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arXiv:2304.01581 · Physical Review A

Generation of rotational-ground-state HD+ ions in an ion trap using a resonance-enhanced threshold photoionization process

Yong Zhang, Qian-Yu Zhang, Wen-Li Bai, Zhi-Yuan Ao, Wen-Cui Peng, Sheng-Guo He, Xin Tong

Published Apr 3, 2023• 16 citations
arXivHTMLPDF

Abstract

We report a method for producing ultracold ${\mathrm{HD}}^{+}$ molecular ions populated in a rotational ground state in an ion trap based on $[2+{1}^{\ensuremath{'}}]$ resonance-enhanced threshold photoionization (RETPI) and sympathetic cooling with the laser-cooled ${\mathrm{Be}}^{+}$ ions. The effects of electric field of the ion trap on the RETPI process of neutral HD molecules and the blackbody radiation (BBR) on the population evolution of rotational states of the generated polar ${\mathrm{HD}}^{+}$ ions have been studied. The initial rotational-ground-state population of ${\mathrm{HD}}^{+}$ ions is 0.93(12). After the cumulation time of 5 s, the rotational-ground-state population is reduced to 0.77(8) due to the BBR coupling. This method of generating ultracold state-selected ${\mathrm{HD}}^{+}$ ions is beneficial for the studies in precision rovibrational spectroscopy, state-controlled cold chemical reaction, and quantum logic spectroscopy.

