Optical breakdown in metal vapours

Mercury, caesium and rubidium vapours have been ionized in the intense radiation flux produced by a Q-switched ruby laser. The technique used is applicable to metals which can be handled in vapour tubes. The characteristic threshold for breakdown of the vapours appears at a density of 13 × 1020 atoms/cm3 for mercury, 12 × 1016 atoms/cm3 for caesium and 5 × 1016 atoms/cm3 for rubidium, for a radiation flux of 5 × 109 w cm-2. These results are compared with measurements by Meyerand and Haught in 1963 which give thresholds at densities of 48 × 1020 atoms/cm3 for argon and 34 × 1021 atoms/cm3 for helium at the same flux. The very low thresholds for breakdown in caesium and rubidium appear to be inconsistent with all theories so far offered to explain the phenomenon.