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Japanese Journal of Applied Physics

Photodissociation of CaOH+ for Regeneration of Ca+ in Linear Paul Trap

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

Published Feb 1, 2006• 3 citations
Publisher

Abstract

The photodissociation of trapped CaOH + produced by Ca +* (3d 2 D J )+H 2 O reaction was performed in order to regenerate Ca + in a linear Paul trap. Most Ca + ions were successfully regenerated from molecular ions by deep UV light irradiation. The photon energy threshold in the CaOH + + h ν→Ca + +OH reaction was estimated to be ∼4.9 eV. The dissociation energies of XH + →X + +H and XOH + →X + +OH (X=Be, Mg, Ca) were calculated using standard quantum chemistry codes to search for the possibility of regenerating light alkaline earth ions from molecular ions. The present experiment suggests that the photodissociation method is applicable to the regeneration of alkaline earth ions lost by chemical reactions with typical residual molecules in an ion trap experiment.

DOI
10.1143/jjap.45.956

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The photodissociation of trapped CaOH + produced by Ca +* (3d 2 D J )+H 2 O reaction was performed in order to regenerate Ca + in a linear Paul trap. Most Ca + ions were successfully regenerated from molecular ions by deep UV light irradiation. The photon energy threshold in the CaOH + + h ν→Ca + +OH reaction was estimated to be ∼4.9 eV. The dissociation energies of XH + →X + +H and XOH + →X + +OH (X=Be, Mg, Ca) were calculated using standard quantum chemistry codes to search for the possibility of regenerating light alkaline earth ions from molecular ions. The present experiment suggests that the photodissociation method is applicable to the regeneration of alkaline earth ions lost by chemical reactions with typical residual molecules in an ion trap experiment.
Molecular ions that are generated by chemical reactions with trapped atomic ions can serve as an accessible testbed for developing molecular quantum technologies. On the other hand, they are also a hindrance to scaling up quantum computers based on atomic ions, as unavoidable reactions with background gases destroy the information carriers. Here, we investigate the single- and two-photon dissociation processes of single CaOH+ molecular ions co-trapped in Ca+ ion crystals using a femtosecond laser system. We report the photodissociation cross section spectra of CaOH+ for single-photon processes at λ = 245-275 nm and for two-photon processes at λ = 500-540 nm. Measurements are interpreted with quantum-chemical calculations, which predict the photodissociation threshold for CaOH+ → Ca+ + OH at 265 nm. This result can serve as a basis for dissociation-based spectroscopy for studying the internal structure of CaOH+. The result also gives a prescription for recycling Ca+ ions in large-scale trapped Ca+ quantum experiments from undesired CaOH+ ions formed in the presence of background water vapor.
Abstract Two-photon dissociation of BeH + ions is studied by detecting the fluorescence changes of Be + –BeH + bi-component Coulomb crystal in a linear Paul trap. BeH + ions generated by an exothermic reaction between electronically excited Be + ions and residual H 2 in the vacuum chamber are photon-dissociated with two photons scanning over the range of 201 nm to 208 nm. Our experiment provides a novel method to maintain the number of Be + ions stable in a Coulomb crystal with a middle ultraviolet band dissociation laser. This two-photon dissociation method extends the wavelength range of the dissociation laser for BeH + compared to the one-photon dissociation, and the method can be utilized to all alkaline earth atomic ions which require suppression of the reaction with residual H 2 gas in vacuum.
The lifetime of Ba+{\mathrm{Ba}}^{+}Ba+ ions confined in a Paul trap is found, under typical conditions, to be limited by chemical reactions with residual background gas. An integrated ion-trap and time-of-flight mass spectrometer are used to analyze the reactions of the trapped Ba+{\mathrm{Ba}}^{+}Ba+ ions with three common gases in an ultrahigh vacuum system (H2{\mathrm{H}}_{2}H2​, CO2{\mathrm{CO}}_{2}CO2​, and H2O{\mathrm{H}}_{2}\mathrm{O}H2​O). It is found that the products of these reactions can all be photodissociated by a single ultraviolet laser at 225 nm, thereby allowing the recovery of the Ba+{\mathrm{Ba}}^{+}Ba+ ions and leading to an increase of the effective trap lifetime. For a Coulomb crystal, the lifetime increased from roughly 6 h to 2 days at room temperature. It is suggested that higher enhancement factors are possible in systems with stronger traps. In addition, photodissociation wavelengths for other common trapped-ion systems are provided.
Radiative emission during cold collisions between trapped laser-cooled Rb atoms and alkaline-earth ions (Ca+^++, Sr+^++, Ba+^++) and Yb+^++, and between Li and Yb+^++, are studied theoretically, using accurate effective-core-potential based quantum chemistry calculations of potential energy curves and transition dipole moments of the related molecular ions. Radiative association of molecular ions is predicted to occur for all systems with a cross section two to ten times larger than the radiative charge transfer one. Partial and total rate constants are also calculated and compared to available experiments. Narrow shape resonances are expected, which could be detectable at low temperature with an experimental resolution at the limit of the present standards. Vibrational distributions are also calculated, showing that the final molecular ions are not created in their ground state level.
We have produced and cooled the molecular ions MgH+{\mathrm{MgH}}^{+}MgH+ and MgD+{\mathrm{MgD}}^{+}MgD+ in a linear Paul trap. These ions were generated by the photochemical reactions ${\mathrm{Mg}}^{+}{(3p}^{2}{P}_{3/2})+{\mathrm{H}}_{2} ({\mathrm{D}}_{2})\ensuremath{\rightarrow}{\mathrm{MgH}}^{+} ({\mathrm{MgD}}^{+})+\mathrm{H} (\mathrm{D}),$ and identified by the radial separation in the trap of ions with different charge-to-mass ratios. The molecular translational motion was cooled sympathetically by Coulomb interaction with laser-cooled Mg+{\mathrm{Mg}}^{+}Mg+ ions to a temperature estimated to be below 100 mK. Ordered structures (ion crystals) containing more than 1000 ions, with more than 9595%95 being molecular ions, were obtained. Such translationally cold and well-localized samples of molecular ions could become very useful for molecular physics and chemistry.
The direct photodissociation of trapped ^{85}Rb_{2}^{+} (rubidium) molecular ions by the cooling light for the ^{85}Rb magneto-optical trap (MOT) is studied, both experimentally and theoretically. Vibrationally excited Rb_{2}^{+} ions are created by photoionization of Rb_{2} molecules formed photoassociatively in the Rb MOT and are trapped in a modified spherical Paul trap. The decay rate of the trapped Rb_{2}^{+} ion signal in the presence of the MOT cooling light is measured and agreement with our calculated rates for molecular ion photodissociation is observed. The photodissociation mechanism due to the MOT light is expected to be active and therefore universal for all homonuclear diatomic alkali metal molecular ions.