Imaging Resonance Effects in C + H2 Collisions Using a Zeeman Decelerator.

An intriguing phenomenon in molecular collisions is the occurrence of scattering resonances, which originate from bound and quasi-bound states supported by the interaction potential at low collision energies. The resonance effects in the scattering behavior are extraordinarily sensitive to the interaction potential, and their observation provides one of the most stringent tests for theoretical models. We present high-resolution measurements of state-resolved angular scattering distributions for inelastic collisions between Zeeman-decelerated C(3P1) atoms and para-H2 molecules at collision energies ranging from 77 cm-1 down to 0.5 cm-1. Rapid variations in the angular distributions were observed, which can be attributed to the consecutive reduction of contributing partial waves and effects of scattering resonances. The measurements showed excellent agreement with distributions predicted by ab initio quantum scattering calculations. However, discrepancies were found at specific collision energies, which most likely originate from an incorrectly predicted quasi-bound state. These observations provide exciting prospects for further high-precision and low-energy investigations of scattering processes that involve paramagnetic species.

Quantum scattering of icosahedron fullerene C60 with noble-gas atoms.

There exist multiple ways to cool neutral molecules. A front runner is the technique of buffer gas cooling, where momentum-changing collisions with abundant cold noble-gas atoms cool the molecules. This approach can, in principle, produce the most diverse samples of cold molecules. We present quantum mechanical and semiclassical calculations of the elastic scattering differential cross sections and rate coefficients of the C60 fullerene with He and Ar noble-gas atoms in order to quantify the effectiveness of buffer gas cooling for this molecule. We also develop new three-dimensional potential energy surfaces for this purpose using dispersion-corrected density functional theory (DFT) with counterpoise correction. The icosahedral anisotropy of the molecular system is reproduced by expanding the potential in terms of symmetry-allowed spherical harmonics. Long-range dispersion coefficients have been computed from frequency dependent polarizabilities of C60 and the noble-gas atoms. We find that the potential of the fullerene with He is about five times shallower than that with Ar. Anisotropic corrections are very weak for both systems and omitted in the quantum scattering calculations giving us a nearly quantitative estimate of elastic scattering observables. Finally, we have computed differential cross sections at the collision energies used in experiments by Han et al. (Chem Phys Lett 235:211, 1995), corrected for the sensitivity of their apparatus, and we find satisfactory agreement for C60 scattering with Ar.

H2O-HCN complex: A new potential energy surface and intermolecular rovibrational states from rigorous quantum calculations.

In this work the H2O-HCN complex is quantitatively characterized in two ways. First, we report a new rigid-monomer 5D intermolecular potential energy surface (PES) for this complex, calculated using the symmetry-adapted perturbation theory based on density functional theory method. The PES is based on 2833 ab initio points computed employing the aug-cc-pVQZ basis set, utilizing the autoPES code, which provides a site-site analytical fit with the long-range region given by perturbation theory. Next, we present the results of the quantum 5D calculations of the fully coupled intermolecular rovibrational states of the H2O-HCN complex for the total angular momentum J values of 0, 1, and 2, performed on the new PES. These calculations rely on the quantum bound-state methodology developed by us recently and applied to a variety of noncovalently bound binary molecular complexes. The vibrationally averaged ground-state geometry of H2O-HCN determined from the quantum 5D calculations agrees very well with that from the microwave spectroscopic measurements. In addition, the computed ground-state rotational transition frequencies, as well as the B and C rotational constants calculated for the ground state of the complex, are in excellent agreement with the experimental values. The assignment of the calculated intermolecular vibrational states of the H2O-HCN complex is surprisingly challenging. It turns out that only the excitations of the intermolecular stretch mode can be assigned with confidence. The coupling among the angular degrees of freedom (DOFs) of the complex is unusually strong, and as a result most of the excited intermolecular states are unassigned. On the other hand, the coupling of the radial, intermolecular stretch mode and the angular DOFs is weak, allowing straightforward assignment of the excitation of the former.

Signatures of Non-universal Quantum Dynamics of Ultracold Chemical Reactions of Polar Alkali Dimer Molecules with Alkali Metal Atoms: Li(2S) + NaLi(a3Σ+) → Na(2S) + Li2(a3Σu+).

