194 nm microplasma lamps driven by excitation transfer: optical sources for the 199Hg ion atomic clock and photochemistry

Abstract A series of miniature, microcavity plasma lamps emitting predominantly at 194 nm has been successfully developed and tested as the optical pump for the microwave 199 Hg-ion atomic clock (40.507 GHz), replacing low pressure, RF-powered Ar/Hg discharges. Intense fluorescence on the 6 p 2 P 1/2 → 6 s 2 S 1/2 transition of the singly-charged 202 Hg ion at 194.23 nm has been generated in arrays of cylindrical microplasmas through electron-impact excitation of He, followed by three-body formation of He 2 ( a 3 Σ u + ) and Penning ionization of Hg. Emission spectroscopy and kinetic modeling of He/Hg vapor plasmas demonstrate that the population of the Hg + (6 2 P 1/2 ) radiating state (16.82 eV), produced by direct or two-step electron impact processes, is < 1% of that generated by excitation transfer to Hg by H e 2 * . Flat, fused silica lamps having emitting areas as small as 4 mm 2 and containing several mg of 202 Hg and 50–800 Torr of He have been fabricated and serve as optical drivers for the Hg + atomic clock cycle. Based on small arrays of 500 μ m–1 mm diameter microcavities, these lamps produce peak and average intensities at 194 nm greater than those associated with the Hg resonance transition at ∼ 254 nm, despite the factor of $?> > 3 difference between the energies of the 6 p 3 P 1 and 6 p 2 P 1/2 states of neutral Hg and Hg + , respectively. These lamps are unique in the sense that the desired radiating species is an excited ion and the background He gas pressure can reach 1 atm, both of which contribute to a dense glow plasma placing severe demands on E / N , power conditioning, and materials selection. Nevertheless, with proper attention given to design, vacuum processing, and preparation of the lamps, lifetimes above 1500 h have been realized to date. When these lamps drive Jet Propulsion Laboratory Hg + clocks, the stability floor has been measured to be ∼ 10 −14 . The implications of this lamp for gas-phase and solid-state photochemistry are also discussed.