Radiative lifetimes of metastable states of Hg+ and Hg2+

Abstract

The natural radiative lifetimes of the 5${\mathit{d}}^{9}$6${\mathit{s}}^{2}$ ${(}^{2}$${\mathit{D}}_{5/2}$ and $^{2}$${\mathit{D}}_{3/2}$) metastable states of ${\mathrm{Hg}}^{+}$ as well as of the 5${\mathit{d}}^{9}$6s${(}^{3}$${\mathit{D}}_{2}$) metastable state of ${\mathrm{Hg}}^{2+}$ have been measured by counting for equal time intervals either the 282-, 198-, or 217-nm photons emitted when the respective metastable ions decay to the ground state. The metastable Hg ions are produced inside a cylindrical electrostatic ion trap by electron bombardment of background Hg vapor maintained at pressures ranging from 4 to 50\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ Torr. The trap consists of a 5.0-cm-diam, 7.5-cm-long cylinder with end caps plus a concentric 0.003-cm-diam central cylinder maintained at a negative potential of 140 V. Following electron impact of the Hg vapor, some of the photons emitted by the decaying metastable ion population are focused onto a narrow-bandwidth interference filter. The photons transmitted by the filter are detected by a photomultiplier tube as a function of time and generate a decay curve. However, the signal detected with the interference filter used to monitor the $^{2}$${\mathit{D}}_{3/2}$ metastable-state population consists of two exponentially decaying components, indicating the presence of another metastable state decaying through the emission of a photon within the bandpass of the filter. The second component is assumed to arise from a cascade originating with the 5${\mathit{d}}^{9}$6s6p (J=9/2) metastable state of ${\mathrm{Hg}}^{+}$. Therefore, as dictated by the composition of the photon decay curve, the pressure-dependent decay rates of the metastable states are obtained from a least-squares fit of either a single- or double-component exponential decay to the natural logarithm of the decay curve.Mean decay rates extrapolated to zero pressure of the background Hg vapor 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 results for the lifetimes of the $^{2}$${\mathit{D}}_{5/2}$, $^{2}$${\mathit{D}}_{3/2}$, and J=9/2 metastable states of ${\mathrm{Hg}}^{+}$ are 87\ifmmode\pm\else\textpm\fi{}4, 8.8\ifmmode\pm\else\textpm\fi{}0.4, and 60\ifmmode\pm\else\textpm\fi{}6 msec, respectively, while the lifetime of the ${\mathrm{Hg}}^{2+}$ ${(}^{3}$${\mathit{D}}_{2}$) metastable state is 34\ifmmode\pm\else\textpm\fi{}3 msec.