Ultracold chemical reactions of weakly bound triplet-state alkali metal dimer molecules have recently attracted much experimental interest. We perform rigorous quantum scattering calculations with a new ab initio potential energy surface to explore the chemical reaction of spin-polarized NaLi(a3Σ+) and Li(2S) to form Li2(a3Σu+) and Na(2S). The reaction is exothermic and proceeds readily at ultralow temperatures. Significantly, we observe strong sensitivity of the total reaction rate to small variations of the three-body part of the Li2Na interaction at short range, which we attribute to a relatively small number of open Li2(a3Σu+) product channels populated in the reaction. This provides the first signature of highly non-universal dynamics seen in rigorous quantum reactive scattering calculations of an ultracold exothermic insertion reaction involving a polar alkali dimer molecule, opening up the possibility of probing microscopic interactions in atom+molecule collision complexes via ultracold reactive scattering experiments.

Elastic and glancing-angle rate coefficients for heating of ultracold Li and Rb atoms by collisions with room-temperature noble gases, H2, and N2.

Trapped ultracold alkali-metal atoms can be used to measure pressure in the ultra-high-vacuum and XHV pressure regimes, those with p < 10-6 Pa. This application for ultracold atoms relies on precise knowledge of collision rate coefficients of alkali-metal atoms with residual room-temperature atoms and molecules in the ambient vacuum or with deliberately introduced gasses. Here, we determine combined elastic and inelastic rate coefficients as well as glancing-angle rate coefficients for ultracold 7Li and 87Rb with room-temperature noble gas atoms as well as H2 and 14N2 molecules. Glancing collisions are those processes where only little momentum is transferred to the alkali-metal atom and this atom is not ejected from its trap. Rate coefficients are found by performing quantum close-coupling scattering calculations using ab initio ground-state electronic Born-Oppenheimer potential energy surfaces. The potentials for Li and Rb with noble gas atoms and also for Rb(2S)-H2(XΣg +) and Rb(2S)-N2(X1Σg +) systems are based on the non-relativistic spin-restricted coupled-cluster method with single, double, and noniterative triple excitations [RCCSD(T)]. For Li(2S)-N2(X1Σg +), the potential is computed at the explicitly correlated spin-restricted RCCSD(T)-F12 level. For Rb, Kr, and Xe atoms, scalar relativistic corrections to the core electrons have been included, while second-order spin-orbit corrections from the valence electrons have been estimated. Data for Li-H2 and Li-He were taken from the existing literature. We estimate standard uncertainties of the rate coefficients by comparing rate coefficients calculated using potentials found with electronic basis sets of increasing size, including estimates of relativistic spin-orbit corrections and the uncertainty of the van der Waals coefficients. The relative uncertainties of rate coefficients are 1%-2% with the exception of 7Li or 87Rb colliding with 20Ne. Those have relative uncertainties of 9% and 8%, respectively. We also show that a commonly used semiclassical approximation for the total elastic rate coefficient agrees with the quantum calculations to 10% with the exception of 7Li and 87Rb collisions with H2, where the semiclassical value underestimates the quantum value by 20%.

Can armchair nanotubes host organic color centers?

We use time-dependent density functional theory to investigate the possibility of hosting organic color centers in (6, 6) armchair single-walled carbon nanotubes, which are known to be metallic. Our calculations show that in short segments of (6, 6) nanotubes∼5nm in length there is a dipole-allowed singlet transition related to the quantum confinement of charge carriers in the smaller segments. The introduction ofsp3defects to the surface of (6, 6) nanotubes results in new dipole-allowed excited states. Some of these states are redshifted from the native confinement state of the defect-free (6, 6) segments; this is similar behavior to what is observed withsp3defects to exciton transitions in semiconducting carbon nanotubes. This result suggests the possibility of electrically wiring organic color centers directly through armchair carbon nanotube hosts.

Quantum Defects: What Pairs with the Aryl Group When Bonding to the sp2 Carbon Lattice of Single-Wall Carbon Nanotubes?

