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 ({He^3) }^+-Cs system, the rate equations for the magnetic sublevel populations for an ion with I=12, J=12 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 =0 transitions of ({He^3) }^+ 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 310^-8 Torr. The =0 lines obtained with long integration times had a signal-to-noise ratio which indicated that the =1 transitions should be observable, as has since been demonstrated.