C.T. Reimann

Erosion and molecule formation in condensed gas films by electronic energy loss of fast ions

Abstract Fast ions erode films of molecular and rare gas solids as a result of electronic energy loss of the incident particles. The dependence of the low temperature erosion yield on electronic stopping power is nonlinear and approximately quadratic for light ions. Thermal spike and Coulomb repulsion models for erosion in this temperature regime are discussed. Measurements of ejected species from D 2 O ice eroded by MeV He + ions show a constant and dominant yield of D 2 O molecules below 135 K and a monotonically increasing yield of D 2 and O 2 molecules from 55 to 150 K. The new molecules must result from rearrangement of fragments produced in the film by ionization. The temperature dependence is believed to be due to diffusion controlled migration in the film.

Electronic sputtering of low temperature molecular solids

Electronically stimulated sputtering of insulating molecular gas solids is a remarkably efficient process at excitation densities accessible by MeV light ions and keV electrons. This paper concentrates on the cases of CO and H2O (D2O). The approximately quadratic dependence of sputtering yield on the excitation density along individual particle tracks observed earlier for incident MeV ions has also been found for incident keV electrons in the case of CO. Coupled with time-of-flight energy spectra of ejected D2O from solid D2O, this behavior leads to a picture of rapid electronic relaxation with molecular repulsion involving pairs of molecular ions. We also report a dependence of CO sputtering yield on incident angle for MeV He ions which varies as (cosθ)−1.6, in qualitative support of the multiple ion picture. In addition to ejection of the principal molecular species of a solid film, electronic excitation of molecular solids even at very low temperatures leads to formation of new molecular species by bond disruption and fragment rearrangement. In the case of D2O the dominant new molecules are D2 and O2 whose ejection from the film is strongly thermally activated.

Ion-induced molecular ejection from D2O ice

Abstract Studies of molecules ejected from water ice by fast ions provide insight into the electronic relaxation processes and subsequent chemistry occurring in ice at very low temperatures. The ion-induced ejection of D 2 O, D 2 , and O 2 molecules from thin films of D 2 O ice has been measured as a function of the fluence of incident MeV ions at temperatures between 10 and 140 K. For a given beam current, the O 2 yield exhibits initial transients which are slow compared with the prompt ejection of D 2 O. We interpret these results as due to the build-up of O 2 in the films following fragmentation of D 2 O molecules by incident ions. The fragmens re-form into new molecular species which diffuse to and escape from the surface, aided by subsequent bombardment. The D 2 transient has a prompt component, which we postulate is due to rapid formation during electronic recombination near the surface. A slow component of the D 2 transient is also observed, which may arise through a two-step process similar to that of O 2 . Time-of-flight energy spectra of the ejected D 2 O molecules have also been measured. Incident ion masses and energies range from those for which nuclear elastic energy deposition dominates (50 keV argon) to those for which electronic energy deposition dominates (1.5 MeV helium). The spectra cannot be described by models typically employed for the sputtering of metals. For instance, the spectra do not have maxima characteristic of the sublimation energy of the solid. In addition, the sputtering yield in the high energy part of the ejection spectrum of D 2 O is too large to arise from nuclear elastic energy deposition. It must result instead from relatively energetic non-radiative relaxation of electronic excitation. For incident MeV ions that deposit their energy predominantly in electronic excitation, the low energy part of the D 2 O ejection spectrum is greatly enhanced, indicative of a weakly antibinding region formed along an incident particle track. Enhanced ion yields are also found in the collision cascade region which are attributed to electronic processes.

Ion-induced chemistry in condensed gas solids

Abstract Solids of H2O, D2O, CO2, and SO2 have been bombarded by MeV and keV ions, and the sputtering of these materials investigated by measuring the total yield and the mass and energy spectra of ejected particles. One observes the results of considerable chemical activity in these low temperature solids. With MeV ions incident on D2O and CO2, the production of D2, O2 and CO, O2 respectively are found, while keV ions on SO2 produce SO, SO3 and O2.

