Radiative lifetime of the 2s2p35S2metastable state ofN+

The natural radiative lifetime of the 2s2${\mathit{p}}^3$ ${(}^5$ ${\mathit{S}}_2$) metastable state of ${\mathrm{N}}^{+}$ has been measured by counting for equal time intervals the 214-nm photons emitted when the metastable ions decay to the 2${\mathit{s}}^2$2${\mathit{p}}^2$ ${(}^3$ ${\mathit{P}}_{2,1}$) levels of ${\mathrm{N}}^{+}$. Although forbidden to decay by the LS selection rule ΔS=0, the ${}_{}{}^5{}_{}{}^{}$ ${\mathit{S}}_2$ state decays primarily as a consequence of spin-orbit mixing with other levels of the 2s2${\mathit{p}}^3$ configuration. The metastable ${\mathrm{N}}^{+}$ ions are produced inside a cylindrical electrostatic ion trap by electron bombardment of a parent ${\mathrm{N}}_2$ vapor maintained at pressures ranging from 5 to 25×${10}^{\mathrm{-}8}$ Torr. The ion trap consists of a 5.0-cm-diam, 7.5-cm-long outer cylinder with end caps and a concentric 0.003-cm-diam central wire maintained at a negative potential of about 140 V. Some of the photons emitted by the decaying ${}_{}{}^5{}_{}{}^{}$ ${\mathit{S}}_2$ ions are focused onto a 12-nm-bandwidth interference filter in front of a photomultiplier tube and counted. The mean lifetime is obtained from the slope of a straight-line least-squares fit to the appropriate region of a logarithmic plot of the decay counts. Our result, extrapolated to zero ${\mathrm{N}}_2$ pressure, for the natural radiative lifetime of the ${\mathrm{N}}^{+}$ ${(}^5$ ${\mathit{S}}_2$) metastable state is 5.4±0.3 msec.