The semiclassical perturbation formalism applied here as well as the corresponding computer code (Sahal-Bréchot 1969a,b), have been updated and improved several times (Sahal-Bréchot 1974; Fleurier et al. 1977; Dimitrijevic & Sahal-Bréchot 1984; Dimitrijevic et al. 1991a; Dimitrijevic & Sahal-Bréchot 1996b). The semiclassical perturbation method used here, has been reviewed e.g. in Dimitrijevic & Sahal-Bréchot (1996c) and Dimitrijevic (1996). Atomic energy levels of Si XI and Si XIII needed for calculations, have been taken from Martin & Zalubas (1983). Oscillator strengths have been calculated within the Coulomb approximation (Bates & Damgaard 1949, and the tables of Oertel & Shomo 1968). For higher levels, the method of Van Regemorter et al. (1979) has been used.
Our results for electron-, proton-, and
He III-impact line widths and shifts for 4 Si XI and 61 Si XIII
multiplets are shown in Table 1 (accessible only in electronic form),
for perturber densities 1018-1023 cm-3 and
1016-1023 cm-3 respectively. The temperature range is 
K for Si XI and 
K for
Si XIII. For Si XIII, the complete set of data is given for the
perturber density of 1019 cm-3.
For lower densities,
only data needed for better
interpolation are given. Stark broadening
parameters for densities lover than tabulated, or for transitions not
tabulated for perturber densities lower than 1019 cm-3 for Si
XIII, are
linear with perturber density.
We present in the Table 1 as well, a parameter c (Dimitrijevic &
Sahal-Bréchot 1984),
which gives an estimate for the maximum perturber density for which
the line may be treated
as isolated when it is divided by the
corresponding full width at half maximum. For
each value given in Table 1, the collision
volume (V) multiplied by the perturber density (N) is much
less than one and the impact approximation is valid
(Sahal-Bréchot 1969a,b). Values for NV > 0.5 are not given
and values for 
are denoted by an asterisk.
When the impact approximation is not valid, the ion
broadening contribution may be estimated by using
quasistatic approach (Sahal-Bréchot 1991 or Griem
1974). In the region between where neither of these two
approximations is valid, a unified type theory should be used. For example
in Barnard et al. (1974), a simple analytical formulas for such
a case are given. The accuracy of the results obtained decreases when
broadening by ion interactions becomes important.
Presented results may be of interest for the modelling and research of subphotospheric layers and the considerations of radiation transfer in stellar, fusion and laser produced plasmas, as well as for the investigation and modeling of soft X-ray lasers. They may be of significance as well for further development and rafinement of the Stark broadening theory for multicharged ion lineshapes, as well as for the investigation of regularities and systematic trends of Stark broadening parameters along isoelectronic sequences. The corresponding reliable experimental data will be certainly of particular interest.
Acknowledgements
This work is a part of the project "Astrometrical, Astrodynamical and Astrophysical Investigations", supported by Ministry of Science and Technology of Serbia.