Radiative lifetimes of several metastable states of doubly and triply ionized Ar, Kr, and Xe.

Abstract

The natural radiative lifetimes of the ${\mathit{ns}}^{2}$${\mathit{np}}^{4}$${(}^{1}$${\mathit{S}}_{0}$) metastable states of ${\mathrm{Ar}}^{2+}$, ${\mathrm{Kr}}^{2+}$, and ${\mathrm{Xe}}^{2+}$ with n=3, 4, or 5, respectively, have been measured using an ion-trapping technique. In addition, we have measured the natural radiative lifetimes of the 4${\mathit{s}}^{2}$4${\mathit{p}}^{3}$${(}^{2}$${\mathit{P}}_{1/2}$) metastable state of ${\mathrm{Kr}}^{3+}$ and also of the 5${\mathit{s}}^{2}$5${\mathit{p}}^{3}$${(}^{2}$${\mathit{P}}_{1/2}$ and $^{2}$${\mathit{P}}_{3/2}$) metastable states of ${\mathrm{Xe}}^{3+}$. The decay rate of each state was determined by counting for equal time intervals the uv photons emitted as the metastable ion population decayed to a lower level of the ground-state configuration. A population of ions was produced inside a cylindrical electrostatic ion trap by electron bombardment of the appropriate parent gas maintained at pressures ranging from 4 to 80\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ Torr. The trap consisted of a 5.0-cm-diam, 7.5-cm-long cylinder with end caps plus a concentric 0.003-cm-diam central cylinder maintained at a negative potential of 150 V. Following electron impact of the parent gas, some of the photons emitted by the decaying metastable ion population were focused onto a narrow-bandwidth interference filter.Photons transmitted by the filter were detected by a photomultiplier tube as a function of time and generated a decay curve. As dictated by the composition of the photon decay curve, the pressure-dependent decay rate of each metastable state was obtained from a least-squares fit of either a single- or double-component exponential decay to the natural logarithm of its decay curve. The mean decay rate was extrapolated to zero pressure of the parent gas using a straight-line least-squares fit, and the inverse of the zero-pressure intercept yields the natural radiative lifetime of the relevant metastable state. Our measurements for the $^{1}$${\mathit{S}}_{0}$ metastable states of ${\mathrm{Ar}}^{2+}$, ${\mathrm{Kr}}^{2+}$, and ${\mathrm{Xe}}^{2+}$ yield 133\ifmmode\pm\else\textpm\fi{}24, 14.8\ifmmode\pm\else\textpm\fi{}0.8, and 4.6\ifmmode\pm\else\textpm\fi{}0.3 msec for the natural radiative lifetime of the respective metastable state. Our result for the lifetime of the $^{2}$${\mathit{P}}_{1/2}$ metastable state of ${\mathrm{Kr}}^{3+}$ is 47\ifmmode\pm\else\textpm\fi{}5 msec, while the lifetimes of the $^{2}$${\mathit{P}}_{1/2}$ and $^{2}$${\mathit{P}}_{3/2}$ metastable states of ${\mathrm{Xe}}^{3+}$ are 15.6\ifmmode\pm\else\textpm\fi{}0.9 and 5.3\ifmmode\pm\else\textpm\fi{}0.5 msec, respectively.