SPUTTERING OF ICES BY HIGH ENERGY PARTICLE IMPACT

Abstract Electronic excitation of thin films of molecular and rare gas solids by MeV He and H ions is efficient in causing sputtering. The sputtering yields vary widely for different species, even for those with very similar surface binding energy. The sputtering yields for CO, Ar, and N 2 , for example, differ by a factor of 10. Even more striking for these three is the dependence of the sputtering yield on the electronic stopping power of the bombarding particle. The dependence is found to be quadratic for CO, linear for Ar and it exhibits a linear-to-quadratic transition in N 2 . The experimental results indicate the importance of the specific routes for nonradiative electronic deexcitation and the importance of cooperative effects associated with neighboring excitations produced along the tracks of individual bombarding particles.

Energy transfer in solid argon during ion bombardment

Solid argon is a model system for studies of radiative and nonradiative processes associated with electronic excitation by MeV light ion bombardment. The electronic excited states, excitons and excimers, have been extensively studied in this material and careful studies have also been made of the ejection (sputtering) of atoms from the surface. The sputtering requires- transfer of energy from electronic excitation to kinetic energy of atomic motion. Ultraviolet luminescence spectra show a rich structure including features due to molecules ejected while still in an electronically excited state. The luminescence and sputtering results can be described together in terms of exciton formation, diffusion and decay.

Secondary electron, ion and photon emission during ion beam irradiation of polymer and condensed gas films

Abstract During heavy ion irradiation of polymer and condensed gas films large fluxes of ions, excited neutrals and ultraviolet radiation are liberated. Because the fluxes are so large, they can contribute secondary currents comparable to (or even larger than) the current of incident ions unless they are carefully suppressed. For polymer films the secondary fluxes vary with the polymers used and with the primary ion dose. While films like HPR-204, and polyimides like PIQ show a gradual decrease of the ionic species emission, poly(methyl methacrylate), PMMA, shows a peak in the ionic species emission with increasing dose. These observations suggest that for typical ion implantations through polymer masks, the error in the charge integration at the target may be a function of the polymer as well as the dose if proper care is not exercised in the suppression of secondaries. For condensed gas solids, efficient ultraviolet emission can produce photoelectron currents which are a strong function of film thickness. Argon is a particularly striking example. We comment on secondary suppression techniques that can be used to minimize or eliminate beam integration errors in these cases or that alternatively can provide information about the secondary fluxes and the processes which produce them.

Electronically Induced Desorption and Luminescence from Multilayer Argon Films

The rare gas solids provide special opportunities for study of desorption induced by electronic transitions (DIET) because so much is already known about the processes of electronic excitation and decay in the bulk of these weakly bonded low-temperature systems [1]. Their importance was evident in ion induced sputtering work of OLLERHEAD, et al [2] on xenon and BESENBACHER, et al [3] on argon and in electron induced sputtering of neon by BORGESEN, et al [4] and argon by COLETTI, et al [5]. We have concentrated on the study of solid argon and report on it here. We have examined both the ejection of neutral argon atoms from the surface and the emission of ultraviolet light from the full thickness of the films. These reflect the non-radiative and radiative parts of the decay of electronic excitation starting with Ar+ -electron (hole-electron) pairs. A part of this work has been reported by REIMANN et al [6]. Using the particularly simple conditions of uniform excitation throughout the film thicknesses that are provided by MeV light ions, we have been able to deduce diffusion lengths for excitons as well as surface and interface boundary conditions for these efficient carriers of electronic excitation. It is clear that the desorption observed does not arise solely or even dominantly from electronic excitation in the surface monolayer of multi-layer films. It arises instead from electronic excitation generated in tide bulk of the film and diffusion of the excited species over distances ~200A. In the electronic deexcitation chain of these excited species there are repulsive, non-radiative, energy releases which produce desorption when the releases occur within ~10A of the surface.