Electronic stopping of slow light ions in matter

The project Electronic Stopping of Slow Light Ions in Matter (ESOSLIM) aimed at an im-proved understanding of the slowing down of ions (hydrogen ions, helium ions) at low energies (typically 1 - 10 keV) in matter due to interaction with the electrons of the material. This slowing down process ("electronic stopping") is a fundamental quantity, and is well understood for simple metals like Al, where a stopping force proportional to the velocity of the ion is predicted from theory and has been experimentally observed. ESOSLIM aimed at the electronic stopping of slow ions in more complex metals (with a more complex electronic structure) like gold (Au) and in an excellent insulator like Lithium fluoride (LiF). Results for Au: Au has a more complex band structure, but at sufficiently high velocities the stopping force is known to be proportional to velocity. For part of the conduction electrons in Au, the so called 5d-electrons, an excitation threshold exists for these electrons (about 2 eV), and it was known that due to this threshold, deviations from the velocity proportional stopping force occur below a certain proton velocity. Now, we developed an experimental technique which permits to measure the slowing down process at even lower ion velocities (energies): the stopping of protons and deuterons in polycrystalline gold has been studied in the range 200 eV/atom - 10 keV, using atomic and molecular ions as projectiles. As a result, we found that below a characteristic velocity the 5d- electrons do not contribute to electronic stopping anymore and the stopping force becomes proportional to velocity again, due to stopping by the free electrons of Au, the so called 6s electrons. This means, that vth is the velocity threshold for contribution of the 5d electrons to the stopping process. The stopping force was shown to be due to binary collisions of the ion with individual conduction electrons. Results for LiF: Insulators like LiF have a very large threshold energy for electronic excitation (14 eV) and are therefore transparent (they cannot absorb visible light). Ionic crystals like LiF have high erosion rates under particle bombardment (electronic sputtering). Therefore, an experimental method was developed to obtain a spectrum by use of a mini-mum amount of ions ("marker method"). Despite the large threshold energy for electronic excitation, a velocity proportional stopping force had been observed experimentally; in other words, the insulator had been found to act like a simple metal. In ESOSLIM, we wanted to learn how the stopping force would be at even lower ion velocities. As a result, we found that below a given ion velocity the stopping force ceases to be proportional to the ion velocity, but points towards to a threshold velocity, below which the stopping force might vanish. The underlying mechanism of the stopping force is not yet fully clear, but from the data is it obvious that the energy loss mechanism is different from binary collisions which are important in metals. Thus, we made a major step forward in the understanding of electronic stopping of low velocity ions by proofing that the energy loss mechanism in insulators is different from that in metals.