DOI
10.1103/physreva.107.043101

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We report a method for producing ultracold HD+{\mathrm{HD}}^{+}HD+ molecular ions populated in a rotational ground state in an ion trap based on [2+1\ensuremath′][2+{1}^{\ensuremath{'}}][2+1\ensuremath′] resonance-enhanced threshold photoionization (RETPI) and sympathetic cooling with the laser-cooled Be+{\mathrm{Be}}^{+}Be+ ions. The effects of electric field of the ion trap on the RETPI process of neutral HD molecules and the blackbody radiation (BBR) on the population evolution of rotational states of the generated polar HD+{\mathrm{HD}}^{+}HD+ ions have been studied. The initial rotational-ground-state population of HD+{\mathrm{HD}}^{+}HD+ ions is 0.93(12). After the cumulation time of 5 s, the rotational-ground-state population is reduced to 0.77(8) due to the BBR coupling. This method of generating ultracold state-selected HD+{\mathrm{HD}}^{+}HD+ ions is beneficial for the studies in precision rovibrational spectroscopy, state-controlled cold chemical reaction, and quantum logic spectroscopy.
We present a new method for the generation of rotationally and vibrationally state-selected, translationally cold molecular ions in ion traps. Our technique is based on the state-selective threshold photoionization of neutral molecules followed by sympathetic cooling of the resulting ions with laser-cooled calcium ions. Using N₂(+) ions as a test system, we achieve >90% selectivity in the preparation of the ground rovibrational level and state lifetimes on the order of 15 minutes limited by collisions with background-gas molecules. The technique can be employed to produce a wide range of apolar and polar molecular ions in the ground and excited rovibrational states. Our approach opens up new perspectives for cold quantum-controlled ion-molecule-collision studies, frequency-metrology experiments with state-selected molecular ions and molecular-ion qubits.
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This thesis looks into developing strategies that maintain state selection in molecular ions, allowing one to observe state-selected reactions in cold environments, in particular the state-selected reaction between C2H+2 and ND3. Examining reactive ion molecule collisions under cold conditions provides insight into fundamental reaction dynamics, which are thermally averaged out at higher temperatures. A theoretical model is used to investigate laser-driven, blackbody-mediated, rotational cooling schemes for several 1Σ and 2Π diatomic species. The rotational cooling is particularly effective for DCl+ and HCl+, for which 92% and >99% (respectively) of the population can be driven into the rovibrational ground state. For the other systems a broadband optical pumping source is found to enhance the population that can be accumulated in the rovibrational ground state by up to 29% more than that achieved when exciting a single transition. 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In order to identify states of the acetylene ion that could be trapped sufficiently long enough for state-selected reactions in the ion trap with decelerated ND3, the theoretical work has been complemented by experimental investigations into the production of C2H+2 in selected states, and ion trapping of the same using sinusoidal and digital trapping voltages. Appropriate (2+1) REMPI (Resonance Enhanced Multiphoton Ionization) schemes are used to produce C2H+2 in different quantum states, with (1+1) Resonance Enhanced Multiphoton Dissociation (REMPD) employed to detect the ion thus produced. The concept of digital ion trapping for ejection onto MCPs is introduced. A comprehensive comparison between sinusoidal and digital trapping fields has been performed with respect to trap depth and stability regions. Programs have been developed to calculate the stability regions for different ions under varying experimental conditions. 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This technique is found to be well mass-resolved, with the signal arising from CaOH+ (57 amu) and CaOD+ (58 amu) resolvable on the TOF trace. This technique would enable one to monitor a reaction in a Coulomb crystal where the reactant and product species are both either lighter or heavier than calcium, such as the reaction between C2H+2 and ND3, something which has not been previously possible. It is, also, potentially a very important technique for reactions with many product channels.
We present a new method for the generation of rotationally and vibrationally state-selected, translationally cold molecular ions in ion traps. Our technique is based on the state-selective threshold photoionization of neutral molecules followed by sympathetic cooling of the resulting ions with laser-cooled calcium ions. Using N2+_2^+2+​ ions as a test system, we achieve > 90 % selectivity in the preparation of the ground rovibrational level and state lifetimes on the order of 15 minutes limited by collisions with background-gas molecules. The technique can be employed to produce a wide range of apolar and polar molecular ions in the ground and excited rovibrational states. Our approach opens up new perspectives for cold quantum-controlled ion-molecule-collision studies, frequency-metrology experiments with state-selected molecular ions and molecular-ion qubits.
Hydrogen molecular ions play an important role for accurate quantum electrodynamics calculations of rovibrational energy levels [1] and the comparison with laser spectroscopy results [2] allowed probing fundamental physics constants and laws. Advances are driven by an accurate control of the external and internal molecular degrees of freedom with a wide range of techniques [3]. Experimental setups with HD+ions are based on ions in a linear Paul trap and sympathetically cooled to the Lamb-Dick confinement regime by a laser-cooled atomic ion (Be+). Optical transitions allowed rotational cooling [4] and state-resolved detection by resonant-enhanced-multiphoton dissociation (REMPD) [5]. This contribution reports numerical results on the use of a two-photon rotational transition in the terahertz frequency range for preparation of trapped HD+ions in the rovibrational ground state and for high-resolution spectroscopy.
We report a method of efficiently generating ultracold hydrogen molecular ions HD+ in a segmented linear ion trap. HD+ ions are created by [2+1′] resonance-enhanced multiphoton ionization (REMPI) of an HD supersonic molecular beam, and sympathetically cooled by laser-cooled Be+ ions. The HD+ ions are generated species-selectively using 201 nm and 395 nm pulse lasers with energies of 0.08 mJ (5.1 × 106 W/cm2) and 1.5 mJ (9.5 × 107 W/cm2) respectively. The average loading rate of 4.3(2) HD+ ions per second in the ion trap is determined by comparing the experimental images with the simulated images of Be+-HD+ bi-component Coulomb crystals. This method paves a way to produce the state-selected HD+ ions, which can improve the signal strengths of the selected rovibrational transitions of HD+ ions.
We give a detailed characterization of a recently developed method to prepare translationally cold, internally state-selected molecular ions in ion traps [X. Tong, A. H. Winney, and S. Willitsch, Phys. Rev. Lett. 105, 143001 (2010)]. The technique relies on the generation of molecular ions in a well-defined rotational-vibrational quantum state using threshold photoionization followed by sympathetic cooling of the translational motion with laser-cooled Ca+{}^{+}+ ions. We discuss the experimental requirements for the successful generation and sympathetic cooling of state-selected ions, explore the influence of collisional and radiative processes on the population redistribution dynamics, and give an assessment of the scopeof the method.