Aryl diazonium reactions are widely used to covalently modify graphitic electrodes and low-dimensional carbon materials, including the recent creation of organic color centers (OCCs) on single-wall carbon nanotube semiconductors. However, due to the experimental difficulties in resolving small functional groups over extensive carbon lattices, a basic question until now remains unanswered: what group, if any, is pairing with the aryl sp3 defect when breaking a C═C bond on the sp2 carbon lattice? Here, we show that water plays an unexpected role in completing the diazonium reaction with carbon nanotubes involving chlorosulfonic acid, acting as a nucleophilic agent that contributes -OH as the pairing group. By simply replacing water with other nucleophilic solvents, we find it is possible to create OCCs that feature an entirely new series of pairing groups, including -OCH3, -OC2H5, -OC3H7, -i-OC3H7, and -NH2, which allows us to systematically tailor the defect pairs and the optical properties of the resulting color centers. Enabled by these pairing groups, we further achieved the synthesis of OCCs with sterically bulky pairs that exhibit high purity defect photoluminescence effectively covering both the second near-infrared window and the telecom wavelengths. Our studies further suggest that these diazonium reactions proceed through the formation of carbocations in chlorosulfonic acid, rather than a radical mechanism that typically occurs in aqueous solutions. These findings uncover the unknown half of the sp3 defect pairs and provide a synthetic approach to control these defect color centers for quantum information, imaging, and sensing.

High-Resolution Imaging of C + He Collisions using Zeeman Deceleration and Vacuum-Ultraviolet Detection.

High-resolution measurements of angular scattering distributions provide a sensitive test for theoretical descriptions of collision processes. Crossed beam experiments employing a decelerator and velocity map imaging have proven successful to probe collision cross sections with extraordinary resolution. However, a prerequisite to exploit these possibilities is the availability of a near-threshold state-selective ionization scheme to detect the collision products, which for many species is either absent or inefficient. We present the first implementation of recoil-free vacuum ultraviolet (VUV) based detection in scattering experiments involving a decelerator and velocity map imaging. This allowed for high-resolution measurements of state-resolved angular scattering distributions for inelastic collisions between Zeeman-decelerated carbon C(3P1) atoms and helium atoms. We fully resolved diffraction oscillations in the angular distributions, which showed excellent agreement with the distributions predicted by quantum scattering calculations. Our approach offers exciting prospects to investigate a large range of scattering processes with unprecedented precision.

Surface Studies on Glass Powders Used in Commercial Glass-Ionomer Dental Cements.

The surface properties of three commercial ionomer glass powders, i.e., Fuji IX, Kavitan Plus and Chemadent G-J-W were studied. Samples were analyzed by X-ray fluorescence spectroscopy (XRF), and the density was determined by gas pycnometry. Morphology was studied using scanning electron microscopy (SEM) and laser diffraction (LD) technique, whereas low-temperature nitrogen sorption measurements determined textural parameters like specific surface area and pore volume. Thermal transformations in the materials studied were evaluated by thermogravimetric analysis (TGA), which was carried out in an inert atmosphere between 30 °C and 900 °C. XRF showed that Fuji IX and Kavitan Plus powders were strontium-based, whereas Chemadent G-J-W powder was calcium-based. Powders all had a wide range of particle sizes under SEM and LD measurements. Specific surface areas and pore volumes were in the range 1.42-2.73 m2/g and 0.0029 to 0.0083 cm3/g, respectively, whereas densities were in the range 2.6428-2.8362 g/cm3. Thermogravimetric analysis showed that the glass powders lost mass in a series of steps, with Fuji IX powder showing the highest number, some of which are attributed to the dehydration and decomposition of the polyacrylic acid present in this powder. Mass losses were more straightforward for the other two glasses. All three powders showed distinct losses at around 780 °C and 835 °C, suggesting that similar dehydration steps occur in all these glasses. Other steps, which differed between glass powders, are attributed to variations in states of water-binding on their surfaces.

Quantum Spin State Selectivity and Magnetic Tuning of Ultracold Chemical Reactions of Triplet Alkali-Metal Dimers with Alkali-Metal Atoms.

We demonstrate that it is possible to efficiently control ultracold chemical reactions of alkali-metal atoms colliding with open-shell alkali-metal dimers in their metastable triplet states by choosing the internal hyperfine and rovibrational states of the reactants as well as by inducing magnetic Feshbach resonances with an external magnetic field. We base these conclusions on coupled-channel statistical calculations that include the effects of hyperfine contact and magnetic-field-induced Zeeman interactions on ultracold chemical reactions of hyperfine-resolved ground-state Na and the triplet NaLi(a^{3}Σ^{+}) producing singlet Na_{2}(^{1}Σ_{g}^{+}) and a Li atom. We find that the reaction rates are sensitive to the initial hyperfine states of the reactants. The chemical reaction of fully spin-polarized, high-spin states of rotationless NaLi(a^{3}Σ^{+},v=0,N=0) molecules with fully spin-polarized Na is suppressed by a factor of 10-100 compared to that of unpolarized reactants. We interpret these findings within the adiabatic state model, which treats the reaction as a sequence of nonadiabatic transitions between the initial nonreactive high-spin state and the final low-spin states of the reaction complex. In addition, we show that magnetic Feshbach resonances can similarly change reaction rate coefficients by several orders of magnitude. Some of these resonances are due to resonant trimer bound states dissociating to the N=2 rotational state of NaLi(a^{3}Σ^{+},v=0) and would thus exist in systems without hyperfine interactions.

