Trapping, Cooling, and Photodissociation Analysis of State-Selected ${\mathrm{H}}_{2}^{+}$ Ions Produced by ($3+1$) Multiphoton Ionization

Trapping, Cooling, and Photodissociation Analysis of State-Selected $H_{2}^{+}$ Ions Produced by ( $3 + 1$ ) Multiphoton Ionization

Julian Schmidt, Thomas Louvradoux, Johannes Heinrich, Nicolas Sillitoe, Malcolm Simpson, Jean-Philippe Karr, and Laurent Hilico

Phys. Rev. Applied 14, 024053 – Published 19 August 2020

Abstract

We report on the production of cold, state-selected $H_{2}^{+}$ molecular ions in a linear rf trap. The ions are produced by ( $3 + 1$ ) resonance-enhanced multiphoton ionization (REMPI) of $H_{2}$ , and sympathetically cooled by laser-cooled ${Be}^{+}$ 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 $H_{2}^{+}$ ion samples. Moreover, the large difference between the photodissociation efficiencies of ions created in the $v = 0$ and $v = 1$ levels provides a way to detect a $v = 0 \to 1$ transition. These results pave the way towards high-resolution vibrational spectroscopy of $H_{2}^{+}$ for fundamental metrology applications.

Received 28 April 2020
Revised 4 June 2020
Accepted 9 July 2020

DOI:https://doi.org/10.1103/PhysRevApplied.14.024053

Physics Subject Headings (PhySH)

Atom & ion cooling Cold and ultracold molecules Molecular spectra Molecule trapping & guiding Multiphoton or tunneling ionization & excitation Photodissociation

Atomic, Molecular & Optical

Authors & Affiliations

Julian Schmidt ¹, Thomas Louvradoux ¹, Johannes Heinrich¹, Nicolas Sillitoe ¹, Malcolm Simpson ², Jean-Philippe Karr ^1,3, and Laurent Hilico ^1,3,*

¹Laboratoire Kastler Brossel, Sorbonne Université, CNRS, ENS-PSL Research University, Collège de France, 4 place Jussieu, F-75005 Paris, France
²Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria
³Université d’Evry-Val d’Essonne, Université Paris-Saclay, Boulevard François Mitterrand, F-91000 Evry, France

^*laurent.hilico@spectro.jussieu.fr

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Vol. 14, Iss. 2 — August 2020

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Atomic and Molecular Physics

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Figure 1
Schematic of the experimental setups. (a) Molecular-beam setup, which can be connected to one of two UHV chambers containing different ion traps. $N$ , pulsed valve with a 150- $μ m$ nozzle; $S_{1}$ , 1-mm skimmer; $S_{2}$ , 150- $μ m$ skimmer. Typical pressures in the different pumping stages are indicated. The pressures in the ion traps are typically $10^{- 10}$ mbar. (b) Hyperbolic rf ion trap. EG, electron gun (home-made tungsten hair pin); MCP, microchannel plate. (c) Segmented linear rf ion trap with laser-cooled ${Be}^{+}$ ions. EG, Kimball Physics electron gun FRA-2X1-2. Imaging of the ${Be}^{+}$ fluorescence on the EMCCD camera is done using a $f = 70$ -mm objective with a magnification of 9. Another part of the fluorescence is collected by an in-vacuum lens ( $f = 30$ mm) and focused on a photomultiplier tube (PMT).
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Figure 2
Mean ion signal as a function of the REMPI laser wavelength, obtained after 3 s of REMPI pulses synchronized with $H_{2}$ pulses (corresponding to 60 pulses). The ion signal is averaged over 10 to 20 experimental cycles. Error bars represent the standard deviation. The positions of the resonances are in good agreement with the data shown in Ref. [26], allowing their identification.
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Figure 3
Mean ion signal from the $v = 0$ , R(0)–R(1) line (see Fig. 2) as a function of pulse energy. Each data point is an average of 10 to 60 experiments. Horizontal error bars represent a 5% uncertainty of the pulse energy. Vertical error bars include the statistical deviation within one measurement run and a conservative estimate of the stability over the duration of the entire experiment. The stability is in part limited by the fluctuating $H_{2}$ background gas pressure. The solid line is a fit with a cubic model, ion signal $= 0.136 E^{3}$ .
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Figure 4
Mean ion signal in the $v = 0$ state as a function of the time of flight, i.e., the delay between the opening of the pulsed valve and the REMPI laser pulse. Each data point is the average of 10 to 20 experimental sequences. Error bars represent the standard deviation. The backing pressure is 2 bar and the valve opening duration is estimated to be shorter than $10 μ s$ . The dashed line is a fit by a Gaussian function with a FWHM of $35 (2) μ s$ .
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Figure 5
Sympathetic cooling and detection of $H_{2}^{+}$ ions produced by REMPI. (a) Fluorescence image of a ${Be}^{+}$ ion Coulomb crystal in the linear trap. The dark region along the horizontal axis is due to the presence of sympathetically cooled $H_{2}^{+}$ ions. (b) Typical detection signals of $H_{2}^{+}$ ions. The fluorescence of the ${Be}^{+}$ ions is measured as a function of the excitation frequency $f$ . Three 0.5-s sweeps (light gray) are averaged. The peak at about 1.8 MHz corresponds to excitation of the radial secular motion of $H_{2}^{+}$ . In the second signal, the peak around 1.2 MHz corresponds to the appearance of $H_{3}^{+}$ ions due to chemical reactions of $H_{2}^{+}$ with $H_{2}$ (see text). The $H_{2}^{+}$ ion signal is the shaded area below the curve. (c) Experimental sequence for measuring the photodissociation efficiency. The $H_{2}^{+}$ ion number is measured nondestructively before and after interaction with the 213-nm laser.
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Figure 6
Dissociation of $H_{2}^{+}$ ions produced by REMPI in the $v = 0$ and in the $v = 1$ states and sympathetically cooled in the linear trap. The normalized remaining ion fraction $F$ (see text) is plotted as a function of the dissociation duration $t_{d}$ during which the 213-nm laser is applied. Each data point is an average of 6 to 25 measurements. Error bars correspond to the standard deviation. The solid lines indicate a common fit of the data with $P_{d} = 9.8$ mW using Eq. (3). The dashed line is a prediction for the data with $P_{d} = 22.6$ mW. Squares, $λ = 302.47$ nm, $P_{d} = 9.8$ mW; circles, $λ = 295.61$ nm, $P_{d} = 9.8$ mW; triangles, $λ = 295.61$ nm, $P_{d} = 22.6$ mW.
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Physical Review Applied

Trapping, Cooling, and Photodissociation Analysis of State-Selected H2+ Ions Produced by (3+1) Multiphoton Ionization

Julian Schmidt, Thomas Louvradoux, Johannes Heinrich, Nicolas Sillitoe, Malcolm Simpson, Jean-Philippe Karr, and Laurent Hilico

Phys. Rev. Applied 14, 024053 – Published 19 August 2020

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Trapping, Cooling, and Photodissociation Analysis of State-Selected $H_{2}^{+}$ Ions Produced by ( $3 + 1$ ) Multiphoton Ionization