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Trapped Ultracold Molecular Ions: Candidates for an Optical Molecular Clock for a Fundamental Physics Mission in Space

Bernhard Roth, Jeroen C. J. Koelemeij, H. Daerr, Ingo Ernsting, Solvejg Jørgensen, Maksim V. Okhapkin, Andreas Wicht, Alexander Yu. Nevsky, Stephan Schiller

Published Jun 1, 2006• 1 citation
OpenAlex

Abstract

Narrow ro-vibrational transitions in ultracold molecules are excellent candidates for frequency references in the near-IR to visible spectral domain and interesting systems for fundamental tests of physics, in particular for a satellite test of the gravitational redshift of clocks. We have performed laser spectroscopy of several ro-vibrational overtone transitions υ = 0 → υ = 4 in HD + ions at around 1.4 μm. 1+1 REMPD was used as a detection method, followed by measurement of the number of remaining molecules. The molecular ions were stored in a linear radiofrequency trap and cooled to millikelvin temperatures, by sympathetic cooling using laser-cooled Be + ions simultaneously stored in the same trap.

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Narrow ro-vibrational transitions in ultracold molecules are excellent candidates for frequency references in the near-IR to visible spectral domain and interesting systems for fundamental tests of physics, in particular for a satellite test of the gravitational redshift of clocks. We have performed laser spectroscopy of several ro-vibrational overtone transitions υ = 0 → υ = 4 in HD + ions at around 1.4 μm. 1+1 REMPD was used as a detection method, followed by measurement of the number of remaining molecules. The molecular ions were stored in a linear radiofrequency trap and cooled to millikelvin temperatures, by sympathetic cooling using laser-cooled Be + ions simultaneously stored in the same trap.
Narrow ro-vibrational transitions in ultracold molecules are excellent candidates for frequency references in the near-IR to visible spectral domain and interesting systems for fundamental tests of physics, in particular for a satellite test of the gravitational redshift of clocks. We have performed laser spectroscopy of several ro-vibrational overtone transitions υ = 0 → υ = 4 in HD+ ions at around 1.4 μm. 1+1 REMPD was used as a detection method, followed by measurement of the number of remaining molecules. The molecular ions were stored in a linear radiofrequency trap and cooled to millikelvin temperatures, by sympathetic cooling using laser-cooled Be+ ions simultaneously stored in the same trap.
We have measured the ro-vibrational infrared transition (v = 0, J = 2) → (v = 4, J = 1) in ultracold HD+ molecular ions embedded inside Be+ Coulomb crystals, by 1+1 REMPI spectroscopy. The molecular ions, trapped in a linear radiofrequency trap, were cooled to temperatures of few tens of mK, via sympathetic cooling using laser-cooled Be+ ions. For detection, we have used secular excitation of the HD+ ions to monitor their loss rate. The transition frequency agrees with a theoretical prediction.
A single, trapped and ultracold molecular hydrogen ion is an attractive quantum system for exploring various aspects of fundamental physics, such as the determination of fundamental constants and testing their time-independence. Here we demonstrate, for the first time, controlled loading, sympathetic cooling, mass spectrometric identification, and vibrational excitation of ultracold single HD+ ions trapped in a tightly confining radiofrequency trap using single laser-cooled Be+ ions for sympathetic cooling. The apparatus can be used also for preparing other single ions, both lighter and heavier than the coolant ion.
We report a high-resolution spectroscopic study of molecular ions at millikelvin temperatures. We measured several rovibrational infrared transitions in HD+\mathrm{H}{\mathrm{D}}^{+}HD+ molecular ions, stored in a radio-frequency trap and sympathetically cooled to $\ensuremath{\approx}20\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ by laser-cooled Be+{\mathrm{Be}}^{+}Be+ ions. We observed hyperfine splitting of the lines, in good agreement with theoretical predictions. The transitions were detected by monitoring the decrease in ion number after selective photodissociation of HD+\mathrm{H}{\mathrm{D}}^{+}HD+ ions in the upper vibrational state. The method described here is expected to be generally applicable.
We have produced large samples of ultracold (<20 mK) ArH + , ArD + , N 2 H + , N 2 D + , H 3 + , D 3 + , D 2 + , H 2 D + and D 2 D + molecular ions, by sympathetic cooling and crystallization via laser-cooled Be + ions in a linear radio-frequency trap. As technique, we used chemical reactions with sympathetically cooled noble gas atomic ions or N 2 + and O 2 + molecular ions. These ultracold molecules are interesting targets for high-precision measurements in fundamental physics and may open new routes for the study of state-selective chemical reactions relevant to interstellar chemistry.
We propose to use laser spectroscopy of ultracold molecules to establish improved limits on the time independence of electron-to-nuclear and nuclear-nuclear mass ratios by comparing, via an optical frequency comb, the frequencies of suitable sets of transitions in the ground electronic state. Hydrogen molecular ions trapped in a radiofrequency trap, sympathetically cooled by atomic ions, are identified as an accessible system. We show that the dipole-allowed rovibrational transition frequencies of HD+\mathrm{H}{\mathrm{D}}^{+}HD+ are suitable probes for a time dependence of me∕mp{m}_{e}∕{m}_{p}me​∕mp​ or mp∕md{m}_{p}∕{m}_{d}mp​∕md​. Separate bounds on the time independence of these constants can be obtained from a comparison of HD+\mathrm{H}{\mathrm{D}}^{+}HD+ and H2+\mathrm{H}_{2}{}^{+}H2​+ transitions frequencies. Spectroscopy of single molecular ions via a quantum jump method is proposed as an approach toward ultrahigh precision.