Cold chemistry with electronically excited Ca+ Coulomb crystals

Abstract

Rate constants for chemical reactions of laser-cooled Ca(+) ions and neutral polar molecules (CH(3)F, CH(2)F(2), or CH(3)Cl) have been measured at low collision energies (<E(coll)>/k(B)=5-243 K). Low kinetic energy ensembles of (40)Ca(+) ions are prepared through Doppler laser cooling to form "Coulomb crystals" in which the ions form a latticelike arrangement in the trapping potential. The trapped ions react with translationally cold beams of polar molecules produced by a quadrupole guide velocity selector or with room-temperature gas admitted into the vacuum chamber. Imaging of the Ca(+) ion fluorescence allows the progress of the reaction to be monitored. Product ions are sympathetically cooled into the crystal structure and are unambiguously identified through resonance-excitation mass spectrometry using just two trapped ions. Variations of the laser-cooling parameters are shown to result in different steady-state populations of the electronic states of (40)Ca(+) involved in the laser-cooling cycle, and these are modeled by solving the optical Bloch equations for the eight-level system. Systematic variation of the steady-state populations over a series of reaction experiments allows the extraction of bimolecular rate constants for reactions of the ground state ((2)S(1/2)) and the combined excited states ((2)D(3/2) and (2)P(1/2)) of (40)Ca(+). These results are analyzed in the context of capture theories and ab initio electronic structure calculations of the reaction profiles. In each case, suppression of the ground state rate constant is explained by the presence of a submerged or real barrier on the ground state potential surface. Rate constants for the excited states are generally found to be in line with capture theories.

Production of cold bromine atoms at zero mean velocity by photodissociation

“…By mismatching the fragment recoil and parent molecule velocities, thus creating samples with different mean velocities, it should be possible to create a source of cold atoms with tunable energy, which would be suitable for collisional studies. The atom densities demonstrated in this work are already comparable with neutral molecule densities used in recent ion-molecule experiments in our group 40,41 . Figure 5 shows simulations (using the same parameters as in the simulations of the experimental images in this paper) of the axial velocity distribution of a slow beam of Br atoms created in the entrance of a 5 mm radius, 1 T depth magnetic guide (equivalent to a temperature depth of 1.34 K).…”

Design and characterization of a linear quadrupole ion trap for high-resolution Coulomb-crystal time-of-flight mass spectrometry

“…Previously, images of Coulomb crystals were used to calculate the crystal volume assuming that the crystal is rotationally symmetric in a standard LQT. From the change of the crystal volume over time, reaction rates could be extracted [9,[15][16][17]. This method is difficult to use when several ionic species are involved in a reaction and rates have to be determined for all channels.…”

“…Thus, part of the Ca + ions are in electronically excited states from where they can react with different rates than from the ground state [9,15,16,36]. To determine the ground-state reaction rate, we repeated the experiment by blocking the 397 nm laser during the reaction to only have Ca + ions in the electronic ground state.…”

“…Laser-and sympathetically-cooled ions in traps form ordered structures termed Coulomb crystals [6]. Rate constants for molecules reacting with the laser cooled or sympathetically cooled ions can be determined by observing changes in the crystal structure [6][7][8][9][10]. The mass-to-charge ratio of the ionic reaction products can be determined by resonant-excitation mass spectrometry (REMS) [8,11,12].…”

Control of the conformations of ion Coulomb crystals in a Penning trap

“…4). Beyond this, other forms of ICCs in RF traps have been used for a variety of experiments, including cavity quantum electrodynamics (QED) 5 and the measurement of reaction rates of ions with molecules 6 . Many further proposals have been made for the use of ICCs in direct quantum simulation of a wide variety of physical systems and processes, from quantum spin systems 7 to Hawking radiation 8 .…”