The 109.1-nm transition in neutral Cs is the prototype of a class of transitions that originate from doubly excited quasi-metastable quartet levels. In this Letter we describe experiments that use tunable VUV radiation to determine the identity of this transition, to measure its oscillator strength, and to estimate its hyperfine splitting.In recent work' it was postulated that particular coreexcited quartet levels of alkali and alkalilike ions should have radiative rates that are comparable with their autoionizing rates. This occurs since, by LS selection rules, these levels may couple only to doublet levels, which are themselves prohibited from autoionizing by angular momentum and parity considerations. Levels of this type have been termed quasimetastable and are specified by the condition that IJ -Li = 3/2 and that parity and angular momentum be both even or odd; they can occur even in heavy atoms.The 109.1-nm transition in Cs (Fig. 1) is a prototype of the class of transitions that originate from levels of this type. The upper level of this transition is Cs(5p55d6s)4P;/ 2 and is expected to radiate to the valence fine-structure levels Cs(5p 6 5d) 2 P 5 / 2 and Cs(5p 6 5d) 2 P 3 / 2 . Holmgren et al. 2 recently observed intense emission at 109.1 nm using a pulsed hollowcathode discharge and, on the basis of the fine-structure splitting and the intensity ratio of its fine-structure components, ascribed the radiation as originating from this transition. Somewhat earlier, Aleksakhin et al.3 associated radiation at 108.5 nm with the same transition.In this Letter we describe an experiment to confirm the identity of this transition through accurate measurements of the fine-structure splitting and from estimates of the oscillator strength and hyperfine splitting. A pulsed hollow cathode was used to populate the lower level of this transition, and tunable VUV radiation, generated by four-wave mixing, was used to make absorption measurements at near-Doppler-limited resolution.resonant level between 73 228 and 74 074 cm-'. Referring to Fig. 2(a), the desired level was found by observing degenerate 4WSFM (tripling) through successive levels in the Zn 4sns 'So Rydberg series.Based on a quantum-defect extrapolation from the (tabulated) 6 lower members of the series, we identify the level at 73 747.7 cm-1 as Zn 4slOs 'So. No other two-photon levels were observed within the necessary energy range.A single Q-switched, frequency-doubled Nd:YAG laser was used to pump two dye-laser systems. The first was operated with an intracavity 6talon to narrow the linewidth to 0.05 cm-, tuned to 542.2 nm, and frequency doubled to provide the two-photon resonant wave. The second dye laser was tuned around a small region near 558 nm and had a linewidth of 0.3 cm-'. The two outputs were combined by using a dichroic mirror and focused into the Zn cell at a power density of about 2 X 1010 W cm-2 . The Zn cell consisted of a 2.5-cm-diameter stainless-steel tube (horizontal) with a 6-cm-long hot zone produced by an external (vertical) sodium hea...