Two-step Optical Excitation for Doppler Linewidth Reduction and Motion Study of Ions Stored in a Paul Trap

Abstract

Atomic thorium ions were stored in an r .f. quadrupole trap and excited by tunable laser light . One-step and two-step excitation schemes were applied; a four to eightfold reduction of the Doppler linewidth is obtained for the two-step scheme with essentially no loss in signal strength . The Doppler-free two-photon transition was also observed . The lineshapes were calculated using a simple model ; the observed profiles were well reproduced proving that the velocity and acceleration features of stored ions are properly accounted for . A further reduction of the Doppler linewidth is expected for the two-step method if a short lived intermediate level is used . . IntroductionIn previous experiments [1, 2], laser spectroscopic investigations were performed on thorium ions for the determination of isotope shifts and hyperfine splittings of several thorium isotopes . For these measurements, the ions were confined for many hours in an r .f . trap [1][2][3][4][5][6][7] . Laser spectroscopy of stored ions has been shown to be highly sensitive [5], since the ions can be stored for a long time allowing for many photon scattering processes for each trapped ion . The method was found [1] to be well suited for the study of radioactive atomic ions ; in the case of Th, samples of only 10 pg, i .e . 3 x 10 10 atoms, were required for the measurements .The resolution, however, is rather low because of the relatively large Doppler broadening due to the fast motion of the ions in the confining electric field . The low resolution had the disadvantage that, for example, the hyperfine splitting of odd Th isotopes [1] could not be resolved . In order to improve the resolution, we have investigated two-step excitation as a means of reducing the Doppler broadening of spectral lines of stored ions . The methods developed proved to be useful for the study of details of the motion of stored ions allowing a deeper insight into their kinematics . . Experimental detailsThe technique of storing ions in a Paul trap is well known ; reviews are found, e .g ., in references [6,7] . For our experiments we have used an ion trap with a radius parameter r0 = 1 cm . The electrodes of the trap were made of stainless steel . It is mounted inside an ultra high vacuum chamber that is evacuated to a residual gas

Energy levels of Th+between 7.3 and 8.3 eV

“…Nevertheless, the fluorescence signals from Th + are only observable for a few hundred seconds [15] due to the formation of molecular compounds. Similar observations have been made in other experiments with trapped Th + and Th 3+ ions [31,32].…”

Laser ablation loading of a radiofrequency ion trap

“…about 10 11 atoms, and a detected ion number below 10 5 per ablation pulse, an overall ion production and trapping efficiency of order of magnitude 10 −7 to 10 −6 can be deduced for the experiments reported here. This may be compared to an efficiency of about 10 −8 to 10 −7 that has been reported for the loading of Th + ions from a heated wire source [26].…”

Production and investigations of negative osmium ions for fundamental applications: REOSTRAP

“…Several positive ionic species have been laser cooled up to date. The most remarkable cases with references to the original publications are: 24 Mg + (λ = 280 nm) [15], Ba + (λ = 493.4 nm) [16], 9 Be + (λ = 313 nm) [17], 198 Hg + (λ = 194 nm) [18], Th + (λ = 583.9 nm) [19] and In + (λ = 230.6 nm) [20].…”