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Physical Review Letters

Observation of a Phase Transition of Stored Laser-Cooled Ions

F. Diedrich, E. Peik, J. M. Chen, W. Quint, HERBERT WALTHER

Published Dec 28, 1987• 414 citations
Publisher

Abstract

Clouds of two to about fifty simultaneously stored, laser-cooled ${\mathrm{Mg}}^{+}$ ions in a Paul trap were observed in two phases, which are clearly distinguishable by their excitation spectra. Transitions between these phases can be induced either by a variation of the power of the laser radiation used to cool the ions or by a change of the size of the radio-frequency voltage applied to the trap. Transitions between a "crystalline" phase and a "gaseous" phase can be repeatedly observed by variation of the appropriate parameters. The two phases and the transitions between them have also been recorded by a photon-counting image system.

DOI
10.1103/physrevlett.59.2931

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Clouds of two to about fifty simultaneously stored, laser-cooled Mg+{\mathrm{Mg}}^{+}Mg+ ions in a Paul trap were observed in two phases, which are clearly distinguishable by their excitation spectra. Transitions between these phases can be induced either by a variation of the power of the laser radiation used to cool the ions or by a change of the size of the radio-frequency voltage applied to the trap. Transitions between a "crystalline" phase and a "gaseous" phase can be repeatedly observed by variation of the appropriate parameters. The two phases and the transitions between them have also been recorded by a photon-counting image system.
We observed phase transitions and ordered structures of laser-cooled Mg+{\mathrm{Mg}}^{+}Mg+ ions stored in a radio-frequency quadrupole storage ring with a diameter of 115 mm. The ions are confined two dimensionally by a quadrupole field and can move freely along the ring circumference. By cooling the ions with tangentially intersecting laser beams, we observed a linear string of ions aligned along the center line of the quadrupole field. For higher ion densities the linear structure changes into a helical structure. The dynamics of the stored ions was studied.
The first observation of Pr+ ions stored in a Paul trap is reported.Initially the ions were observed by the electronic detection method, and further the laser induced fluorescence, following resonance absorption at a certain optical transition in Pr+ ion, which is excited from the ground state, was recorded.Moreover, fluorescence signal following the excitation from a low-lying metastable state could be detected.The Paul trap system and some other parts of tle experimental setup were constructed within the frame of this work and thus are briefly described in the present contribution.PACS numbers: 32.80.Pj of the equipment of the laser-spectroscopic laboratory to our group, and for having partially involved one of the authors (D.S.) in the experiments in the Paul trap, particularly with HΣ+ ions.Many valuable hints concerning the practical aspects of the experiments in the Paul trap are also deeply appreciated.
We have performed laser cooling of Mg+{}^{+}+ ions confined in a Penning trap. The externally produced ions were captured in flight, stored, and laser cooled. Laser-induced fluorescence was observed perpendicular to the cooling laser axis. Optical detection down to the single-ion level together with the electronic detection of the ion oscillations inside the Penning trap have been used to acquire information on the ion storage time, ion number, and ion temperature. Evidence for the formation of ion crystals has been observed. These investigations are an important prerequisite for the sympathetic cooling of simultaneously stored highly charged ions and the precision laser spectroscopy of forbidden transitions in these.
Fluorescence line profiles and their implications on the cooling dynamics of the Mg+{\mathrm{Mg}}^{+}Mg+ ions stored in a linear rf trap are studied. The line profile is dictated by the temperature of the ion cloud at different laser detunings. The upper bound of the lowest temperature was estimated for different values of the rf trapping potential amplitude and the buffer gas pressure. A general trend of this ultimate temperature to increase with the rf trapping voltage and buffer gas pressure is expected, with an abrupt change at some critical value corresponding to the transition to and from a strongly correlated liquid or crystal state. While on the one hand this expectation was confirmed when the buffer gas pressure was varied; on the other hand the influence of the amplitude of the trapping voltage on the ultimate temperature shows an interesting new feature of first dipping down before the sharp increase occurs.