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 /=},/==} 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 AF=0 transitions of (He 3 ) 4 " 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 X10"" 8 Torr. The AF = 0 lines obtained with long integration times had a signal-to-noise ratio which indicated that the AF= ±1 transitions should be observable, as has since been demonstrated.

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