The spectral radiant intensity (without imaging) is observed between 5000 Å and 1250 Å with gas pressures 1–8 atm in argon and 10–40 atm in helium. Tungsten and carbon electrodes with 1-mm gaps were used at breakdown voltages of 1.5–6.5 kV and C = 0.85 µF and L = 16.8 nHy. After reaching opacity and saturation within the visible range, the continuum radiation shifts toward shorter wavelengths with increasing voltage and gas pressure and reaches the intensity of the broad A+, A++, and He+ lines. Saturation of spectral intensity and self-reversal of lines is observed; intensities at 1250 Å may be higher by a factor of 25 than those at 5000 Å. Mean gas temperatures of approximately 60,000 K in argon and 100,000 K in helium are determined from the spectral lines of the electrode vapor in the case of relatively low gas pressures and foremost transparent channels by means of the Norm-temperature method. Channel pressures and particle densities are ca culated from the channel expansion. Theoretical values of the continuum radiant intensity agree with the experimental results reasonably well.
Lothar Michel and Heinz Fischer, "Vacuum Ultraviolet Emission from High Density Spark Discharges in Argon and Helium," Appl. Opt. 11, 899-906 (1972)