Probing isotope effects in chemical reactions using single ions

Diabatic Potential Energy Surfaces of SrH2+ and Dynamics Studies of the Sr+(5s2S) + H2 Reaction

Based on the ab initio energy points of both ground and excited states, a neural network fitting method combined with a specific function was successfully used to construct the diabatic potential energy surfaces (PESs) of the SrH2+ system. The topographical features of the diabatic PESs were examined in detail. The results indicate that the nonadiabatic transition characteristics between ground and excited states are accurately described by the newly constructed diabatic PESs. To verify the validity and applicability of the diabatic PESs, as well as the nonadiabatic effects during the reaction process, the quantum dynamics studies of the Sr+(5s2S) + H2 reaction were performed based on both adiabatic and diabatic PESs. The dynamics results indicate that adiabatic dynamics results are dozens of times larger than those of nonadiabatic. This illustrates the significant effect of nonadiabaticity, indicating that adiabatic dynamics results often overestimate the actual values. The integral cross sections (ICSs) were calculated and compared with the experimental data. The diabatic ICSs are in good agreement with the experimental results. The reasonable dynamics results indicate that the newly constructed diabatic PESs are suitable for the relevant dynamics studies.

A Globally Accurate Neural Network Potential Energy Surface and Quantum Dynamics Studies on Be+(2S) + H2/D2 → BeH+/BeD+ + H/D Reactions

Chemical reactions between Be+ ions and H2 molecules have significance in the fields of ultracold chemistry and astrophysics, but the corresponding dynamics studies on the ground-state reaction have not been reported because of the lack of a global potential energy surface (PES). Herein, a globally accurate ground-state BeH2+ PES is constructed using the neural network model based on 18,657 ab initio points calculated by the multi-reference configuration interaction method with the aug-cc-PVQZ basis set. On the newly constructed PES, the state-to-state quantum dynamics calculations of the Be+(2S) + H2(v0 = 0; j0 = 0) and Be+(2S) + D2(v0 = 0; j0 = 0) reactions are performed using the time-dependent wave packet method. The calculated results suggest that the two reactions are dominated by the complex-forming mechanism and the direct abstraction process at relatively low and high collision energies, respectively, and the isotope substitution has little effect on the reaction dynamics characteristics. The new PES can be used to further study the reaction dynamics of the BeH2+ system, such as the effects of rovibrational excitations and alignment of reactant molecules, and the present dynamics data could provide an important reference for further experimental studies at a finer level.

Trapping and Sympathetic Cooling of Conformationally Selected Molecular Ions

We report the generation, trapping, and sympathetic cooling of individual conformers of molecular ions with the example of cis- and trans- meta-aminostyrene. Following conformationally selective photoionization, the incorporation of the conformers into a Coulomb crystal of laser-cooled calcium ions was confirmed by fluorescence imaging, mass spectrometry, and molecular dynamics simulations. We deduce the molecules to be stable in the trap environment for more than ten minutes. The present results pave the way for the spectroscopy and controlled chemistry of distinct ionic conformers in traps.

Cold Ion-Molecule Reactions in the Extreme Environment of a Coulomb Crystal

Coulomb crystals provide a unique environment in which to study ion-neutral gas-phase reactions. In these cold, trapped ensembles, we are able to study the kinetics and dynamics of small molecular systems. These measurements have connections to chemistry in the Interstellar Medium (ISM) and planetary atmospheres. This Feature Article will describe recent work in our laboratory that uses Coulomb crystals to study translationally cold, ion-neutral reactions. We provide a description of how the various affordances of our experimental system allow for detailed studies of the reaction mechanisms and the corresponding products. In particular, we will describe quantum-state resolved reactions, isomer-dependent reactions, and reactions with a rarely studied, astrophysically relevant ion, CCl+.

