Role of the He I and He II metastables in the resonance 2p ²P^o<inf>1/2,3/2</inf> B III level population

Abstract

Aims. The aim of this work is to present atomic processes which lead to an extra population of the 2p 2Po1/2,3/2 B III resonance levels in helium plasma generating intense radiation in the B III 206.578 nm and 206.723 nm lines. Methods. The line profiles were recorded using a step-by-step (7.3 pm) technique which provides monitoring of the line shapes continually during the plasma decay and gives the possibility to compare line shapes at various times in the same plasma. Results. On the basis of the line intensity decays of the doubly ionized boron resonance spectral lines in laboratory nitrogen and helium plasmas, we have found the existence of a permanent energy transfer from He I and He II metastables to the 2p 2Po1/2,3/2 B III resonance levels. The shapes of the mentioned lines are also observed. At electron temperatures of about 18 000 K and electron densities about 1.1 × 1023 m-3, the Stark broadening was found as a main B III line broadening mechanism. The measured Stark widths (W) are compared with the Doppler width (WD) and with the splitting in the hyperfine structure (Δhfs). Our measured W data are found to be much higher than results obtained by means of various theoretical approaches. Conclusions. The He I and He II metastables over populate the B III resonance levels leading to populations higher than predicted by LTE model. Consequently, the emitted B III resonance lines are more intense than expected from LTE model. This fact can be of importance if B III resonance line intensities are used for abundance determination purposes in astrophysics. Similar behavior can be expected for some lines emitted by astrophysical interesting emitters: Al III, Si III, Sc III, Cr III, V III, Ti III, Fe III, Co III, Ni III, Ga III, Zr III, Y III, Nb III, In III, Sn III, Sb III, Au III, Pb III and Bi III in hot and dense helium plasmas. © ESO 2007.