Lifetimes of the metastable 2D levels of Ca+ ions stored in a Paul ion trap

Abstract

Measurement of the mean lifetime of the $sp2$D metastable levels of Ca$sp+$ ions has been completed by detecting the spontaneous electric quadrupole (E2) transitions from the levels. A beam of calcium atoms effused from an oven through the center of a cylindrically symmetric Paul ion trap (radio frequency, or rf ion trap). A crossed electron beam passed through the trap to ionize the atoms. The ions were then trapped by the radio frequency voltage and dc voltage applied to the trap ring electrode. The Ca$sp+$ and other ions with low charge states were trapped for several seconds or longer at a background pressure near 2 $times$ 10$sp{-8}$ torr. Spontaneous decay from the metastable levels produced during the ionization was observed using an optical detection system. The lifetime was obtained from the time-dependence of the wavelength-selected photon decay intensity, measured during the ion storage. Data were taken as a function of residual H$sb2$ pressure in order to study the effect of the quenching of metastable states by collisions of neutral gas molecules. The mean lifetime of the two $sp2$D levels was then determined. Our measurement of the metastable lifetimes of the Ca$sp+$ levels $3psp63d sp2Dsb{3/2}$ and $3psp63d sp2Dsb{5/2}$ produced the combined lifetime of $tausb{E2}$ = 0.908s, with uncertainty from 0.781s to 1.086s. Corrections for the wavelength dependence of the transition rates and the optical system yield the final results $tau(3d sp2Dsb{5/2}$) = 0.901s and $tau(3d sp2Dsb{3/2}$) = 0.921s. These results are compared with recent calculations based on many-electron theories. The data are found to be in good agreement with the most recent calculations but differ from the earlier calculations and the most recent relativistic MBPT calculation.