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arXiv:2109.05174 · Physical Review A

Increase of the barium ion-trap lifetime via photodissociation

Hao Wu, Michael Mills, Elizabeth P. West, Michael C. Heaven, Eric R. Hudson

Published Dec 3, 2021• 9 citations
arXivHTMLPDF

Abstract

The lifetime of ${\mathrm{Ba}}^{+}$ 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 ${\mathrm{Ba}}^{+}$ ions with three common gases in an ultrahigh vacuum system (${\mathrm{H}}_{2}$, ${\mathrm{CO}}_{2}$, and ${\mathrm{H}}_{2}\mathrm{O}$). 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 ${\mathrm{Ba}}^{+}$ 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.

DOI
10.1103/physreva.104.063103

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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.
Ultra-high vacuum conditions are ideal for the study of trapped ions. They offer an almost perturbation-free environment, where ions confined in traps can be studied for extended periods of time-facilitating precision measurements and allowing infrequent events to be observed. However, if one wishes to study processes involving molecular ions, it is important to consider the effect of blackbody radiation (BBR). The vast majority of molecular ions interact with BBR. At 300 K, state selection in trapped molecular ions can be rapidly lost (in a matter of seconds). To address this issue, and to maintain state selectivity in trapped molecular ions, a cryogenic ion trap chamber has been constructed and characterized. At the center of the apparatus is a linear Paul ion trap, where Coulomb crystals can be formed for ion-neutral reaction studies. Optical access is provided, for lasers and for imaging of the crystals, alongside ion optics and a flight tube for recording time-of-flight mass spectra. The ion trap region, encased within two nested temperature stages, reaches temperatures below 9 K. To avoid vibrations from the cryocooler impeding laser cooling or imaging of the ions, vibration-damping elements are explicitly included. These components successfully inhibit the coupling of vibrations from the cold head to the ion trap-confirmed by accelerometer measurements and by the resolution of images recorded at the trap center (at 9 and 295 K). These results confirm that the cryogenic ion trap apparatus meets all requirements for studying ion-neutral reactions under cold, controlled conditions.
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.
Using a laser-cooled ion trapping apparatus, we have investigated laser-induced chemical reactions between cold trapped Ba+{\mathrm{Ba}}^{+}Ba+ ions and several neutral molecular gases at room temperature, O2{\mathrm{O}}_{2}O2​, CO2\mathrm{C}{\mathrm{O}}_{2}CO2​, and N2O{\mathrm{N}}_{2}\mathrm{O}N2​O, leading to the production of cold trapped $(\ensuremath{\approx}20\phantom{\rule{0.3em}{0ex}}\mathrm{mK})$ BaO+\mathrm{Ba}{\mathrm{O}}^{+}BaO+ ions. The BaO+\mathrm{Ba}{\mathrm{O}}^{+}BaO+ ions were converted back to Ba+{\mathrm{Ba}}^{+}Ba+ ions via reaction with room-temperature CO. Reaction rates were determined by employing molecular dynamics simulations. The cold mixed-species ion ensembles produced were used for studying the efficiency of sympathetic cooling, by variation of the ratio of laser-cooled to sympathetically cooled ion numbers. In one extreme case, 20 laser-cooled 138Ba+^{138}\mathrm{Ba}^{+}138Ba+ ions were capable of maintaining the translational temperature of 120 sympathetically cooled barium isotopes (135−−137Ba+)(^{135--137}\mathrm{Ba}^{+})(135−−137Ba+) and 430 138Ba16O+^{138}\mathrm{Ba}^{16}{\mathrm{O}}^{+}138Ba16O+ molecules at approximately 25mK25\phantom{\rule{0.3em}{0ex}}\mathrm{mK}25mK.
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.
We describe a versatile experimental apparatus designed for investigating ion-molecule reaction dynamics at low temperatures under ultra-high vacuum conditions (<8 × 10-11 Torr). The system integrates laser-cooled atomic ions confined in a linear Paul trap with a pulsed molecular beam featuring simultaneous quantum-state and velocity selection. Reaction products are identified using fluorescence imaging and a time-of-flight mass spectrometer. The setup accommodates a broad range of ionic and neutral species-including Be+, C+, H2, N2, O2, H2O, and others-enabling high-sensitivity studies of reaction dynamics. As a benchmark, we present measurements of the Be+ + H2 reaction across a range of collision energies, including reactions with H2 vibrationally excited to the v = 1 level. The apparatus offers fine control over both internal and external degrees of freedom, providing a general platform for cold ion-molecule reaction studies.
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.