Roaming pathways and survival probability in real-time collisional dynamics of cold and controlled bialkali molecules.

Perfectly controlled molecules are at the forefront of the quest to explore chemical reactivity at ultra low temperatures. Here, we investigate for the first time the formation of the long-lived intermediates in the time-dependent scattering of cold bialkali [Formula: see text]Rb molecules with and without the presence of infrared trapping light. During the nearly 50 nanoseconds mean collision time of the intermediate complex, we observe unconventional roaming when for a few tens of picoseconds either NaRb or [Formula: see text] and [Formula: see text] molecules with large relative separation are formed before returning to the four-atom complex. We also determine the likelihood of molecular loss when the trapping laser is present during the collision. We find that at a wavelength of 1064 nm the [Formula: see text] complex is quickly destroyed and thus that the [Formula: see text]Rb molecules are rapidly lost.

Effects of conical intersections on hyperfine quenching of hydroxyl OH in collision with an ultracold Sr atom.

The effect of conical intersections (CIs) on electronic relaxation, transitions from excited states to ground states, is well studied, but their influence on hyperfine quenching in a reactant molecule is not known. Here, we report on ultracold collision dynamics of the hydroxyl free-radical OH with Sr atoms leading to quenching of OH hyperfine states. Our quantum-mechanical calculations of this process reveal that quenching is efficient due to anomalous molecular dynamics in the vicinity of the conical intersection at collinear geometry. We observe wide scattering resonance features in both elastic and inelastic rate coefficients at collision energies below [Formula: see text]. They are identified as either p- or d-wave shape resonances. We also describe the electronic potentials relevant for these non-reactive collisions, their diabatization procedure, as well as the non-adiabatic coupling between the diabatic potentials near the CIs.

Photon-mediated charge exchange reactions between 39K atoms and 40Ca+ ions in a hybrid trap.

We present experimental evidence of charge exchange between laser-cooled potassium 39K atoms and calcium 40Ca+ ions in a hybrid atom-ion trap and give quantitative theoretical explanations for the observations. The 39K atoms and 40Ca+ ions are held in a magneto-optical (MOT) and a linear Paul trap, respectively. Fluorescence detection and high resolution time of flight mass spectra for both species are used to determine the remaining number of 40Ca+ ions, the increasing number of 39K+ ions, and 39K number density as functions of time. Simultaneous trap operation is guaranteed by alternating periods of MOT and 40Ca+ cooling lights, thus avoiding direct ionization of 39K by the 40Ca+ cooling light. We show that the K-Ca+ charge-exchange rate coefficient increases linearly from zero with 39K number density and the fraction of 40Ca+ ions in the 4p 2P1/2 electronically-excited state. Combined with our theoretical analysis, we conclude that these data can only be explained by a process that starts with a potassium atom in its electronic ground state and a calcium ion in its excited 4p 2P1/2 state producing ground-state 39K+ ions and metastable, neutral Ca (3d4p 3P1) atoms, releasing only 150 cm-1 equivalent relative kinetic energy. Charge-exchange between either ground- or excited-state 39K and ground-state 40Ca+ is negligibly small as no energetically-favorable product states are available. Our experimental and theoretical rate coefficients are in agreement given the uncertainty budgets.

State-to-state scattering of highly vibrationally excited NO at broadly tunable energies.

Experimental developments continue to challenge the theoretical description of molecular interactions. One key arena in which these advances have taken place is in rotationally inelastic scattering. Electric fields have been used with great success to select the initial quantum state and slow molecules for scattering studies, revealing novel stereodynamics, diffraction oscillations and scattering resonances. These have enjoyed excellent agreement with quantum scattering calculations performed on state-of-the-art coupled-cluster potential energy surfaces. To date these studies have largely employed reactants in the ground vibrational state (v = 0) and the lowest low-field-seeking quantum state. Here we describe the use of stimulated emission pumping to prepare NO molecules in arbitrary single rotational and parity states of v = 10 for inelastic scattering studies. These are employed in a near-copropagating molecular beam geometry that permits the collision energy to be tuned from above room temperature to 1 K or below, with product differential cross-sections obtained by velocity map imaging. This extremely nonequilibrium condition, not found in nature, tests current theoretical methods in a new regime.

