Emission at 1091 A in neutral core-excited Cs

Certain quartet levels in alkalilike systems retain metastability against autoionization while acquiring large radiative yields. This quasi-metastability occurs through selective coupling to nonautoionizing doublet levels by the spin-orbit interaction. An example of such a level is the 5 p 5 5 d 6 s 4 p 5/2 level of neutral Cs, which has a calculated branching ratio for radiation at 1091 A of 43 percent. Experimentally, it is found that this line has an emission intensity equal to 1/6 of that of the strongest ion line of Cs(+), and is a promising candidate for an extreme ultraviolet laser.