Exchange-Collision Technique for the rf Spectroscopy of Stored Ions

A description is given of a technique, whose application to 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 (He³)⁺-Cs system, the rate equations for the magnetic sublevel populations for an ion with I=1/2, J=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 ΔF=0 transitions of (He³)⁺ indicate that a Cs spin polarization of 0.5 was achieved in the optically pumped atomic beam. The He⁺ polarization approached that of the Cs atoms. With on-off modulation of the Cs polarization, a total 2% change in the 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×{}10^-8 Torr. The ΔF=0 lines obtained with long integration times had a signal-to-noise ratio which indicated that the ΔF=±1 transitions should be observable, as has since been demonstrated.