Line broadening in the Si I, Si II, Si III, and Si IV spectra in the helium plasma

Abstract

Context. The neutral and ionized silicon spectral line shapes have been investigated in the laboratory helium plasma at electron densities ranging between $3.7\times 10^{22}$ m -3 and $1.1\times 10^{23}$ m -3 and electron temperatures between 12 500 K and 19 000 K, both interesting for astrophysics. Aims. The aim of this work is to present experimental Stark FWHM (full-width at half of the maximum line intensity, W ) for number of spectral lines from neutral (Si I), singly (Si II), doubly (Si III), and triply (Si IV) ionized silicon spectra emitted by the pulsed helium discharge, which is optically thin at the wavelengths of the investigated ionic silicon lines. A specific method for estimating self-absorbtion is presented in detail. For investigated Si I spectral lines, applying the proposed method, an optical depth of less than 0.38 is found. Appropriate corrections of the Si I Stark widths were made. The Stark widths of different ionic species, presented in this paper, are measured for the first time in the essentially same laboratory plasma. Methods. The silicon atoms were evaporated from the walls of the specially designed pyrex discharge tube in the pulsed helium discharge at a pressure of 665 Pa in a flowing regime. The Si I, Si II, Si III, and Si IV spectral line profiles were recorded using the McPherson model 209 spectrograph and the Andor ICCD camera as the detection system. Results. The Stark FWHM s of 13 Si I, 15 Si II, 28 Si III, and 9 Si IV spectral lines were measured in the wavelength interval between 206 nm and 640 nm. Five Si I, four Si II, eleven Si III, and one Si IV W values from the above set not had measured or calculated. Our W values are compared with the existing theoretical and experimental data. Conclusions. At the mentioned plasma parameters tolerable agreement was found (within the accuracy of the experiment and uncertainties of the theoretical approaches used) between measured and calculated Stark FWHM values. We recommend the Stark FWHM s of the intense 254.182 nm, 308.624 nm, and 309.342 nm Si III, and 314.956 nm and also 316.571 nm Si IV lines for the plasma diagnostic purposes.