The impact of non-adiabatic effects on reaction dynamics: a study based on the adiabatic and non-adiabatic potential energy surfaces of CaH2. Phys Chem Chem Phys 2023;25:22744-22754. [PMID: 37605513 DOI: 10.1039/d3cp02416d] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The two-state non-adiabatic potential energy matrices of the CaH2+ system are calculated via a diabatization approach by using a neural network model. Subsequently, the adiabatic and non-adiabatic potential energy surfaces (PESs) are constructed based on these non-adiabatic potential energy matrices. Furthermore, based on the adiabatic and non-adiabatic PESs, the Ca+(4s2S) + H2(X1Σ+g) → H(2S) + CaH+(X1Σ+) reaction is studied using the time-dependent wave packet method. Comparative analysis of the experimental and theoretical integral reaction cross-sections (ICSs) indicates that the maximum deviation between the results obtained from the adiabatic PES and the corresponding experimental value is 12.7 bohr2; in contrast, the maximum discrepancy between the theoretical result derived from the non-adiabatic PES and the experimental value is merely 0.42 bohr2. The potential well along the reaction path acts as a 'filter', selectively guiding intermediates with longer lifetimes in the potential well back to the reactant channel. This phenomenon indicates that the non-adiabatic effects significantly influence the reaction dynamics of the CaH2+ system.

Conformational and state-specific effects in reactions of 2,3-dibromobutadiene with Coulomb-crystallized calcium ions

Recent advances in experimental methodology enabled studies of the quantum-state- and conformational dependence of chemical reactions under precisely controlled conditions in the gas phase. Here, we generated samples of selected gauche and s-trans 2,3-dibromobutadiene (DBB) by electrostatic deflection in a molecular beam and studied their reaction with Coulomb crystals of laser-cooled Ca⁺ ions in an ion trap. The rate coefficients for the total reaction were found to strongly depend on both the conformation of DBB and the electronic state of Ca⁺. In the (4p)²P_1/2 and (3d)²D_3/2 excited states of Ca⁺, the reaction is capture-limited and faster for the gauche conformer due to long-range ion-dipole interactions. In the (4s)²S_1/2 ground state of Ca⁺, the reaction rate for s-trans DBB still conforms with the capture limit, while that for gauche DBB is strongly suppressed. The experimental observations were analysed with the help of adiabatic capture theory, ab initio calculations and reactive molecular dynamics simulations on a machine-learned full-dimensional potential energy surface of the system. The theory yields near-quantitative agreement for s-trans-DBB, but overestimates the reactivity of the gauche-conformer compared to the experiment. The present study points to the important role of molecular geometry even in strongly reactive exothermic systems and illustrates striking differences in the reactivity of individual conformers in gas-phase ion-molecule reactions.

Efficiently loading 40Ca+-27Al+ ion crystal using sympathetic cooling and pulsed laser ablation

We demonstrate a method to efficiently load a pair of ⁴⁰Ca⁺-²⁷Al⁺ ion crystals with sympathetic cooling and pulsed laser ablation, serving as a starting step for the ²⁷Al⁺ clock. We achieved a technique to rapidly detect the loading of hot ions by monitoring the ²S_1/2 → ²D_5/2 narrow transition of ⁴⁰Ca⁺ that couples to the shared motional modes between the two ions. The sympathetic cooling time of the ⁴⁰Ca⁺-²⁷Al⁺ ion pair is measured. Two traps are employed to compare the loading time from two directions and it was found that the loading from the axial direction takes much shorter time than loading from the radial direction of the trap. With the help of adaptively controlled trap potential, our method reduced the average loading time of a ⁴⁰Ca⁺-²⁷Al⁺ pair from 26 to 1 min. This research is an important step for increasing the uptime ratio of the ²⁷Al⁺ optical clock and is useful for other mixed-species ion crystals based on sympathetic cooling.

Neural network potential energy surface and quantum dynamics studies for the Ca+(2S) + H2 → CaH+ + H reaction

Reactive collisions of Ca+ ion with H2 molecule play a crucial role in ultracold chemistry, quantum information and other cutting-edge fields, and have been widely concerned experimentally, but the corresponding...

