2 Experiment

A modified version of the linear-low pressure pulsed arc (Djenize et al. 1992, 1998) has been used as a plasma source at two different discharge conditions. A pulsed discharge was driven in a quartz discharge tube of 5 mm inner diameter and effective plasma lengths of 5.8 cm and 6.3 cm (Fig. 1 in Djenize et al. 1998). The tube has end-on quartz windows. On the opposite side of the electrodes the glass tube was expanded in order to reduce erosion of the glass wall and also sputtering of the electrode material onto the quartz windows. The working gas was a nitrogen-oxygen mixture ($83\%$ N$_2 + 17\%$ O₂) at 70 Pa filling pressure (Experiment a) and CO₂ at 133 Pa filling pressure (Experiment b) with a constant flux flowing regime. The chosen flux and pressure provide minimal self-absorption of the investigated spectral lines. Spectroscopic observation of isolated spectral lines were made end-on along the axis of the discharge tube. A capacitor of 14 $\mu$F was charged up to 3.0 kV and 2.8 kV, in experiments a and b, respectively. The line profiles were recorded using a step-by-step technique with 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 system was calibrated by using the EOA-101 standard lamp. The instrumental FWHM of 0.008 nm was determined by narrow spectral lines emitted from the hollow cathode discharge. The spectrograph exit slit (10 $\mu$m) with the calibrated photomultiplier was micrometrically moved along the spectral plane in small wavelength steps (0.0073 nm). The photomultiplier signal was digitalized and averaged (five shots at each position) using an oscilloscope interfaced to a computer. Total line intensity (I) corresponds to the area under the line profile.

Plasma reproducibility was monitored by the O II and OIII lines and, also, by the discharge current using the Rogowski coil signal (it was found that the signal is reproducible within $\pm 3\%$).

The plasma parameters were determined using standard diagnostic methods (Rompe & Steenbeck 1967). Thus, in the case of the Exp. b, the electron temperature was determined from the Boltzmann plot of twelve O II lines (394.505; 395.437; 407.216; 407.587; 407.886; 408.512; 409.294; 408.716; 413.281; 432.577; 418.546; 418.979 nm) within an energy interval of 5.88 eV for corresponding upper-levels with an estimated error within $\pm 5\%,$ assuming the existence of LTE, according to criterion from Griem (1974). The Boltzmann plot, as an example, obtained at 15 $\mu$s after the beginning of the discharge is presented in Fig. 1. In the case of Exp. a, the electron temperature was determined from the ratios of the relative intensities of 348.49 nm N IV to 393.85 nm N III and the previous N III to the 399.50 nm N II spectral line, assuming the existence of LTE, with an estimated error of $\pm 12\%.$ The necessary atomic data were taken from the available literature (Wiese et al. 1966; Lide 1994; NIST 2000; Kurucz 2000). Forms of the electron temperature decays are presented in Fig 2.

The electron density decay was measured using a well-established single laser interferometry technique (Ashby et al. 1965) for the 632.8 nm He-Ne laser wavelength with an estimated error of $\pm 5\%.$ The electron density decays are presented in Fig. 3.