The radiative lifetime of the 3s3${\mathit{p}}^3$${(}^5$${\mathit{S}}_2^{\mathit{o}}$) metastable level of ${\mathrm{P}}^{+}$ has been measured by counting, for equal time intervals, the ∼221-nm photons emitted when the metastable ions decay to the 3${\mathit{s}}^2$3${\mathit{p}}^2$${(}^3$${\mathit{P}}_{2,1}$) levels of ${\mathrm{P}}^{+}$. This is the first lifetime measurement of the 3s3${\mathit{p}}^3$${(}^5$${\mathit{S}}_2^{\mathit{o}}$) 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 ${}_{}{}^5{}_{}{}^{}$${\mathit{S}}_2^{\mathit{o}}$ level decays via intercombination electric dipole transitions by spin-orbit mixing with other levels of the 3s3${\mathit{p}}^3$ configuration. A metastable ${\mathrm{P}}^{+}$ ion population was produced inside a cylindrical radio-frequency ion trap by electron bombardment of ${\mathrm{PH}}_3$ vapor at pressures ranging from 5×${10}^{\mathrm{-}8}$ to 40×${10}^{\mathrm{-}8}$ Torr. The trap, operated with potential-well depths ranging from 12 to 20 eV, consists of two circular, wire-mesh end caps and a cylindrical, wire-mesh ring electrode with physical dimensions ${\mathit{r}}_0$=${\mathit{z}}_0$=1.67 cm. Some of the light emitted by the decaying ${}_{}{}^5{}_{}{}^{}$${\mathit{S}}_2^{\mathit{o}}$ 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 3s3${\mathit{p}}^3$${(}^5$${\mathit{S}}_2^{\mathit{o}}$) metastable level of ${\mathrm{P}}^{+}$ is 167±12 μs.

• Received 11 October 1991

DOI:https://doi.org/10.1103/PhysRevA.45.2716