Radiative lifetime of the 3s3p3(5So2) metastable level of P+

The radiative lifetime of the 3s3p³(⁵S^o₂) metastable level of P⁺ has been measured by counting, for equal time intervals, the ~221-nm photons emitted when the metastable ions decay to the 3s²3p²(³P_2,1) levels of P⁺. This is the first lifetime measurement of the 3s3p³(⁵S^o₂) level belonging to a low-charge-state ion in the Si i isoelectronic sequence. Although forbidden to decay by the LS selection rule ΔS=0, the ⁵S^o₂ 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 5×10^-8 to 40×10^-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 ⁵S^o₂ 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 the 3s3p³(⁵S^o₂) metastable level of P⁺ is 167+/-12 μs.