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Physical Review A

YbH+ formation in an ytterbium ion trap

Thai M. Hoang, Yuan‐Yu Jau, Richard E. Overstreet, Peter D. D. Schwindt

Published Feb 14, 2020• 14 citations
Publisher

Abstract

The trapped $^{171}\mathrm{Yb}^{+}$ ion is a promising candidate for portable atomic clock applications. However, with buffer-gas cooled ytterbium ions, the ions can be pumped into a low-lying $^{2}F_{7/2}$ state or form ${\mathrm{YbH}}^{+}$ molecules. These dark states reduce the fluorescence signal from the ions and can degrade the clock stability. In this work, we study the dynamics of the populations of the $^{2}F_{7/2}$ state and ${\mathrm{YbH}}^{+}$ molecules under different operating conditions of our $^{171}\mathrm{Yb}^{+}$ ion system. Our study indicates that $^{2}F_{7/2}$-state ions can form ${\mathrm{YbH}}^{+}$ molecules through interactions with hydrogen gas. As observed previously, dissociation of ${\mathrm{YbH}}^{+}$ is observed at wavelengths around 369 nm. We also demonstrate ${\mathrm{YbH}}^{+}$ dissociation using 405 nm light. Moreover, we show that the population in the dark states can be limited by using a single repump laser at 935 nm. Our study provides insights into the molecular formation in a trapped ion system.

DOI
10.1103/physreva.101.022705

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Studies of trapped, cooled ion ensembles

David Offenberg

This work presents various novel results in the field of experimental and theoretical trapped ion studies. All investigations involve laser-cooled 138Ba+ ions confined in a linear quadrupole ion trap, which serve as coolant for complex molecular ions, as target for collisions with neutral atoms, or as model system in theoretical analyses. The special feature of the apparatus used in this work, is a molecular ion source based on electrospray ionization that allows for the production of gas-phase molecular ions of almost arbitrary species with a maximum mass-to-charge ratio of 2,000 Da. Trapped together with laser-cooled 138Ba+ ions, the molecular ions can be cooled down, kept cold and investigated for many minutes and in principle for up to hours in a nearly collisionless environment. In this work, the sympathetic cooling of complex molecular ions to translational temperatures below 1 K is demonstrated for different types of molecules covering a mass range from 182 to 12,400 Da. Using the example of the protein cytochrome c, with a mass of about 12,400 Da the most massive molecular species sympathetically cooled in an ion trap so far, the methods for detection of successful trapping and cooling, and temperature determination are presented in detail. In one case, multiply protonated molecules of the protein cytochrome c have been cooled to less than 0.75 K. Furthermore, two methods for measuring photodestruction rates of cold, trapped complex molecular ions have been developed that can, for example, be applied in destructive spectroscopy schemes. Both techniques are demonstrated using the example of singly protonated glycyrrhetinic acid molecules (GAH+) dissociated by an ultraviolet (UV) laser. Measurements of the photodestruction rate were performed at different intensities of the UV laser, and rates as low as 0.05 s-1 and less have been determined, which is only possible due to the long ion storage times that can be achieved with this apparatus. For the UV laser wavelength of 266 nm, a destruction cross section of (1.1±0.1)∙10E-17 cm2 for GAH+ has been determined. In this work, feasibility studies towards internal cooling of trapped complex molecular ions have been performed. Such an additional internal cooling could be realized via collisions with laser-cooled neutral atoms. To test their suitability, neutral atoms of Li, Na, K, Rb, Cs, and Yb have been collided with trapped laser-cooled and not cooled Ba+ ions as well as with complex molecular ions. For all species charge exchange reactions with Ba+ ions have been observed excluding these species for collisional cooling. While K, Rb, and Cs showed charge exchange reactions with ground state and laser-excited Ba+ ions, the charge exchange rates of Yb and Ba+ were found to be state dependent. Collisions of neutral Yb atoms were found to be reactive and lead to the formation of adducts and fragments, in contrast to the alkali metals for which no such reactions have been observed. In addition, the motion of cooled, trapped ions has been studied theoretically via simulations. When the translational energy of a trapped ion ensemble is sufficiently reduced, a first-order phase transition from a liquid to a solid state occurs with the ion ensemble changing from a disordered ion cloud to an ordered ion crystal. As ions in a linear quadrupole trap arrange in shell structures, a free motion in radial direction is disabled, and ions can only move to neighboring shells when their kinetic energy is high enough to overcome a certain potential barrier. The simulations have shown that intershell diffusion rates in the considered ion crystals increase exponentially when the ion temperatures exceed values that well agree with the predictions of other established models. Ion ensembles of different ion numbers (300, 500, and 1000 138Ba+ ions) and symmetries (prolate and spherical) have been investigated showing a clear tendency for the phase transition temperatures: The higher the ion number, the lower is the transition temperature.

