The modified version of the linear low pressure pulsed arc
(Djenize et al. 1991) has been used as a plasma source. A pulsed discharge driven in a quartz
discharge tube of 5 mm inner diameter and has an effective plasma length of 5.8
cm (Milosavljevic 1996). The tube has end-on quartz windows. On the opposite
side of the electrodes the glass tube is expanded in order to reduce erosion of
the glass wall and also sputtering of the electrode material onto the quartz
windows. The working gas is a nitrogen and oxygen mixture (83
N2 + 17
O2) at
70 Pa filling pressure in flowing regime. Spectroscopic observation of isolated
spectral lines is made end-on along the axis of the discharge tube. A capacitor
of 14 
F is charged up to 3.0 kV and supplied discharge currents up to 7.7 kA.
The line profiles is recorded by a shot-by-shot technique using a
photomultiplier (EMI 9789 QB) and a grating spectrograph (Zeiss PGS-2,
reciprocal linear dispersion 0.73 nm/mm in the first order) system. The
instrumental HWHM of 0.004 nm is obtained by using of the narrow spectral lines
emitted by the hollow cathode discharge. The recorded profile of these lines
have been of the Gaussian type within 7 accuracy in the range of the
investigated spectral line wavelengths. The exit slit (10 m) of the
spectrograph with the calibrated photomultiplier is micrometrically traversed
along the spectral plane in small wavelength steps (0.0073 nm). The
photomultiplier signal is digitized using an oscilloscope, interfaced to a
computer. A sample output, as example, is shown in Fig. 1 (click here).
Figure 1: Recorded spectrum at 11 s after the beginning of
the discharge (when the spectral line profiles were analyzed) with the
investigated NIV spectral lines
Plasma reproducibility was monitored by the NIII line radiation and also by the
discharge current (it was found to be within 8). The measured profiles were
of the Voigt type due to the convolution of the Lorentzian Stark and
Gaussian profiles caused by Doppler and instrumental broadening. For electron
density and temperature obtained in our experiment the Lorentzian fraction in
the Voigt profile was dominant (over 80). Van der Waals and resonance
broadening were estimated to be smaller by more than an order of magnitude in
comparison to Stark, Doppler and instrumental broadening. A standard
deconvolution procedure (Davies & Vaughan 1963) was used. The deconvolution
procedure was computerized using the least square algorithm. The Stark widths
were measured with 
error. Great care was taken to minimize the
influence
of selfabsorption on Stark width determination. The opacity was checked by
measuring line-intensity ratios within multiplets No. 3 in the cases of the NII
and NIII spectral lines. The values obtained were compared with calculated
ratios of the products of the spontaneous emission probabilities and the
corresponding statistical weights of the upper levels of the lines. It turns
out that these ratios differed by less than 
The Stark shifts were
measured relative to the unshifted spectral lines emitted by the same plasma
(Puric & Konjevic 1972). The Stark shift of spectral line can be measured
experimentally by evaluating the position of the spectral line centre recorded
at two various electron density values during the plasma decay. In principle,
the method requires recording of the spectral line profile at the high electron
density (N1) that causes an appreciable shift and then later when the
electron
concentration has dropped to the value (N2) lower for at least an order of
magnitude. The difference of the line center positions in the two cases is
, so that the shift d1 at the higher electron density N1 is:
The Stark shift data was corrected for the electron temperature decay
(Popovic et al. 1992). Stark shift data are determined with 
nm error at
a given N and T. The plasma parameters were determined using standard
diagnostic methods. The electron temperature was determined from the
ratios of the relative intensities of the 348.49 nm NIV to 393.85 nm NIII and the
previous NIII to 399.50 nm NII spectral lines, assuming the existence of LTE,
with an estimated error of 
All the necessary atomic parameters were taken
from Wiese et al. (1966). The electron density decay was
measured using a single
wavelength He-Ne laser interferometer (Ashby et al. 1965)
for the 632.8 nm
transition with an estimated error of 
Electron temperature and density
decays are presented in Fig. 2 (click here)
Figure 2: Temporal evolution of the electron density (N) and temperature (T)
in the decaying plasma.