Fast Metastable Helium Research Articles

Formation of fast metastable helium atoms in electron capture by He+ions in alkali-metal vapours

A beam-attenuation technique has been used to study the formation of fast metastable atoms in electron capture by 5-100 keV He+ ions in passage through both thick and thin targets of sodium, potassium and caesium vapour. Lower limits to the fraction of metastable atoms formed in the beam are obtained and used to determine cross section for metastable atom formation. Spectroscopic observations indicate that the 21S/23S ratio in the collision products is very small. Cross sections for the collisional destruction of fast metastable helium atoms in H2, He, Ar, and N2 have also been obtained from the attenuation plots and compared with previous experimental data.

He(21S) and He-formation in fast charge-changing collisions and evidence for a long-lived doublet state of He-

The formation of fast metastable helium atoms in two-electron capture by 200 keV 4He2+ ions in passage through hydrogen has been studied by means of the beam-attenuation technique. The results have been compared with metastable formation in one-electron capture by He+ ions. The cross section for the process He2++H2 to He(21S)+H++H+ which necessarily leads to 21S rather than 23S formation has been determined. The formation of He- ions in one-electron capture by 200 keV He(21S) and (23S) atoms has also been studied. The cross section for capture by 21S atoms is shown to be approximately one half that for capture by 23S atoms.

Crossed-molecular-beams investigation of the excitation of ground-state neon atoms by 4.6-eV helium metastables

A molecular beam source has been developed which produces fast metastable helium atoms and was used in a crossed-molecular-beams experiment to measure the absolute differential elastic-collision cross section of these atoms with a velocity-selected beam of ground-state neon atoms. The results at 4.6 eV c.m. show strong evidence of excitations around 5° and 8° c.m. The total excitation cross section was estimated at 1.45 Å2 and probably originates from potential-energy curve crossings at 0.15 and 0.3 eV.

Electron loss from and destruction of fast metastable helium ions passing through atomic hydrogen

The cross sections for electron loss from and destruction of fast metastable helium ions. He+(2s) passing through ground state atomic hydrogen are calculated using the first Born approximation. The theoretical predictions are in reasonable agreement with measurements of Gilbody and Shah (1974).

Formation of He-ions in collisions involving 80-200 keV helium metastable atoms in hydrogen

The formation of He- ions in collisions involving one-electron capture by fast metastable helium atoms in hydrogen has been studied experimentally. In the energy range 80-200 keV lower bounds on the cross section sigma 0*1 have been obtained which are about an order of magnitude greater than the only previous estimates of the He- formation cross sections.

Production and Loss of Fast Metastable Helium Atoms in Collisions with Permanent Gases

Cross sections for production and destruction of fast ${\mathrm{He}}^{0}$ atoms in triplet states have been measured for 10- to 30-keV ${\mathrm{He}}^{+}$ ions incident on gas targets of Kr, ${\mathrm{N}}_{2}$, and Ne. Values of the fractions of neutral ${\mathrm{He}}^{0}$ atoms in the triplet metastable state emerging from thin targets of Kr, ${\mathrm{N}}_{2}$, and Ne are 0.12\ifmmode\pm\else\textpm\fi{}0.005, 0.06\ifmmode\pm\else\textpm\fi{}0.03, and 0.03\ifmmode\pm\else\textpm\fi{}0.02, respectively, for incident ${\mathrm{He}}^{+}$ energies of 30 keV.

Production and Loss of Fast Metastable Helium Atoms in Collisions with Xe,H2, Ar, and He

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Formation of He- ions in charge-changing collisions during passage of fast helium beams through hydrogen

The role of fast metastable helium atoms in the formation of He- ions by charge-changing collisions during the passage of 100 kev helium beams through hydrogen has been investigated quantitatively. It is shown that He- ions are formed by collisions involving one-electron capture by fast metastable helium atoms while all other possible production mechanisms are found to be relatively insignificant. A lower bound of 54 ± 04 × 10-18 cm2 is obtained for the cross section for He- formation from He(2 3S) atoms.

Formation and destruction of fast metastable helium atoms in charge-changing collisions

Collisions involving one-electron capture and loss during the passage of 60-450 kev helium beams through hydrogen have been investigated by a technique which allows the fast metastable atom population of the beam to be determined. One-electron loss cross sections σ0*1 and σ01 for metastable and ground-state helium atoms have been separately determined. σ0*1 is found to be between 13 and 3 times greater than σ01 in the present energy range. It is shown that at energies between 80 and 250 kev, where the formation of He2+ is relatively unlikely, a satisfactory description of the charge equilibration of fast helium beams can be obtained in terms of charge-changing collisions involving He+ ions and ground-state and metastable helium atoms. Cross sections σ10 and σ10* for one-electron capture into metastable and ground states of helium have been estimated in the range 75-250 kev where it is found that σ10* is between 30% and 50% of σ10.

A quantitative analysis of fast metastable helium atoms in collisions involving electron loss

The attenuation of a fast beam of helium atoms by electron-loss collisions during passage through hydrogen has been carefully investigated. The measurements allow the metastable atom population of the beam to be determined and, for the first time, permit separate measurements of the electron-loss cross sections for both ground-state and metastable helium atoms. At 120 kev these are 7.3 ± 0.8 and 48 ± 8 cm2 × 10-17 respectively.