Theoretical investigation of laser cooling for BN- anion by ab inito calculation

A theoretical investigation for the feasibility of laser cooling BN^-anion is presented. An ab initio calculation on the three low-lying states Χ²Σ⁺, Α²Π and Β²Σ⁺ are performed at the CASSCF/MRCI + Q level. The calculated spectroscopic constants are in good agreement with the available theoretical and experimental data. Radiative properties including Franck-Condon factor, Einstein coefficients and radiative lifetimes are determined. The calculation shows that the transition B²Σ⁺(v^')↔X²Σ⁺(v^'') has highly diagonal FCFs, especially f₀₀ = 0.9898, and enough short radiative lifetimes. A cooling scheme by three laser beams is proposed, which requires one main pumping laser(λ₀₀ = 474.67 nm) and two repumping lasers (λ₀₁ = 514.64 nm, λ₁₂= 514.90 nm). The population dynamics of cooling is investigated with the rate equation approach. The simulation demonstrates that the population does not remain trapped within the intermediate Α²Π state. The resultant scattered photons are about2.5×10⁴, which is expected to stop BN^-anion molecule in a cryogenic beam theoretically.

Optical Mass Spectrometry of Cold RaOH^{+} and RaOCH_{3}^{+}

We present an all-optical mass spectrometry technique to identify trapped ions. The new method uses laser-cooled ions to determine the mass of a cotrapped dark ion with a sub-dalton resolution within a few seconds. We apply the method to identify the first controlled synthesis of cold, trapped RaOH^{+} and RaOCH_{3}^{+}. These molecules are promising for their sensitivity to time and parity violations that could constrain sources of new physics beyond the standard model. The nondestructive nature of the mass spectrometry technique may help identify molecular ions or highly charged ions prior to optical spectroscopy. Unlike previous mass spectrometry techniques for small ion crystals that rely on scanning, the method uses a Fourier transform that is inherently broadband and comparatively fast. The technique's speed provides new opportunities for studying state-resolved chemical reactions in ion traps.

Quantum Dynamics Studies of the Significant Intramolecular Isotope Effects on the Nonadiabatic Be+(2P) + HD → BeH+/BeD+ + D/H Reaction

Quantum time-dependent wave packet dynamics studies on the nonadiabatic Be⁺(²P) + HD → BeH⁺/BeD⁺ + D/H reaction are performed for the first time employing recently constructed diabatic potential energy surfaces. Strong intramolecular isotope effects and unusual results are presented, which are attributed to the dynamic effects of shallow wells induced by avoided crossing on the diagonal V₂₂^d surface. The BeH⁺ + D and BeD⁺ + H channels are dominated by high-J and low-J partial waves, respectively. The BeD⁺/BeH⁺ branching ratio is larger than 10 at low energy and gradually decreases with increasing collision energy. The BeH⁺ product is primarily distributed at low vibrational states, whereas there exists an obvious population inversion of vibrational states on the BeD⁺ product. The results of differential cross sections suggest that the formation of the BeH⁺ + D channel favors a direct reaction process, while the BeD⁺ + H channel is mainly generated by the complex-forming mechanism.

Quantum dynamics studies of isotope effects in the Mg+(3p) + HD → MgH+/MgD+ + D/H insertion reaction

The time-dependent wave packet quantum dynamics studies for the Mg+(3p) + HD → MgH+/MgD+ + D/H diabatic reaction are carried out for the first time on recently developed diabatic YHWCH potential energy surfaces [Phys. Chem. Chem. Phys., 2018, 20, 6638-6647]. The results of reaction probabilities and total integral cross sections show a dramatic preference to the formation of MgD+ over MgH+ owing to the insertion reaction mechanism in the title reaction. The MgD+/MgH+ branching ratio witnesses a monotonic decrease from 10.58 to 3.88 at collision energy range of 0.01 to 0.20  eV, and at the collision energy of 0.114  eV, it is close to the experimental value of 5. The rovibrational state-resolved ICSs of the two channels show the products MgD+ have higher vibrational excitation and hotter rotational state distributions. The opacity function P(J) suggests that the MgH+ + D channel and MgD+ + H channel are dominated by high-b and low-b collisions, respectively. Both forward and backward scattering peaks are found in the differential cross section curves, whereas the angle distributions of products are not strictly forward-backward symmetric because of the short lifetime of the complex in the reaction.