•Jan 1, 2010•1 citation
The trapped 171Yb+^{171}\mathrm{Yb}^{+}171Yb+ ion is a promising candidate for portable atomic clock applications. However, with buffer-gas cooled ytterbium ions, the ions can be pumped into a low-lying 2F7/2^{2}F_{7/2}2F7/2​ state or form YbH+{\mathrm{YbH}}^{+}YbH+ molecules. These dark states reduce the fluorescence signal from the ions and can degrade the clock stability. In this work, we study the dynamics of the populations of the 2F7/2^{2}F_{7/2}2F7/2​ state and YbH+{\mathrm{YbH}}^{+}YbH+ molecules under different operating conditions of our 171Yb+^{171}\mathrm{Yb}^{+}171Yb+ ion system. Our study indicates that 2F7/2^{2}F_{7/2}2F7/2​-state ions can form YbH+{\mathrm{YbH}}^{+}YbH+ molecules through interactions with hydrogen gas. As observed previously, dissociation of YbH+{\mathrm{YbH}}^{+}YbH+ is observed at wavelengths around 369 nm. We also demonstrate YbH+{\mathrm{YbH}}^{+}YbH+ dissociation using 405 nm light. Moreover, we show that the population in the dark states can be limited by using a single repump laser at 935 nm. Our study provides insights into the molecular formation in a trapped ion system.
It is known that fluorescence of trapped Yb+{\mathrm{Yb}}^{+}Yb+ ions fades with continuous driving of the 2^{2}2 S1/2{\mathrm{S}}_{1\mathrm{/}2}S1/2​${\mathrm{\ensuremath{-}}}^{2}$P1/2{\mathrm{P}}_{1\mathrm{/}2}P1/2​ transition in the presence of buffer gases. This phenomenon has been interpreted as being caused by population trapping in the 2^{2}2 F7/2{\mathrm{F}}_{7\mathrm{/}2}F7/2​ state of Yb+{\mathrm{Yb}}^{+}Yb+ . As another cause of the fluorescence disappearance, we found the production of YbH+{\mathrm{YbH}}^{+}YbH+ by chemical reaction of Yb+{\mathrm{Yb}}^{+}Yb+ in excited states with H2{\mathrm{H}}_{2}H2​ gas. We found that the wavelengths of 369.482, 369.202, and 368.947 nm, formerly assigned as wavelengths for deexcitation of Yb+{\mathrm{Yb}}^{+}Yb+ in the 2^{2}2 F7/2{\mathrm{F}}_{7\mathrm{/}2}F7/2​ state to the 2^{2}2 S1/2{\mathrm{S}}_{1\mathrm{/}2}S1/2​ ground state, are actually photodissociation lines of YbH+{\mathrm{YbH}}^{+}YbH+ . The change of the resonance point of rf resonance absorption signals indicates the production and dissociation of YbH+{\mathrm{YbH}}^{+}YbH+ ions. The production of YbH+{\mathrm{YbH}}^{+}YbH+ may be a problem in establishing frequency standards using buffer-gas-cooled Yb+{\mathrm{Yb}}^{+}Yb+ , owing to difficulty in removing H2{\mathrm{H}}_{2}H2​ gas from vacuum systems.
We report on a study to determine the efficiency of various buffer gases in quenching the population trapping states of laser-excited ytterbium ions. The ions were confined in an rf-dc hybrid trap, and their 2^{2}2S1/2{\mathit{S}}_{1/2}S1/2​${\mathrm{\ensuremath{\leftrightarrows}}}^{2}$P3/2{\mathit{P}}_{3/2}P3/2​ transition was excited with 329-nm laser light. Buffer gases used include H2{\mathrm{H}}_{2}H2​, N2{\mathrm{N}}_{2}N2​, CO2{\mathrm{CO}}_{2}CO2​, Ar, and He at several different pressure regimes. We have identified N2{\mathrm{N}}_{2}N2​ to be highly efficient in quenching the population of the trapping states and thus be the most suitable buffer gas to use with ytterbium for the development of a trapped-ion microwave frequency standard.
Long-lived metastable states of Yb+{\text{Yb}}^{+}Yb+ ions are used for atomic frequency standards, precision measurements, and quantum information research. The effect of population trapping and transfer in these states must be well understood. We report here the transfer of Yb+{\text{Yb}}^{+}Yb+ ions into the long-lived 2F7/2{^{2}F}_{7/2}2F7/2​ state by means of collisions between He buffer gas and Yb+{\text{Yb}}^{+}Yb+ ions held in a linear Paul trap. Transfer rates were measured as functions of buffer-gas pressure and repump-laser power, and the collisional population transfer rates were extracted. The measured transfer rate coefficients are $8.32(75)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ and $8.65(33)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }{\text{cm}}^{3}/\text{s}$ for the collisional processes ${^{2}P}_{1/2}\ensuremath{\rightarrow}{^{2}D}_{5/2}$ and ${^{2}D}_{3/2}\ensuremath{\rightarrow}{^{2}F}_{7/2}$, respectively.
We report on a study to determine the efficiency of various buffer gases in quenching the population trapping states of laser excited ytterbium ions. The ions were confined in an rf/dc hybrid trap, and their /sup 2/S/sub 1/2//spl hArr//sup 2/P/sub 3/2/ transition was excited with 329 nm laser light. Buffer gases used include H/sub 2/, N/sub 2/, CO/sub 2/, Ar, and He at several different pressure regimes. We have identified N/sub 2/ to be highly efficient in quenching the population of the trapping states, and thus be the most suitable buffer gas to use with ytterbium for the development of a trapped ion microwave frequency standard.
