Abstract

In an extension of previous work^1,2 on the efficient (up to 1%) generation of vacuum ultraviolet radiation in mercury vapor, we now report the generation of continuously tuneable, narrow-band (0.02 cm⁻¹) radiation in the spectral region of 1218Å to 1197Å. The mechanism is a resonantly enhanced 4-wave sum-mixing process $6{\text{s}}^{21}{\text{s}}_{\text{o}}\underrightarrow{2{\text{ω}}_1}6{\text{s7s}}^1{\text{s}}_{\text{o}}\underrightarrow{{\text{ω}}_2}6{\text{snp}}^1{\text{P}}_1\underrightarrow{{\text{ω}}_3}6{\text{s}}^{21}{\text{s}}_{\text{o}}$ where the ω₃ radiation is generated about the 12p through the 16p¹P₁ transitions of Hg. The ω₂ coumarin 520 dye laser is pumped by the third harmonic of a Nd:Yag laser, currently producing only 0.2 mJ of laser light which is tuneable from 5000 to 5500Å. The ω₁ laser at 6250Å is pumped by the remaining second harmonic of the Nd:Yag giving 0.8 mJ when frequency doubled to 3125Å. We are confident that these input energies will he increased substantially when the appropriately coated optics are acquired. Using a 25 cm focal length achromat to focus the beams into a Hg cell at 140°C (2 Torr), with 10 Torr of helium as buffer gas, we measure the generated photons in an NO ionisation chamber. The generated power peaks up substantially in the immediate vicinity of the np ¹P₁ transition frequencies and, even though hydrogen Lyman α is almost exactly midway between the 11p and the 12p transition, we measure 2×10⁹ photons per pulse in this 10 pps system. At the nearby 12p ¹P₁ frequency (1212.65Å) we see 2×10¹¹ photons per pulse. We will show absorption profiles scanned through hydrogen Ly α (1215.67Å), deuterium Ly α (1215.34Å) and also through the resonance triplet of atomic nitrogen at 1199.55Å, 1200.22Å, and 1200.71Å. A careful examination of the wavelengths involved lead to the conclusion that the transition line frequencies listed in the Moore tables for the 15p and the 16p¹P₁ levels of Hg are off. We measure the 15p level to be at 83273.5 cm⁻¹ whereas the Moore tables give 83280.5 cm⁻¹. Similarly, the 16p is measured at 83407.7 cm⁻¹ in comparison with the tabulated frequency of 83420.3 cm⁻¹.

© 1982 Optical Society of America