Hybrid electrostatic ion beam trap (HEIBT): Design and simulation of ion-ion, ion-neutral, and ion-laser interactions

Using dichroic electrostatic mirrors, which can reflect a fast ion beam while transmitting a counterion beam, allows extending the field of electrostatic ion trapping. We present the design and simulations of a hybrid electrostatic ion beam trap that allows simultaneous trapping of velocity matched cation and anion beams. The possible merged beam ion-ion, ion-neutral, and ion-laser experiments are discussed.

Isotope-selective chemistry in the Be+(2S1/2) + HOD → BeOD+/BeOH+ + H/D reaction

Low temperature reactions between laser-cooled Be+(2S1/2) ions and partially deuterated water (HOD) molecules have been investigated using an ion trap and interpreted with zero-point corrected quasi-classical trajectory calculations on a highly accurate global potential energy surface for the ground electronic state. Both product channels have been observed for the first time, and the branching to BeOD+ + H is found to be 0.58 ± 0.14. The experimental observation is reproduced by both quasi-classical trajectory and statistical calculations. Theoretical analyses reveal that the branching to the two product channels is largely due to the availability of open states in each channel.

Global diabatic potential energy surfaces for the BeH2 + system and dynamics studies on the Be+(2P) + H2(X1Σg +) → BeH+(X1Σ+) + H(2S) reaction

The Be+(2P) + H2(X1Σg +) → BeH+(X1Σ+) + H(2S) reaction has great significance for studying diabatic processes and ultracold chemistry. The first global diabatic potential energy surfaces (PESs) which are correlated with the lowest two adiabatic states 12A' and 22A' of the BeH2 + system are constructed by using the neural network method. Ab initio energy points are calculated using the multi-reference configuration interaction method with the Davidson correction and AVQZ basis set. The diabatic energies are obtained from the transformation of ab initio data based on the dipole moment operators. The topographical characteristics of the diabatic PESs are described in detail, and the positions of crossing between the V d 11 and V d 22 are pinpointed. On new diabatic PESs, the time-dependent quantum wave packet method is carried out to study the mechanism of the title reaction. The results of dynamics calculations indicate the reaction has no threshold and the product BeH+ is excited to high vibrational states easily. In addition, the product BeH+ tends to backward scattering at most collision energies.

Rovibronic Spectroscopy of Sympathetically Cooled 40CaH. J Phys Chem A 2018. [PMID: 29521505 DOI: 10.1021/acs.jpca.7b12823]

We measure the rovibronic transitions X ¹Σ⁺, v″ = 0, J″ → A ¹Σ⁺, v' = 0-3, J' of CaH⁺ and obtain rotational constants for the A ¹Σ⁺ state. The spectrum is obtained using two-photon photodissociation of CaH⁺ cotrapped with Doppler cooled Ca⁺. The excitation is driven by a mode-locked, frequency-doubled Ti:Sapph laser, which is then pulse shaped to narrow the spectral bandwidth. The measured values of the rotational constants are in agreement with ab initio theory.

The ground and low-lying excited states and feasibility of laser cooling for GaH+ and InH+ cations

The potential energy curves and transition dipole moments of 1²Σ⁺ and 1²Π states of GaH⁺ and InH⁺ cations are performed by employing ab initio calculations. Based on the potential energy curves, the rotational and vibrational energy levels of the two states are obtained by solving the Schrödinger equation of nuclear movement. The spectroscopic parameters are deduced with the obtained rovibrational energy levels. The spin-orbit coupling effect of the ²Π states for both the GaH⁺ and InH⁺ cations are also calculated. The feasibility of laser cooling of GaH⁺ and InH⁺ cations are examined by using the results of the electronic and spectroscopic properties. The highly diagonal Franck-Condon factors and appropriate radiative lifetimes are determined by using the potential energy curves and transition dipole moments for the ²Π_{1/2, 3/2}↔1²Σ⁺ transitions. The results indicate that the ²Π_{1/2, 3/2}↔1²Σ⁺ transitions of both GaH⁺ and InH⁺ cations are appropriate for the close cycle transition of laser cooling. The optical scheme of the laser cooling is constructed for the GaH⁺ and InH⁺ cations.

