement with molecular dynamics simulations over the entire calculated energy range. The model surfaces were prepared by D, D2 and D3 impacts in a way that mimics the experiment. For D2 incident at energies below 15 eV/D, the simulations show a strong dependence of the sputtering yields on the vibrational state of the incident projectile.

Lifetimes of heavy-Rydberg ion-pair states formed through Rydberg electron transfer

The lifetimes of K(+)..Cl(-), K(+)..CN(-), and K(+)..SF(6)(-) heavy-Rydberg ion-pair states produced through Rydberg electron transfer reactions are measured directly as a function of binding energy using electric field induced detachment and the ion-pair decay channels discussed. The data are interpreted using a Monte Carlo collision code that models the detailed kinematics of electron transfer reactions. The lifetimes of K(+)..Cl(-) ion-pair states are observed to be very long, >100 micros, and independent of binding energy. The lifetimes of strongly bound (>30 meV) K(+)..CN(-) ion pairs are found to be similarly long but begin to decrease markedly as the binding energy is reduced below this value. This behavior is attributed to conversion of rotational energy in the CN(-) ion into translational energy of the ion pair. No long-lived K(+)..SF(6)(-) ion pairs are observed, their lifetimes decreasing with increasing binding energy. This behavior suggests that ion-pair loss is associated with mutual neutralization as a result of charge transfer.