Observation of Cooper minima in excited-s-state photoionization cross sections in neon and argon

Observations of Cooper minima in excited-state photoionization cross sections for the Ne 3s and Ar 4s levels are reported. Prebreakdown Ne and Ar discharges under irradiation by light in the 200-400-nm wavelength range yield spectral dependences of photoionization cross sections with minima at the ionization threshold of these states in Ne and slightly distant from them in Ar. They are in close agreement with certain theoretical predictions. Autoionization structure which exists between the two ionization limits p322 and p122 was not observed in any of the Ne or Ar photoionization spectra due to both low spectral resolution and the high density of levels in this spectral region. Application of these measurements to ultraviolet-radiation detection is examined. High detection sensitivity of ultraviolet light is indicated.