Diabatic potential energy surfaces of MgH2+ and dynamic studies for the Mg+(3p) + H2 → MgH+ + H reaction

The global diabatic potential energy surfaces for the Mg⁺(3p) + H₂ → MgH⁺ + H reaction are structured for the first time.

Configuration interaction study on the low-lying electronic states of strontium hydride cation including spin-orbit coupling

Ab initio calculations on low-lying electronic states of strontium hydride cations, SrH⁺, have been performed using the internally contracted multi-reference configuration interaction (icMRCI) method with Davidson correction (+Q). Spin-orbit coupling (SOC) effect between the singlet and triplet states of SrH⁺ has been investigated for the first time. The potential energy curves (PECs) of a total of 12 Λ-S states, as well as the 23 Ω states generated from the Λ-S states after considering the SOC effect, have been calculated. The spectroscopic constants and transition properties, including the transition dipole moments, the Franck-Condon factors, and the radiative lifetimes, have been obtained based on the calculated PECs. It indicates that the SOC effect plays a non-negligible role in electronic states of SrH⁺. Our study should shed light on the structure and behavior of low-lying electronic states and should pave further experimental studies on the spectroscopy of strontium hydride cations.

Rydberg-Stark deceleration of atoms and molecules

The large electric dipole moments associated with highly excited Rydberg states of atoms and molecules make gas-phase samples in these states very well suited to deceleration and trapping using inhomogeneous electric fields. The methods of Rydberg-Stark deceleration with which this can be achieved are reviewed here. Using these techniques, the longitudinal motion of beams of atoms and molecules moving at speeds as high as 2500 m/s have been manipulated, with changes in kinetic energy of up to |Δ E _kin|=1.3×10^-20 J (|Δ E _kin|/e=80 meV or |Δ E _kin|/h c=650 cm ^-1) achieved, while decelerated and trapped samples with number densities of 10⁶- 10⁷ cm ^-3 and translational temperatures of ∼150 mK have been prepared. Applications of these samples in areas of research at the interface between physics and physical chemistry are discussed.

Ejection of Coulomb Crystals from a Linear Paul Ion Trap for Ion-Molecule Reaction Studies

Coulomb crystals are being increasingly employed as a highly localized source of cold ions for the study of ion-molecule chemical reactions. To extend the scope of reactions that can be studied in Coulomb crystals-from simple reactions involving laser-cooled atomic ions, to more complex systems where molecular reactants give rise to multiple product channels-sensitive product detection methodologies are required. The use of a digital ion trap (DIT) and a new damped cosine trap (DCT) are described, which facilitate the ejection of Coulomb-crystallized ions onto an external detector for the recording of time-of-flight (TOF) mass spectra. This enables the examination of reaction dynamics and kinetics between Coulomb-crystallized ions and neutral molecules: ionic products are typically cotrapped, thus ejecting the crystal onto an external detector reveals the masses, identities, and quantities of all ionic species at a selected point in the reaction. Two reaction systems are examined: the reaction of Ca(+) with deuterated isotopologues of water, and the charge exchange between cotrapped Xe(+) with deuterated isotopologues of ammonia. These reactions are examples of two distinct types of experiment, the first involving direct reaction of the laser-cooled ions, and the second involving reaction of sympathetically-cooled heavy ions to form a mixture of light product ions. Extensive simulations are conducted to interpret experimental results and calculate optimal operating parameters, facilitating a comparison between the DIT and DCT approaches. The simulations also demonstrate a correlation between crystal shape and image shape on the detector, suggesting a possible means for determining crystal geometry for nonfluorescing ions.

