Threshold in the Stopping of Slow Protons Scattered from the Surface of a Wide-Band-Gap Insulator

Abstract

We have measured the energy loss of slow protons scattered with energies from 300 eV to 28 keV from a clean and flat LiF(001) surface under a grazing angle of incidence. Our data reveal a threshold behavior of stopping at low projectile energies. The effect on the outgoing charge state indicates that electron capture and loss are dominant mechanisms for the stopping of slow protons interacting with a wide-band-gap insulator. Our data allow one also to deduce information on charge transfer in front of the surface of an insulator. [S0031-9007(98)07770-9] PACS numbers: 34.50.Bw, 79.20.Rf Stopping of atomic projectiles in matter is an important subject in fundamental and, in particular, applied research. An interesting problem in this field, that has been paid increasing attention to recently, is the (electronic) stopping of light ions with low velocities (y , y 0 , y 0 is the Bohr velocity) in insulators. From simple intuitive arguments one would expect that the band gap of insulators will suppress electronic excitation phenomena by the projectiles. Then stopping of slow light ions in insulators should clearly differ from stopping in metals, where excitations of conduction electrons close to the Fermi energy play a decisive role [1]. Similar as for the stopping by noble gas atoms, where substantial electronic excitation energies give rise to threshold effects for projectile stopping [2], one should observe similar effects also with insulator targets.In a recent paper Eder et al.[3] reported on the energy loss of slow protons traversing thin foils made from large-band-gap insulators. No threshold effects for projectile stopping in insulators (Al 2 O 3 , SiO 2 , or LiF) are observed down to energies of about 2 keV. Instead, the stopping power shows the linear dependence 2dE͞dx ϳ y. The authors interpret their data in terms of excitations owing to a local reduction of the band gap of the target in collisions of the projectiles with target atoms.Motivated by these studies, we investigated the stopping of slow protons by an insulator for projectile energies down to 300 eV. We observe a threshold behavior in the stopping by the wide-band-gap insulator LiF ͑E g ഠ 14 eV͒. Since defined energy losses of atomic projectiles at those low energies with solids can hardly be measured via transmission through solid matter, we have scattered the incident ions (protons) from the surface of a crystal target under a grazing angle of incidence. Under this condition the projectiles do not penetrate the solid but are reflected specularly from the surface with a distance of closest approach of typically 2-3 a.u. The overall energy loss of scattered projectiles is generally much lower than the initial energy (typically some percent). Data on projectile stopping obtained via bulk transmission or surface scattering can be compared to some extent, since (1) at low velocities small impact parameters resulting in larger angular deflections play a negligible role, and (2) the electronic structure of the LiF target is practically identical in t...

Vanishing gap in LiF for electronic excitations by slow antiprotons

“…2,5,14 Only in a recent stopping-power experiment using a backscattering geometry 1 a deviation from a linear v p dependence was found for proton energies below about 3.8 keV in good agreement with our estimate. A deviation from metallic behavior was also found for grazing scattering of low-energy protons at an LiF surface 15 where the onset of the deviation at about 3 keV coincides with our predicted values. In the latter case, however, detailed calculations using a surface cluster ͑as opposed to bulk clusters studied in this work͒ are necessary for a direct comparison between theory and experiment.…”

“…[14][15][16] In analogy to the Fano-Lichten effect 17 for positively charged ions and the Fermi-Teller effect 18 for antiprotons in the gas phase, energy levels of the crystal are shifted due to the perturbation by ͑anti͒protons. Consequently, the energy gaps between the bands may be altered.…”

“…For comparison, we also perform calculations using proton projectiles where we focus on the dominant energy-loss channel corresponding to the conversion of the proton to neutral hydrogen. 15 Using an embedded-cluster approach we determine the electronic excitation gap in LiF when the solid is quasistatically perturbed by the external charge. We will show that the change in the excitation energy due to the perturbation by an antiproton is dramatic.…”

The treatment of electronic excitations in atomistic models of radiation damage in metals

“…Pruneda et al [138] examined the case of protons and anti-protons penetrating the insulator lithium fluoride. Experimental data for the electronic stopping power of LiF for various penetrating particles suggests the existence of a threshold effect [139,140,141]. Below a certain velocity (0.1 v 0 for protons in LiF) the electronic stopping power drops nearly to zero, violating the standard dE/dx ∝ v behaviour.…”

Ab initioelectronic stopping power and threshold effect of channeled slow light ions inHfO2

“…Instead, however, no threshold effect was originally observed in most systems [21][22][23], with the exception that a threshold velocity of v ≃ 0.2 a.u. in LiF under grazing incidence [24]. Recent theoretical and experimental research managed to measure velocities in extremely low velocity regime (v ≤ 0.1 a.u) displaying a clear velocity threshold [20,[25][26][27][28].…”