PRECISION-MEASUREMENT OF THE ENERGY OF THE 4D(9)5S(2)D-2(5/2) METASTABLE LEVEL IN AG-I

The energy of the 4d(9) 5s(2) D-2(5/2) metastable level in Ag I, which is the upper level of the very narrow 5s(2)S(1/2)-4d(9)5s(2) D-2(5/2) two-photon transition at 661.2 nm, has been determined from precision measurements of the wavelengths of the 206.1 nm (5s S-2(1/2)-6p P-2(3/2)0) and 547.5 nm (4d(9) 5s(2) D-2(5/2)-6p P-2(3/2)0) lines emitted from a hollow-cathode discharge. The measured energy of the 4d(9) 5s(2) D-2(5/2) level, 30242.286(7) cm(-1), is combined with the known hyperfine splittings and the estimated Ag-107-Ag-109 isotope shift to obtain accurate absolute frequencies for the hyperfine components of the 661.2 nm transition. These results should help in the detection of the narrow 661.2 nm two-photon transition, which has been proposed as a new optical frequency standard.