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Chinese Journal of Physics

An efficient preparation of HD+ molecular ions in an ion trap by REMPI

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

Published Jan 13, 2023• 7 citations
Publisher

Abstract

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.

DOI
10.1016/j.cjph.2023.01.004

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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 report on the production of cold, state-selected H2+{\mathrm{H}}_{2}^{+}H2+​ molecular ions in a linear rf trap. The ions are produced by (3+13+13+1) resonance-enhanced multiphoton ionization (REMPI) of H2{\mathrm{H}}_{2}H2​, and sympathetically cooled by laser-cooled Be+{\mathrm{Be}}^{+}Be+ ions. After demonstrating and characterizing the REMPI process, we use photodissociation by a deep UV laser at 213 nm to verify the high vibrational purity of the produced H2+{\mathrm{H}}_{2}^{+}H2+​ ion samples. Moreover, the large difference between the photodissociation efficiencies of ions created in the v=0v=0v=0 and v=1v=1v=1 levels provides a way to detect a $v=0\ensuremath{\rightarrow}1$ transition. These results pave the way towards high-resolution vibrational spectroscopy of H2+{\mathrm{H}}_{2}^{+}H2+​ for fundamental metrology applications.
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.
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 $[2+{1}^{\ensuremath{'}}]$ 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.
The high-resolution ro-vibrational spectroscopy of the H2+ molecular ion by resonance-enhanced multiphoton dissociation (REMPD) requires a state-selective source of H2+. In this thesis we present work on a functional state selective H2+ ion source using resonance enhanced multiphoton ionisation (REMPI) with a 303 nm pulsed laser.The second part of the thesis presents numerical simulations of sympathetic cooling in linear RF traps, whose main application is the GBAR project (Gravitational Behaviour of Antihydrogen at Rest) which involves sympathetic cooling of an antimatter H̄+ ion by laser-cooled Be+ ions. We developed a GPU code using a variable timestep technique enabling a fast description of Coulomb interactions. We discuss the influence of RF heating and scaling laws between cooling times, initial energy and ion numbers in the cooling crystal. We show that the H̄ sympathetic cooling step of GBAR could be feasible using a rotationally asymmetric two-component Be+/HD+ crystal which appears more effective than a single-component Be+ crystal. We find that the H̄+ ion’s capture by this crystal could be detected experimentally by Fourier analysis of the fluorescence data.
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.
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.