A cloud of ultracold atoms confined in a magneto-optical trap (MOT) has a temperature greater than that of a low-density vapour in the same light field. It has been observed (Drewsen M. et al., AppL Phys. B, 59 (1994) 283) that this temperature excess is proportional to the cube root of the total number of trapped atoms (N1/3). We present an explanation for this effect in terms of the scattering which arises when photons spontaneously emitted by atoms have an appreciable probability of being reabsorbed within the cloud. This hypothesis has been tested by time-of-flight measurements of the temperature of clouds of atoms released from a MOT into l-dimensional and 3-dimensional optical molasses in the g + -r -and lin I lin configuration.
Isotope shifts and hyperfine splittings in optical transitions for atomic ions of the thorium isotopes 227Th to 23~ and 232Th have been measured by laser spectroscopy on stored ions. From the isotope shift data, changes of the mean square charge radii are determined. A continuous increase of the charge radius with mass number A is observed, in agreement with droplet model calculations. The results indicate that the odd-even staggering for Th is different from that one of the neighbouring isotones of Fr and Ra. There is some empirical evidence from systematics for an inversion of the staggering and the appearance of an octupole deformation at N< 137. The hyperfine splitting for 229Th for 3 electronic levels is given.
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
We use the results of a Lagrangian formulation of the dynamics of ions in a Penning trap to calculate the motional frequencies of the ions as a function of the trapping parameters. We then add realistic perturbations to the ideal trapping fields in the formalism and deduce the effects of these perturbations on the ion motion. For an ideal trap there exist values of the applied trapping fields that result in a degeneracy in the ion oscillation frequencies associated with different types of motion. We show that this motional frequency degeneracy is lifted by the inclusion of the effect of a tilt of the trap axis with respect to the axis of the applied magnetic field, leading to an ''avoided crossing'' between the oscillation frequencies. We calculate typical ion orbits for trap parameters that give oscillation frequencies near the avoided crossing between the axial and modified cyclotron frequencies. We generalize the analysis to include the motion of ions in a combined ͑Penning-Paul͒ trap and perform an experiment to test the predictions of the theory for a degeneracy between the modified cyclotron frequency and the axial frequency for Mg ϩ ions held in a tilted combined trap. The oscillation frequencies are measured for a range of tilt angles using a photon-photon correlation technique. There is good agreement between the experimental results and the theoretical predictions. The method we describe may prove to be a useful means by which trap imperfections can be identified and subsequently removed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.