Electrical double layer in molten salts with account of soft repulsions.

Results of Monte Carlo simulations of the electrode-molten salt interface are reported. The system was modeled by soft ions in contact with a soft wall using the Lennard-Jones potential restricted to the repulsion part. The soft wall was formed of C (graphite), Hg, and Pb atoms. Calculations were carried out for the parameter values which would permit making comparison with the real system. The paper presents information on physicochemical properties of the interfacial region, such as the ion singlet distribution functions, the mean electrostatic potential as a function of the distance from the electrode surface, and differential capacitance results as a function of the electrode charges. The differential capacitance curves have a flat and distorted bell shape which vary depending on the kind of the electrode material. The differential capacitance results are discussed and compared with the data obtained from ionic liquid simulations, density functional theory, and mean field calculations.

Quantum Behavior of Spin-Orbit Inelastic Scattering of C-Atoms by D2 at Low Energy.

Fine-structure populations and collision-induced energy transfer in atoms are of interest for many fields, from combustion to astrophysics. In particular, neutral carbon atoms are known to play a role in interstellar media, either as probes of physical conditions (ground state 3P j spin-orbit populations), or as cooling agent (collisional excitation followed by radiative decay). This work aims at investigating the spin-orbit excitation of atomic carbon in its ground electronic state due to collisions with molecular deuterium, an isotopic variant of H2, the most abundant molecule in the interstellar medium. Spin-orbit excitations of C(3P j ) by H2 or D2 are governed by non-adiabatic and spin-orbit couplings, which make the theoretical treatment challenging, since the Born-Oppenheimer approximation no longer holds. Inelastic collisional cross-sections were determined for the C(3P0) + D2 → C(3P j ) + D2 (with j = 1 and 2) excitation process. Experimental data were acquired in a crossed beam experiment at low collision energies, down to the excitation thresholds (at 16.42 and 43.41 cm-1, respectively). C-atoms were produced mainly in their ground spin-orbit state, 3P0, by dissociation of CO in a dielectric discharge through an Even-Lavie pulsed valve. The C-atom beam was crossed with a D2 beam from a second valve. The state-to-state cross-sections were derived from the C(3P j ) (j = 1 or 2) signal measured as a function of the beam crossing angle, i.e., as a function of the collision energy. The results show different quantum behaviors for excitation to C(3P1) or C(3P2) when C(3P0) collides with ortho-D2 or normal-D2. These experimental results are analyzed and discussed in the light of highly accurate quantum calculations. A good agreement between experimental and theoretical results is found. The present data are compared with those obtained for the C-He and C-H2 collisional systems to get new insights into the dynamics of collision induced spin-orbit excitation/relaxation of atomic carbon.

Accurate characterization of the lowest triplet potential energy surface of SO2 with a coupled cluster method.

The near-equilibrium potential energy surface (PES) of the ã 3B1 state of SO2 is developed from explicitly correlated spin-unrestricted coupled cluster calculations with single, double, and perturbative triple excitations with an augmented triple-zeta correlation-consistent basis set. The lowest-lying ro-vibrational energy levels of several sulfur isotopologues have been determined using this PES. It is shown that the new ab initio PES provides a much better description of the low-lying vibrational states than a previous PES determined at the multi-reference configuration interaction level. In particular, the theory-experiment agreement for the three lowest-lying vibrational transitions is within 1-3 cm-1.

Analysis of electrical double layer structure in molten salts.

This paper reports the results of analysis of the electrical double layer (EDL) phenomenon in molten salts to provide information on the influence of short range interaction type on the shape of charge distribution and the effect of the charge distribution shape on capacitance values. A new method of analysis is proposed, which allows a quantitative discussion. It is assumed that EDL can be modelled as a number of capacitor plates connected in series. This paper reports the application of the proposed method in quantitative analysis of the molten salt capacitance data obtained for different short range potentials. The data to be analysed were obtained from the Monte Carlo simulations of the symmetrical molten salt electrolyte for the following short range interaction potentials: hard spheres, Lennard-Jones repulsions, and full Lennard-Jones. The new analysis method gives a more detailed understanding of EDL in molten salts and can become an inspiration for new researches in this field.