2 Experimental set-up and measurements

All measurements have been made in a pulsed discharge lamp. The experimental set-up (shown in Fig. 1) and the methods have already been described in Gigosos et al. (1994) and Aparicio et al. (1998), so that we will only explain details specific to the present experiment.

Figure 1: Experimental set-up

The plasmas were created by discharging a capacitor bank of 20 $\mu$F charged up to 9.5 kV, over a cylindrical Pyrex glass lamp (175 mm in length and 19 mm in interior diameter). During the whole experiment this lamp was working with a continuous flow of neon gas at a rate of 1 cm³/min, and a pressure of 5 mbar, where very small traces of oxygen (actually residual traces) were diluted. In these conditions, self-absorption in OII spectral emission is guaranteed to be completely negligible. The spectral emission lasted for 150 $\mu$s. The gas was pre-ionised in order to have the best discharge reliability. Spectroscopic and interferometric end-on measurements were made simultaneously throughout the plasma life and were taken both 2 mm off the lamp axis, and from symmetrical positions in relation from the axis. The spatially homogenous plasma and the high cylindrical symmetry of electron density and temperature in our lamp allows this configuration (del Val et al. 1998). According to Fig. 1, the lamp is placed in one of the arms of a Twyman-Green interferometer illuminated with an argon ion laser (457.9 and 514.0 nm). The spectroscopic beam is directed to a Jobin-Yvon spectrometer (1.5 m focal length, 1200 lines/mm holographic grating) equipped with an optical multichannel analyser (OMA). The OMA detector array has 512 channels (EG&G 1455R-512-HQ). The dispersions in its first, second and third orders of diffraction at 434.0 nm are 13.08, 5.67 and 2.66 pm/channel respectively. The spectrometer was very carefully calibrated wavelength, as well as intensity (Aparicio et al. 1998; del Val 1997). The OMA was pulsed for 5 $\mu$s in order to have an acceptable signal-to-noise ratio, a sufficient temporal resolution and to be able to follow the temporal plasma evolution. Mirror M4, placed behind the plasma column, was used to measure the optical depth and to detect possible self-absorption effects on each line profile. This can be performed by comparing the spectra taken with and without the light reflected by this mirror (González et al. 1989). In the experimental plasma conditions, self-absorption was found to be completely negligible.

The experiment consists of end-on measurements of the OII spectra emitted by the plasma in the spectral region 400-470 nm. Each spectral interval was recorded at 13 different instants of the plasma lifetime. For each instant at least three runs were made. An example of a spectrum recorded with 6 OII measured lines at the instant 50 $\mu$s of the plasma life is shown in Fig. 2. Also, due to the presence of hydrogen impurities in the neon plasma, it was possible to measure the H$_\alpha$ profiles where Stark broadening allowed us to make a spectroscopic determination of the electron density. The OII profile measurements were registered in second order of diffraction of the spectrometer, the most efficient one for the needed spectral resolution.

Figure 2: OMA recorded spectra showing 6 of the OII lines studied in this work. The spectra, registered in second order of diffraction of the spectrometer, correspond to the plasmalife instant 50 $\mu$s