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Japanese Journal of Applied Physics

Space-Charge Shift in Motional Resonance of Ca+ Induced by Simultaneously Trapped Molecular Ions in Linear Paul Trap

K. Okada, M. Wada, Takashi Nakamura, Toshinobu Takayanagi, Ichiro Katayama, Shunsuke Ohtani

Published Feb 1, 2006• 4 citations
Publisher

Abstract

The space-charge shift in secular motion resonance of Ca + induced by simultaneously trapped molecular ions was observed through the laser-induced fluorescence of buffer-gas-cooled Ca + ions in a linear Paul trap. The shift in the resonance was measured as a function of the production time of molecular ions. A simple pseudopotential model was developed to explain the space-charge shift and the number of trapped molecular ions was estimated using the model. The method can be used to monitor the total amount of invisible ions in principle, as long as the weak coupling condition is satisfied between the observable ions and the invisible ions.

DOI
10.1143/jjap.45.951

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The space-charge shift in secular motion resonance of Ca + induced by simultaneously trapped molecular ions was observed through the laser-induced fluorescence of buffer-gas-cooled Ca + ions in a linear Paul trap. The shift in the resonance was measured as a function of the production time of molecular ions. A simple pseudopotential model was developed to explain the space-charge shift and the number of trapped molecular ions was estimated using the model. The method can be used to monitor the total amount of invisible ions in principle, as long as the weak coupling condition is satisfied between the observable ions and the invisible ions.
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44Ca+, Zn+, or Ga+ ions are sympathetically cooled by Coulomb interaction with laser-cooled 40Ca+ ions, and motional resonances of these ions are observed by monitoring the fluorescence of the 40Ca+ ions. In the experiment of sympathetic cooling of 44Ca+, the ratio of the decrease in the fluorescence signal at the motional resonance of the sympathetically cooled species to that at the motional resonance of the laser-cooled species is almost proportional to the number ratio of the two species. In the case of Ga+ ions as the sympathetically cooled species, the change in the response of sympathetically cooled species becomes larger as the temperature of the ion cloud decreases. Crystallization of an ion cloud which contains both 40Ca+ and Zn+ (or Ga+) is observed.
We demonstrate a simple technique for molecular-ion spectroscopy. BaCl+{}^{+}+ molecular ions are trapped in a linear Paul trap in the presence of a room-temperature He buffer gas and photodissociated by driving an electronic transition from the ground $X {}^{1}{\ensuremath{\Sigma}}^{+}$ state to the repulsive wall of the $A {}^{1}\ensuremath{\Pi}$ state. The photodissociation spectrum is recorded by monitoring the induced trap loss of BaCl+{}^{+}+ ions as a function of excitation wavelength. Accurate molecular potentials and spectroscopic constants are determined. A comparison of the theoretical photodissociation cross sections with the measurements shows excellent agreement. This study represents an important step toward the production of ultracold ground-state molecular ions.