We present the design of a compact optical clock based on the 2 S 1/2 → 2 D 3/2 435.5 nm transition in 171 Yb + . The ion trap will be based on a micro-fabricated circuit, with surface electrodes generating a trapping potential to localize a single Yb ion a few hundred μ m from the electrodes. We present our trap design as well as simulations of the resulting trapping pseudo-potential. We also present a compact, multi-channel wavelength meter that will permit the frequency stabilization of the cooling, repumping and clear-out lasers at 369.5 nm, 935.2 nm and 638.6 nm needed to cool the ion. We use this wavelength meter to characterize and stabilize the frequency of extended cavity diode lasers at 369.5 nm and 638.6 nm.
This work presents various novel results in the field of experimental and theoretical trapped ion studies. All investigations involve laser-cooled 138Ba+ ions confined in a linear quadrupole ion trap, which serve as coolant for complex molecular ions, as target for collisions with neutral atoms, or as model system in theoretical analyses. The special feature of the apparatus used in this work, is a molecular ion source based on electrospray ionization that allows for the production of gas-phase molecular ions of almost arbitrary species with a maximum mass-to-charge ratio of 2,000 Da. Trapped together with laser-cooled 138Ba+ ions, the molecular ions can be cooled down, kept cold and investigated for many minutes and in principle for up to hours in a nearly collisionless environment. In this work, the sympathetic cooling of complex molecular ions to translational temperatures below 1 K is demonstrated for different types of molecules covering a mass range from 182 to 12,400 Da. Using the example of the protein cytochrome c, with a mass of about 12,400 Da the most massive molecular species sympathetically cooled in an ion trap so far, the methods for detection of successful trapping and cooling, and temperature determination are presented in detail. In one case, multiply protonated molecules of the protein cytochrome c have been cooled to less than 0.75 K. Furthermore, two methods for measuring photodestruction rates of cold, trapped complex molecular ions have been developed that can, for example, be applied in destructive spectroscopy schemes. Both techniques are demonstrated using the example of singly protonated glycyrrhetinic acid molecules (GAH+) dissociated by an ultraviolet (UV) laser. Measurements of the photodestruction rate were performed at different intensities of the UV laser, and rates as low as 0.05 s-1 and less have been determined, which is only possible due to the long ion storage times that can be achieved with this apparatus. For the UV laser wavelength of 266 nm, a destruction cross section of (1.1±0.1)∙10E-17 cm2 for GAH+ has been determined. In this work, feasibility studies towards internal cooling of trapped complex molecular ions have been performed. Such an additional internal cooling could be realized via collisions with laser-cooled neutral atoms. To test their suitability, neutral atoms of Li, Na, K, Rb, Cs, and Yb have been collided with trapped laser-cooled and not cooled Ba+ ions as well as with complex molecular ions. For all species charge exchange reactions with Ba+ ions have been observed excluding these species for collisional cooling. While K, Rb, and Cs showed charge exchange reactions with ground state and laser-excited Ba+ ions, the charge exchange rates of Yb and Ba+ were found to be state dependent. Collisions of neutral Yb atoms were found to be reactive and lead to the formation of adducts and fragments, in contrast to the alkali metals for which no such reactions have been observed. In addition, the motion of cooled, trapped ions has been studied theoretically via simulations. When the translational energy of a trapped ion ensemble is sufficiently reduced, a first-order phase transition from a liquid to a solid state occurs with the ion ensemble changing from a disordered ion cloud to an ordered ion crystal. As ions in a linear quadrupole trap arrange in shell structures, a free motion in radial direction is disabled, and ions can only move to neighboring shells when their kinetic energy is high enough to overcome a certain potential barrier. The simulations have shown that intershell diffusion rates in the considered ion crystals increase exponentially when the ion temperatures exceed values that well agree with the predictions of other established models. Ion ensembles of different ion numbers (300, 500, and 1000 138Ba+ ions) and symmetries (prolate and spherical) have been investigated showing a clear tendency for the phase transition temperatures: The higher the ion number, the lower is the transition temperature.