Exchange-Collision Technique for the rf Spectroscopy of Stored Ions

TLDR

A technique is described that probes the rf spectrum of field‑confined paramagnetic ions in ultrahigh vacuum by monitoring spin‑dependent collisions with a spin‑polarized neutral beam. The method employs an rf electric quadrupole ion trap, uses the adiabatic approximation to model ion motion, and derives rate equations for the magnetic sublevel populations of the ${\mathrm{He}}^{3+}$–Cs system under simultaneous spin exchange and spin‑dependent charge exchange. The results show a Cs spin polarization of 0.5, He⁺ polarization approaching that of Cs, a 2 % change in He⁺ lifetime with Cs polarization modulation, an 8‑second lifetime at 3×10⁻⁸ Torr, and that ΔF=0 lines with long integration times indicate observable ΔF=±1 transitions.

Abstract

A description is given of a technique, whose application to ${\mathrm{He}}^{+}$ has previously been briefly reported, whereby the rf spectrum of field-confined paramagnetic ions in ultrahigh vacuum is observed through spin-dependent collision processes with a spin-polarized beam of neutral particles. A rf electric quadrupole ion trap is used, and a description of the ion motion, based on the adiabatic approximation, is given, including the effect of randomizing elastic collisions with neutral background particles. With particular reference to the ${({\mathrm{He}}^{3})}^{+}$-Cs system, the rate equations for the magnetic sublevel populations for an ion with $I=\frac{1}{2}$, $J=\frac{1}{2}$ are derived under the simultaneous action of spin exchange and spin-dependent charge exchange with an alkali atom. According to these equations, the relative intensities observed in the $\ensuremath{\Delta}F=0$ transitions of ${({\mathrm{He}}^{3})}^{+}$ indicate that a Cs spin polarization of 0.5 was achieved in the optically pumped atomic beam. The ${\mathrm{He}}^{+}$ polarization approached that of the Cs atoms. With on-off modulation of the Cs polarization, a total 2% change in the ${\mathrm{He}}^{+}$ lifetime was observed, with a signal-to-noise ratio of 4, in an interaction period having a duration of 0.8 sec, the optimum value for the observed 0.4-sec lifetime against Cs-induced ion loss. In the absence of the beam, the lifetime was 8 sec at a residual pressure of 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ Torr. The $\ensuremath{\Delta}F=0$ lines obtained with long integration times had a signal-to-noise ratio which indicated that the $\ensuremath{\Delta}F=\ifmmode\pm\else\textpm\fi{}1$ transitions should be observable, as has since been demonstrated.