Hyperthermal Atoms Research Articles

The ionization of hyperthermal sodium atoms on W(110) as a function of temperature

The neutral backscattered fraction of an incident beam of sodium atoms with energies in the ten or hundred eV range is measured as a function of the temperature of the W(110) surface in the range 300–2000 K for an incident angle of 60°. At temperatures below 1500 K more than 90% of the hyperthermal beam is ionized. The ionization efficiency decreases with increasing surface temperature; the relative increase of the neutral fraction is higher at the lower kinetic energies of the incident beam. The ionization efficiency of the hyperthermal beam is large as compared with a thermal beam, which ionized fraction does not exceed 50%. The theory of resonance surface ionization/neutralization, which has its starting point at chemisorption theory, can explain the experimental results. The charge state of the reflected beam is shown to be determined on the outward path near a “freezing” distance of the order of 5 × 10 −10 m.

The ionization of hyperthermal sodium atoms on W(110) as a function of temperature

Abstract The neutral backscattered fraction of an incident beam of sodium atoms with energies in the ten or hundred eV range is measured as a function of the temperature of the W(110) surface in the range 300–2000 K for an incident angle of 60°. At temperatures below 1500 K more than 90% of the hyperthermal beam is ionized. The ionization efficiency decreases with increasing surface temperature; the relative increase of the neutral fraction is higher at the lower kinetic energies of the incident beam. The ionization efficiency of the hyperthermal beam is large as compared with a thermal beam, which ionized fraction does not exceed 50%. The theory of resonance surface ionization/neutralization, which has its starting point at chemisorption theory, can explain the experimental results. The charge state of the reflected beam is shown to be determined on the outward path near a “freezing” distance of the order of 5 × 10−10 m.

Formation of WF −6 and its dissociative products by collisional ionization

The formation of negative ions in electron transfer reactions between hyperthermal alkali atoms (Na, K) and WF 6 has been studied in the energy range 0–30 eV c.m. Relative cross sections and translational energy thresholds for ion pair formation have been measured, from which the following electron affinities (EA) and bond dissociation energies ( D) have been derived: EA(WF 6) = 3.7 eV, EA(WF 5) = 1.25 eV, D(WF 5—F) = 5.1 eV, D)WF 5—F −) = 5.4 eV, D(WF − 5—F) = 7.6 eV. Several ion molecule reactions are discussed which result in formation of secondary fragmentation ions and WF − 7.

Negative surface ionisation of hyperthermal halogen atoms

Negative surface ionisation of hyperthermal halogen atoms was studied as a function of their kinetic energy on thoriated tungsten and on niobium wires. The transition from a thermal equilibrium process to direct reflection causes the ionisation to increase drastically above thermal energies: efficiencies up to 40% were found for 30 eV Cl atoms impinging on thoriated tungsten.