Observation of vibrational overtones by single-molecule resonant photodissociation

Molecular ions can be held in a chain of laser-cooled atomic ions by sympathetic cooling. This system is ideal for performing high-precision molecular spectroscopy with applications in astrochemistry and fundamental physics. Here we show that this same system can be coupled with a broadband laser to discover new molecular transitions. We use three-ion chains of Ca⁺ and CaH⁺ to observe vibrational transitions via resonance-enhanced multiphoton dissociation detected by Ca⁺ fluorescence. On the basis of theoretical calculations, we assign the observed peaks to the transition from the ground vibrational state, ν=0 to ν=9 and 10. Our method allows us to track single-molecular events, and it can be extended to work with any molecule by using normal mode frequency shifts to detect the dissociation. This survey spectroscopy serves as a bridge to the precision spectroscopy required for molecular ion control.

Studying the spectra of molecules typically requires large samples, which can be difficult to achieve for hard-to-generate ions. Here, the authors obtain spectra from single CaH⁺ molecules in a three-ion Columbic crystal, observing new molecular transitions.

Low-temperature kinetics and dynamics with Coulomb crystals

Coulomb crystals-as a source of translationally cold, highly localized ions-are being increasingly utilized in the investigation of ion-molecule reaction dynamics in the cold regime. To develop a fundamental understanding of ion-molecule reactions, and to challenge existing models that describe the rates, product branching ratios, and temperature dependence of such processes, investigators need to exercise full control over the experimental reaction parameters. This requires not only state selection of the reactants, but also control over the collision process (e.g., the collisional energy and angular momentum) and state-selective product detection. The combination of Coulomb crystals in ion traps with cold neutral-molecule sources is enabling the measurement of state-selective reaction rates in a diverse range of systems. With the development of appropriate product detection techniques, we are moving toward the ultimate goal of examining low-energy, state-to-state ion-molecule reaction dynamics.

Decay rate measurement of the first vibrationally excited state of MgH+ in a cryogenic Paul trap

We present a method to measure the decay rate of the first excited vibrational state of polar molecular ions that are part of a Coulomb crystal in a cryogenic linear Paul trap. Specifically, we have monitored the decay of the |ν = 1, J = 1)(X) towards the |ν = 0, J = 0)(X) level in MgH+ by saturated laser excitation of the |ν = 0, J = 2)(X)-|ν = 1, J = 1)(X) transition followed by state selective resonance enhanced two-photon dissociation out of the |ν = 0, J=2)(X) level. The experimentally observed rate of 6.32(0.69) s(-1) is in excellent agreement with the theory value of 6.13(0.03) s(-1) (this Letter). The technique enables the determination of decay rates, and thus absorption strengths, with an accuracy at the few percent level.

Cryogenic linear Paul trap for cold highly charged ion experiments

Storage and cooling of highly charged ions require ultra-high vacuum levels obtainable by means of cryogenic methods. We have developed a linear Paul trap operating at 4 K capable of very long ion storage times of about 30 h. A conservative upper bound of the H(2) partial pressure of about 10(-15) mbar (at 4 K) is obtained from this. External ion injection is possible and optimized optical access for lasers is provided, while exposure to black body radiation is minimized. First results of its operation with atomic and molecular ions are presented. An all-solid state laser system at 313 nm has been set up to provide cold Be(+) ions for sympathetic cooling of highly charged ions.

Manipulation of individual hyperfine states in cold trapped molecular ions and application to HD+ frequency metrology

Advanced techniques for manipulation of internal states, standard in atomic physics, are demonstrated for a charged molecular species for the first time. We address individual hyperfine states of rovibrational levels of a diatomic ion by optical excitation of individual hyperfine transitions, and achieve controlled transfer of population into a selected hyperfine state. We use molecular hydrogen ions (HD+) as a model system and employ a novel frequency-comb-based, continuous-wave 5  μm laser spectrometer. The achieved spectral resolution is the highest obtained so far in the optical domain on a molecular ion species. As a consequence, we are also able to perform the most precise test yet of the ab initio theory of a molecule.

