Collisional relaxation measurements on Pb+ hyperfine levels

Abstract

Abstract. The time constants for population relaxation of optically pumped Pb + ions in a Paul ion trap have been determined in a He buffer gas atmosphere with additional components of other gases. For the 6 P1/2 ground state of Pb + and an ion temperatur of 104K we find cross sections of 0.72(0.33). 10 -17 cm2; 0.59(0.38). 10 15 cm2; and 2.56 (0.74). 10-14 c m 2 for He, N2 and 02, respectively. The error includes an estimated 20% uncertaincy in the pressure calibration of a residual gas analyser. 31.70.Hq; 32.80.Bx; 32.80.Pj The method of microwave-optical double resonance in radio-frequency ion traps is a well established technique to obtain accurate groundstate hyperfine splittings of ions. Isotopes of alkali-like ions have been measured with very high precision, and uncertainties below 10-1~ are almost routinely obtained [t ]. Those experiments have been performed on single ions as well as on ion clouds. While in the first case the ultimate precision is extremely high and uncertainties below 10 -15 seem feasible, the signal-tonoise ratio is of course much better for large ion clouds, containing up to 106 ions. Consequently this is preferred in experiments, where extreme accuracy is not required. Ion clouds, however, have to be cooled to a certain extend to prevent ions loss from the trap by energy pick-up from the time varying electric trapping fields in particular by those ions, which move at the boundary of large ion clouds. This is most easily performed by collisions of the stored ions with buffer gas atoms, which reduce the ions temperature, if the mass of the neutral atoms is much smaller than the ions mass. Collisions, however, may perturb the ionic eigenstates and thus limit the precision by relaxation processes. PACS:Cross sections for relaxation between optically pumped states have been determined in a number of experiments * Permanent address: Department of Radio-electronics, Peking University, Beijing, PR China on alkali-like ions for S~/2 ground-and P3/2 excited states in collisions with noble gases [2]. In the course of an iontrap experiment for a precision determination of the hyperfine splitting of the 6 2p~/2 ground state of 2°Tpb+ [3] we have performed a number of measurements on the relaxation rates with different background gases. ExperimentAbout 10 6 2°Tpb+ ions were confined continuously for several weeks in a Paul ion trap. They were moderately cooled by collisions with light buffer gases. Selective excitation of the F = 0, mF----0 ground state ])yperfine level by a narrow band tunable laser lead to a complete depletion of this state. This was indicated by the exponential decay of the fluorescence count rate tbllowing the excitation. At 50 mW laser output power the straylight level was reached after about 2 s. The laser than was blocked to allow the depleted state to fill up by coltisional relaxation. After a delay time A t the laser was turned on again and the fluorescence reappeared. The number of counts in a time window of 56 ms, corresponding approximately to the excited state lif...