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AIP conference proceedings

Experiments with single cold molecular ions

Klaus Højbjerre, Peter Frøhlich Staanum, David Offenberg, Jens Lykke Sørensen, A. Mortensen, Michael Drewsen

Published Jan 1, 2006• 1 citation
Publisher

Abstract

In this paper we present ongoing work on single cold molecular ion experiments in a linear Paul trap. In particular, we discuss the properties and prospects of using a non‐destructive identification technique. Measuring the resonance frequency of a common motional mode of a laser cooled atomic ion and a molecular ion the harmonic trap potential, the mass of the latter can be determined. We report recent results using this technique for the cases of 40Ca18O+ and C3H3+.

DOI
10.1063/1.2387936

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In this paper we present ongoing work on single cold molecular ion experiments in a linear Paul trap. In particular, we discuss the properties and prospects of using a non‐destructive identification technique. Measuring the resonance frequency of a common motional mode of a laser cooled atomic ion and a molecular ion the harmonic trap potential, the mass of the latter can be determined. We report recent results using this technique for the cases of 40Ca18O+ and C3H3+.
We demonstrate a simple and nondestructive method for identification of a single molecular ion sympathetically cooled by a single laser cooled atomic ion in a linear Paul trap. The technique is based on a precise nondestructive determination of the molecular ion mass through a measurement of the eigenfrequency of a common motional mode of the two ions. The demonstrated mass resolution is sufficiently high that molecular ion mass doublets can potentially be distinguished from each other. The obtained results represent an important step towards single molecule gas phase chemical physics.
A range of experimental techniques for application in reaction studies between ionic and neutral atoms/molecules and high resolution spectroscopy experiments with sympathetically cooled molecular ions are presented. A novel ion trap loading scheme using the photo-ionisation of atoms generated by the pulsed laser ablation of a solid calcium target has been characterised. We have identified the range of ablation laser fluences that must be used in order to produce a flux of neutral calcium atoms, which is a prerequisite for isotope selective ion trap loading. Calcium ions are trapped and laser cooled in a linear radio-frequency ion trap. We have developed a spectroscopy scheme that allows the entire fluorescence spectrum of trapped ions to be rapidly collected with high precision while maintaining a low ion temperature and good ion localisation throughout interrogation. The scheme has been demonstrated by measuring the saturation intensity of the calcium ion 4S1/2?4P1/2 transition. We have developed a novel scheme to measure the secular motion of trapped ions and demonstrated the application of the technique to ion-neutral collision reaction experiments. Employing pulsed excitation and Doppler velocimetry, we have measured the centre-of-mass mode frequency of single ions as well as large ion crystals with a frequency precision better than 2x10-3 within an interrogation time on the order of seconds. This method has been used to measure the mass of ions and observe charge exchange collisions between trapped calcium isotopes. In particular, we have measured the 44Ca++40Ca!40Ca++44Ca reaction cross section and demonstrated the single-event resolution of the technique. Finally, we have developed a novel all-optical broadband scheme for exciting, amplifying and measuring the secular motion of ions in the trap. Oscillation induced by optical excitation has been coherently amplified to control and measure the ion's secular motion. Requiring only a single interrogation laser, the ion's oscillation amplitude can be precisely controlled. The application of this technique to non-destructive spectroscopy of trapped molecular ions is discussed.
This thesis reports on the setup of a new ion trap apparatus designed for experiments with single ⁴⁰Ca⁺ ions to perform molecular spectroscopy. The calcium ion is laser cooled, allowing for sympathetic cooling of the nonfluorescing molecular ion. The aim of these experiments is to explore loading and identifying molecular ions in RF-Paul traps, as well as developing new spectroscopic tools to measure transitions of molecular ions via the fluorescence of co-trapped ⁴⁰Ca⁺ ions. Ground state cooling of a mixed ion pair is implemented as a first step towards increasing the sensitivity of our technique to the level necessary to measure transitions with low scattering rates (like those present in molecular ions). Doppler cooling on the S(1/2)->P(1/2) transition of the calcium ion results in the formation of a Coulomb crystal, the behavior of which may be used to infer properties of the molecular ion. Following cooling, sideband spectroscopy on the narrow S(1/2)->D(5/2) quadrupole transition of calcium may be used to identify the mass of single molecular ions. This method is verified via a non-destructive measurement on ⁴⁰CaH⁺ and ⁴⁰Ca¹⁶O⁺. The normal modes of the Coulomb crystal can also be used to extract information from the target ion to the control ion. By driving the blue side of a transition, laser induced heating can be put into the two ion system, which leads to changes in fluorescence of the ⁴⁰Ca⁺ ion, first demonstrated with two Ca⁺ isotopes. Increasing the sensitivity of this technique requires ground state cooling of both the ⁴⁰Ca⁺ ion and the ion of interest, enabling the transfer of the ion's motional state into the ground state with high probability. This thesis demonstrates ground state cooling of the atomic ion and sympathetic cooling of a second ion (⁴⁴Ca⁺). Once in the ground state, heating of the Coulomb crystal by scattering photons off of the spectroscopy ion can be measured by monitoring the resolved motional sidebands of the S(1/2)->D(5/2) transition of ⁴⁰Ca⁺, allowing for spectral lines to be inferred. Future experiments will investigate this technique with molecular ions.
The properties of cold ion plasmas with large numbers (>200)(>200)(>200) of laser- and sympathetically cooled species are modeled in detail using molecular dynamics simulations. We describe how to extract temperatures and ion numbers from CCD images. The identification of the ion species by excitation of their oscillation modes is discussed. The sympathetic cooling efficiency, effects of the rf micromotion and of collision heating by a neutral background gas are analyzed, in part experimentally.
Trapped laser-cooled atomic ions are a new tool for understanding cold molecular ions. The atomic ions not only sympathetically cool the molecular ions to millikelvin temperatures, but the bright atomic ion fluorescence can also serve as a detector of both molecular reactions and molecular spectra. We are working towards the detection of single molecular ion spectra by sympathetic heating spectroscopy. Sympathetic heating spectroscopy uses the coupled motion of two trapped ions to measure the spectra of one ion by observing changes in the fluorescence of the other ion. Sympathetic heating spectroscopy is a generalization of quantum logic spectroscopy, but does not require ions in the motional ground state or coherent control of the ion internal states. We have recently demonstrated this technique using two isotopes of Ca+ [Phys. Rev. A, 81, 043428 (2010)]. Limits of the method and potential applications for molecular spectroscopy are discussed.
We investigate the coupling between motional resonances of translationally cold (10mK10\phantom{\rule{0.3em}{0ex}}\mathrm{mK}10mK range) atomic and molecular ions in multispecies Coulomb crystals, stored in a linear Paul trap. The atomic and molecular ions were sympathetically cooled by laser-cooled Be+{\mathrm{Be}}^{+}Be+ ions. The motional resonances of the different ion species were determined nondestructively with high mass-to-charge resolution upon excitation with an oscillating electric field. The experimental results show good agreement with results from molecular dynamics simulations and allow for a precise identification of sympathetically cooled particles in multispecies ion crystals.