1 Introduction

Interest for the doubly ionized oxygen (OIII) spectral lines is in recent years increasing in astrophysics because they are found in the spectra emitted by various kinds of cosmic light sources. In a recent publication by Nelson (2000) it was found that a relationship exists between nuclear black hole mass ( $M_{\rm bh}$) and the OIII spectral line widths for active galactic nuclei (AGNs). In their work, Cuesta & Phillips (2000) report on observations of the planetary nebula NGC 2440 in the transition OIII (436.3 nm and 500.7 nm). Sako et al. (2000) present a preliminary analysis of the first high-resolution X-ray spectrum of a Seyfert 2 galaxy (Markarian 3) and notice that the soft X-ray emission is spatially extended along the OIII ionization cone. Measurements of OIII emission in Lyman-break galaxies (LBGs) have been presented by Teplitz et al. (2000) where the 500.7 nm line emission characteristics are used to estimate the star formation rate (SFR) of the LBGs. del Burgo et al. (2000) have discovered OIII (495.9 nm and 500.7 nm) emission in the inner circumnuclear region of the M 31. Furthermore, the OIII spectral line characteristics have been used in order to find nebular chemical abundances (Oey & Shields 2000; Dopita et al. 2000; Dopita & Sutherland 2000).

Therefore, the knowledge of the OIII spectral line widths and shifts is of interest. If the Stark broadening is the principal pressure-broadening mechanism in plasmas, as e.g. for A and B type stars and white dwarfs, it is possible to obtain the other basic plasma parameters (Griem 1974, 1997) essential in the modelling of various plasma compositions and kinetic processes (Lesage 1994; Zeippen 1995) on the basis of the Stark width and shift values.

From the first experimental measurement of displacements of OIII lines (Pretty 1931), Stark broadening parameters of OIII lines have been measured several times (Platisa et al. 1975; Puric et al. 7itePuric88a,b; Blagojevic et al. 2000). Theoretically, Stark broadening of OIII lines has been investigated by Subrahmaniam (1968), Hey & Bryan (1977), Dimitrijevic & Konjevic (1980, 1981, 1987), Hey & Breger (1980, 1981) and Dimitrijevic (1981, 1988a,b).

We have measured and calculated Stark FWHM (full-width at half intensity maximum, W) and shift (d) of 7 OIII spectral lines that belong to the wavelength range of interest for astrophysical observations. These belong to three transitions in 5 multiplets. Stark widths of 3 lines (334.07, 407.39 and 444.78 nm) have not been measured before (Lesage & Fuhr 1999). Stark shift values have been measured in only one experiment (Puric et al. 1988b). Thus, our five d data have not been measured before. We have calculated W and d values using the known semiclassical perturbation formalism (SCPF) (Sahal-Bréchot 1969a,b) updated several times (Sahal-Bréchot 1974; Fleurier et al. 1977; Dimitrijevic & Sahal-Bréchot 1984; Dimitrijevic et al. 1991; Dimitrijevic & Sahal-Bréchot 1996b). It should be pointed out that for the OIII lines Stark shifts no theoretical predictions exist (Lesage & Fuhr 1999 and references therein).

Our measured and calculated W values have been compared with existing theoretical (Hey & Bryan 1977; Dimitrijevic & Konjevic 1981), experimental (Platisa et al. 1975; Puric et al. 1988a; Blagojevic et al. 2000) and estimated values (Djenize et al. 1990; Djenize & Labat 1996; Djenize 2000). Latest values are obtained on the basis of the regularities of the Stark widths found along the oxygen isonuclear sequences.

Figure 1: Boltzmann plot in the CO2 plasma

Figure 2: Temporal evolution of the electron temperature (T) in the two different plasma compositions

Figure 3: Temporal evolution of the electron density (N) in the two different plasma compositions