Direct and inverse reactions of LiH+ with He(1S) from quantum calculations: mechanisms and rates

The gas-phase reaction of LiH(+) (X(2)Σ) with He((1)S) atoms, yielding Li(+)He with a small endothermicity for the rotovibrational ground state of the reagents, is analysed using the quantum reactive approach that employs the Negative Imaginary Potential (NIP) scheme discussed earlier in the literature. The dependence of low-T rates on the initial vibrational state of LiH(+) is analysed and the role of low-energy Feshbach resonances is also discussed. The inverse destruction reaction of LiHe(+), a markedly exothermic process, is also investigated and the rates are computed in the same range of temperatures. The possible roles of these reactions in early universe astrophysical networks, in He droplets environments or in cold traps are briefly discussed.

Cold chemistry with electronically excited Ca+ Coulomb crystals

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.

Cold heteronuclear atom-ion collisions

We study cold heteronuclear atom-ion collisions by immersing a trapped single ion into an ultracold atomic cloud. Using ultracold atoms as reaction targets, our measurement is sensitive to elastic collisions with extremely small energy transfer. The observed energy-dependent elastic atom-ion scattering rate deviates significantly from the prediction of Langevin but is in full agreement with the quantum mechanical cross section. Additionally, we characterize inelastic collisions leading to chemical reactions at the single particle level and measure the energy-dependent reaction rate constants. The reaction products are identified by in-trap mass spectrometry, revealing the branching ratio between radiative and nonradiative charge exchange processes.

Ion-molecule chemistry at very low temperatures: cold chemical reactions between Coulomb-crystallized ions and velocity-selected neutral molecules

The recent development of a range of techniques for producing cold atoms and molecules at very low translational temperatures T < or = 1 K has provided the opportunity to investigate collisional processes in a new physical regime. We have recently presented a new experimental method to study low-temperature reactive collisions between translationally cold ions and neutral molecules (S. Willitsch et al., Phys. Rev. Lett. 2008, 100, 043203). Our technique relies on the combination of a quadrupole-guide velocity selector for the generation of translationally cold neutral molecules with a facility to produce ordered structures of cold ions (Coulomb crystals) by laser cooling in a linear quadrupole ion trap. The strong localisation of the ions in the trap in combination with the high sensitivity of laser-induced-fluorescence detection enabled us to study chemical reactions on the single-particle level, down to temperatures of T approximately 1 K. In the current paper, we present a detailed characterisation of the scope and limitations of this method based on our study of the reaction between laser-cooled Ca+ ions and velocity-selected CH3F molecules. The properties of our cold-neutrals source and the dependence of the measured rate constant on the shape of the Coulomb crystals, trapping and laser-cooling parameters are discussed. An extension of our technique for the study of low-temperature reactions with sympathetically cooled molecular ions (translational temperature T > 10 mK) is presented and first results on the charge-transfer reaction between OCS+ and ND3 are discussed. Finally, perspectives for further developments of our method are explored.

Rydberg-state-enabled deceleration and trapping of cold molecules

Hydrogen molecules in selected core-nonpenetrating Rydberg-Stark states have been decelerated from a mean initial velocity of 500 m/s to zero velocity in the laboratory frame and loaded into a three-dimensional electrostatic trap. Trapping times, measured by pulsed electric field ionization of the trapped molecules, are found to be limited by collisional processes. As Rydberg states can be deexcited to the absolute ground state, the method can be applied to generate cold samples of a wide range of species.

Power-law distributions for a trapped ion interacting with a classical buffer gas

Classical collisions with an ideal gas generate non-Maxwellian distribution functions for a single ion in a radio frequency ion trap. The distributions have power-law tails whose exponent depends on the ratio of buffer gas to ion mass. This provides a statistical explanation for the previously observed transition from cooling to heating. Monte Carlo results approximate a Tsallis distribution over a wide range of parameters and have ab initio agreement with experiment.