Perspective Cite This: J. Phys. Chem. Lett. 2018, 9, 5797−5804
pubs.acs.org/JPCL
Spectroscopy of Molecular Ions in Coulomb Crystals Aaron T. Calvin† and Kenneth R. Brown*,†,‡ †
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States Departments of Electrical and Computer Engineering, Chemistry, and Physics, Duke University, Durham, North Carolina 27708, United States
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ABSTRACT: In this Perspective, we examine the use of laser-cooled atomic ions and sympathetically cooled molecular ions in Coulomb crystals for molecular spectroscopy. Coulomb crystals are well-isolated environments that provide localization and long storage times for sensitive measurements of weak signals and cold temperatures for precise spectroscopy. Coulomb crystals of molecular and atomic ions enable the detection of singlephoton molecular ion transitions at a range of wavelengths by a change in atomic ion fluorescence at visible wavelengths. We give an overview of the state of the art from action spectroscopy to quantum logic spectroscopy for a wide range of molecular transitions from rotational sublevels separated by 10−7 cm−1 to rovibronic transitions at 25 000 cm−1. We emphasize how this system allows for unparalleled control of the molecular ion state for precision spectroscopy with applications in astrochemistry and fundamental physics. We conclude with an outlook of the use of this control in cold molecular ion reactions.
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neutral counterparts. As an example, CaH was identified in sunspots over a century ago,5 yet endothermic reactions of Ca+ in the ground state with H26 hindered the spectroscopy of the molecular ion before studies in a Coulomb crystal within the past few years.7−12 Trapping molecular ions in a Coulomb crystal allows a high degree of localization and long storage times that enable the observation of weak transitions with even single ions. These molecular ions are translationally cooled to millikelvin temperatures by the Coulomb interaction with the Doppler-cooled atomic ions. Pristine control of all of the ion’s degrees of freedom is driving much innovation in the field. This control will enable the use of quantum logic techniques to probe fundamental physics through precision spectroscopy. The measurement of transitions in polyatomic molecular ions and ion-neutral reaction studies is a promising direction of interest to both the physics and chemistry communities. Laser-Cooled Atomic and Molecular Ions. Mixtures of atomic and molecular ions can be held for long times in Paul13 or Penning traps.14,15 In this Perspective, we focus on Paul traps, which hold the ions using dynamic electric fields. The ion experiences an effective pseudopotential from the dynamic field dependent on the charge-to-mass ratio. Ions with a lower charge-to-mass ratio are more tightly confined. The dynamic electric fields imparts a fast oscillating motion on the ions, known as micromotion. The micromotion amplitude depends on the distance between the ions and the RF null along the trap axis. The motion of the ions about the pseudopotential results in a slower motion called the secular motion. A number of atomic ions can be laser cooled. Typical choices are ions with an unpaired electron in an s-orbital such as alkaline
pectroscopy of molecular ions in Coulomb crystals has gained attention in recent years for many applications because of the high degree of control this platform affords (Figure 1). Despite the inherent challenges of molecular ions,
Figure 1. Advantages of using Coulomb crystals for spectroscopy and reaction studies stem from the high degree of control (localization, quantum states, and selective molecule loading) and versatility in the range of molecules and transitions that may be observed.
such as low densities due to Coulomb repulsion, poor yields, and large reaction cross sections with background gases, spectroscopy of molecular ions is imperative to understand reaction mechanisms such as ionic reactions in organic chemistry1,2 or the reactions proposed to occur in space.3,4 Ion traps provide an isolated environment for the creation and study of reactive species whose spectra are often unknown compared to their © XXXX American Chemical Society
Received: May 1, 2018 Accepted: September 13, 2018 Published: September 13, 2018 5797
DOI: 10.1021/acs.jpclett.8b01387 J. Phys. Chem. Lett. 2018, 9, 5797−5804
The Journal of Physical Chemistry Letters
Perspective
Figure 2. Methods for observing action spectroscopy in Coulomb crystals. (a) Measurement of photon-induced charge transfer reactions of N2+ in the vibrationally excited state with Ar by observing changes in crystal structure. Reprinted with permission from ref 23. Copyright 2014 Nature Publishing Group. (b) Measurement of rovibrational transitions in HD+ by REMPD and detected by mass spectrometry based on changing the fluoresence by exciting the molecular ion motion. Reprinted with permission from ref 24. Copyright 2012 American Physical Society. (c) Measurement of rovibronic transitions in CaH+ by REMPD. The dissociation yields fluorescent Ca+ that can be used to monitor the reaction. Reprinted from ref 11. Copyright 2018 American Chemical Society.
earth ions or Hg+ and Yb+.16 Doppler cooling is the workhorse of laser-cooling and uses the Doppler effect and a strong, cycling transition to transfer energy from the ion’s motion to the difference in energy of absorbed and scattered photons. The cooling efficiency is limited by the natural line width of the cooling transition and is typically on the order of a millikelvin for alkaline earth ions. For a detailed description of laser cooling, see the book Laser Cooling and Trapping by Metcalf and van der Straten.17 Cotrapping molecular ions with laser-cooled atomic ions enables the cooling of molecular ion motion due to the Coulombic interaction. The first cotrapping of molecular ions with laser-cooled atomic ions was shown to sympathetically cool the molecular temperature to
