RADIATIVE LIFETIME OF THE 3S3P(3)(5S2(0)) METASTABLE LEVEL OF P+

The radiative lifetime of the 3s3p3(5S2o) metastable level of P+ has been measured by counting, for equal time intervals, the approximately 221-nm photons emitted when the metastable ions decay to the 3s(2)3p2(3P2,1) levels of P+. This is the first lifetime measurement of the 3s3p3(5S2o) level belonging to a low-charge-state ion in the SiI isoelectronic sequence. Although forbidden to decay by the LS selection rule DELTA-S = 0, the 5S2o level decays via intercombination electric dipole transitions by spin-orbit mixing with other levels of the 3s3p3 configuration. A metastable P+ ion population was produced inside a cylindrical radio-frequency ion trap by electron bombardment of PH3 vapor at pressures ranging from 5 x 10(-8) to 40 x 10(-8) Torr. The trap, operated with potential-well depths ranging from 12 to 20 eV, consists of two circular, wire-mesh end caps and a cylindrical, wire-mesh ring electrode with physical dimensions r0 = z0 = 1.67 cm. Some of the light emitted by the decaying 5S2o population was focused onto a 19-nm-bandwidth interference filter in front of a photomultiplier tube operated in single-photon counting mode. The mean lifetime was obtained from the decay-rate parameter of a nonlinear least-squares fit of a single exponential plus a constant background to the decay counts. Our result for the radiative lifetime of 3s3p3(5S2o) metastable level of P+ is 167 +/- 12-mu-s.