Article pubs.acs.org/JPCA
Identifying Single Molecular Ions by Resolved Sideband Measurements James E. Goeders, Craig R. Clark, Grahame Vittorini, Kenneth Wright, C. Ricardo Viteri, and Kenneth R. Brown* Schools of Chemistry and Biochemistry; Computational Science and Engineering; and Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ABSTRACT: The masses of single molecular ions are nondestructively measured by cotrapping the ion of interest with a laser-cooled atomic ion, 40Ca+. Measurement of the resolved sidebands of a dipole forbidden transition on the atomic ion reveals the normal-mode frequencies of the two ion system. The mass of two molecular ions, 40CaH+ and 40 Ca16O+, are then determined from the normal-mode frequencies. Isotopes of Ca+ are used to determine the effects of stray electric fields on the normal mode measurement. The future use of resolved sideband experiments for molecular spectroscopy is also discussed.
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INTRODUCTION Molecular ions are an important driver of astrochemical processes1,2 as exemplified by H3+, which is ubiquitous in interstellar space and a key proton donor in the formation of cationic hydrocarbons.3 The measurement of hydrocarbon ion concentrations in interstellar clouds can shed light on the local chemical environment.4 Challenges for understanding this distant chemistry comes from both unknown species, as represented by unassigned lines in the diffuse interstellar band5,6 and unknown reaction rates of the vast number of possible collisions.7 Laboratory experiments are necessary to perform the high-resolution spectroscopy required for identifying interstellar species8−10 and to understand the reaction pathways of these ions at temperatures ranging from a few kelvin to 100 K.11−13 Mixtures of trapped molecular and laser-cooled atomic ions are a promising system for the study of cold molecular ion reactivity and spectroscopy.14,15 The strong optical transitions of atomic ions allow for rapid Doppler cooling to millikelvin temperatures, and cotrapped molecular ions may be sympathetically cooled to similar temperatures via Coulombic interaction.16 Due to the wide mass acceptance of linear Paul traps, a broad array of molecular ion masses can be sympathetically cooled. Molecules can either be loaded directly or through gas phase reactions with the trapped atomic ions, and examples ranging in size from HD+ to C60+ have been demonstrated.17,18 The fluorescence of the Doppler cooled atomic ions can be observed by a charge coupled device (CCD) camera, allowing the presence of nonfluorescing molecular ions to be inferred from the positions of fluorescing ions. For systems where the molecular ions are known, the position of the imaged atomic ions may be compared to molecular dynamics simulations to observe reactions.19 Action spectros© 2013 American Chemical Society
copy can then be performed by monitoring the rate of the reaction as a function of the excitation wavelength.20 Due to increased interest in experiments utilizing cold molecules, it is necessary to develop new techniques that can accurately identify molecular ions nondestructively so that further spectroscopy and control experiments may be performed. This is in contrast to common mass spectrometry techniques which are often destructive, thereby eliminating future experiments on the now known ion. The combination of mass and spectroscopic information could enable precise identification of a molecular ion reaction product. In general, direct fluorescence detection is not applicable to molecular ions due to the lack of closed optical transitions. Fortunately, strongly fluorescing atomic ions can serve as sensitive probes of cotrapped molecular ions. Atomic ion fluorescence is sensitive to Doppler shifts and can be used to measure the temperature or kinetic energy of the system.21,22 Observing the change in total fluorescence while exciting mass-dependent motional resonances of trapped ions is widely used for mass spectrometry of large and small ion clouds.23 This method, known as laser-cooled fluorescence mass spectrometry (LCFMS),16 can be used to follow reactions; e.g., the multistep photodestruction of the aniline cation (C6H5NH2+)24 or the slow photodissociation of biomolecules such as glycyrrhetinic acid.25 LCFMS has also been combined with resonantly enhanced multiphoton dissociation (REMPD) to accurately measure vibrational lines with sub-MHz precision.17 Special Issue: Oka Festschrift: Celebrating 45 Years of Astrochemistry Received: December 15, 2012 Revised: April 4, 2013 Published: April 5, 2013 9725
dx.doi.org/10.1021/jp312368a | J. Phys. Chem. A 2013, 117, 9725−9731
The Journal of Physical Chemistry A
Article
Figure 1. False color images of the ion trap used in these experiments. The image on the left shows the axial cross-section of the trap. The RF electrodes are colored dark gray, while the DC electrodes are shown in light gray. The small, circular electrodes colored purple are the compensation rods which run the length of the trap. The larger, cream colored circles are the Macor rods used for alignment and stability. Macor blocks are placed along the top and bottom of the electrodes for further alignment. The image on the right is the side view of the trap showing the Macor holders (cream) and the segmented DC electrodes (shown as light gray).
mm total). DC electrodes are diagonally opposite and consist of nine 3 mm wide electrodes arranged at 0.5 mm intervals with two 10 mm wide end-caps. Additionally, two stainless steel rods of 0.813 mm diameter and 52 mm in length are placed 5.15 mm from the trap axis for the application of compensation voltages. Macor rods are inserted into each electrode and run parallel to the trap axis to mechanically constrain the electrodes. Macor bars lay along the top and bottom of the trap and screw into each electrode and the stabilization end pieces for further stabilization. No voltage is applied to the end pieces, and they are the only mechanical and electrical connections to the vacuum chamber. The quantization axis (y) is defined by applying a 4 gauss magnetic field perpendicular to the trap axis to split the Zeeman levels. All necessary lasers are commercially available (Toptica DL 100). The lasers are introduced to the trap at 45° off-axis to cool transverse as well as axial modes of the atomic ion. The ions are imaged by sending 30% of the collected signal to a CCD camera (SBIG ST-3200ME) and 70% to a photomultiplier tube (Hamamatsu H7360-02). The camera monitors dark ion loss by distinguishing one bright ion and one dark ion from a single bright ion by the position of the bright ion.26 The PMT measures fluorescence for determining the excited state population. Two lasers are utilized for ionization of calcium, a Toptica SHG 110 to isotopically select calcium by exciting the 1S0→1P1 transition at 423 nm and a Nichia laser diode (377 nm) for the transition to the continuum. Toptica DL 100 lasers are used for all of the other transitions. The narrow, dipole forbidden S1/2→ D5/2 transition is driven by a 729 nm laser, which is locked to a temperature stabilized Fabry−Perot etalon via a Pound Drever Hall circuit. The etalon is made of Corning Code 7972, has a Finesse of ∼100 000, and was manufactured by Advanced Thin Films. An electro-optical modulator (EOM) is used for the necessary frequency modulation. A Toptica fast analog lock circuit (FALC) is used to provide feedback to a field effect transistor (FET) that narrows the line width of the laser from ∼500 kHz to
