Session J8: Heavy Particle Collisions: Atoms, Surfaces, and Antimatter

Studying cold collisions and reactions in a hybrid atom--ion trap: towards ultracold molecular ions

We report on our ongoing efforts to study ultra-cold BaCl$^+$ ions [1] and reactions between different cold ions and atoms [2, 3]. We have implemented a new apparatus which joins a time-of-flight mass spectrometer (TOFMS) (a modification of [4]) and a new RF trap. This system allows for both studying ions in Coulomb crystals optically and detecting reaction products using the TOFMS. First experimental data are presented and we discuss further prospects of our development to gain insights into reactions at the quantum level. \\[2ex] [1] W. G. Rellergert et al., Nature~\textbf{495}, 490--494 (2013)\\[0pt] [2] Rellergert et al., Phys.~Rev.~Lett.~\textbf{107}, 243201 (2011)\\[0pt] [3] Sullivan et al., Phys.~Chem.~Chem.~Phys.~\textbf{13}, 18859--18863 (2011)\\[0pt] [4] Schowalter et al., Rev.~Sci.~Instrum.~\textbf{83}, 043103 (2012)   

Charge transfer in cold collisions of Be$^+$ + Be

We study charge transfer in collisions of cold Be$^+$ and Be in an external magnetic field. The atom-ion interaction is modeled by high-level \emph{ab-initio} potential energy curves, including the dipole-dipole terms, as well as Zeeman and hyperfine couplings. The scattering calculations are performed for a range of experimentally accessible magnetic fields and different initial hyperfine states using fully-quantum coupled channel formalism. We report detailed inelastic and charge-exchange cross sections for different isotopes of Be. In addition, we predict a number of magnetic Feshbach resonances and discuss their applications on controlled charge transfer in cold quantum gases and optically trapped systems.   

Quantum Sticking of Atoms on Membranes

A continuum model for low-energy physisorption on a membrane under tension is proposed and studied with variational mean-field theory. A discontinuous change in the energy-dependent sticking coefficient is predicted under certain conditions. This singularity is a result of the bosonic orthogonality catastrophe of the vibrational states of the membrane. The energy-dependent sticking coefficient is predicted to have exponential scaling in 1/E above the singularity. The application of this model to the quantum sticking of cold hydrogen to suspended graphene is discussed. The model predicts that a beam of atomic hydrogen can be completely reflected by suspended graphene at ultralow energies.   

Ion Beam Metastable fraction deduced from Rydberg Spectroscopy Background Levels

The metastable content of U$^{6+}$, Pb$^{4+}$ and Pb$^{2+}$ ion beams from an ECR source was measured as a bi-product of spectroscopic studies of Rydberg levels of U$^{5+}$, Pb$^{3+}$, and Pb$^{+}$ that used the Resonant Excitation Stark Ionization Spectroscopy (RESIS) technique. Autoionization of metastable Rydberg levels within the Stark Ionization detector proves to be the dominant background and noise source with this technique. To reduce this background, a device was introduced to induce autoionization of metastable Rydberg levels prior to the detector. Measurements with this device enabled the deduction of the initial metastable fraction of the ion beam. Results varied from 20 - 50{\%} for these beams.   

Mutual neutralization at low collision energies: the power of imaging

Mutual neutralization studies are generally limited to energies above a few eV, and do not specify the electronic state of the products, merely indicating a band of principal quantum numbers based on time-of-flight intervals (Terao \textit{et al.}, Europhys. Lett. \textbf{1} (1986) 123). We upgraded our merged beam set-up to reach meV collision energies, and incorporated three-dimensional product imaging. Besides providing clear coincidence signals, this technique gives unambiguous identification of the electronic states of the products. Knowing their angular distribution at the different collision energies allows absolute cross sections to be retrieved. Results for the H$^{+}$/H$^{-}$ and He$^{+}$/H$^{-}$ systems will be presented, providing detailed branching ratios for non-degenerate channels.   

Quantum analysis of atom-ion sympathetic cooling in the presence of micromotion

We investigate the problem of a single ion in a radio-frequency trap and immersed in an ultracold Bose gas either in condensed or non-condensed phase. We develop master equation formalism describing the sympathetic cooling and we determine the cooling rates and final energies of ions. We show that cold atomic reservoir modifies the stability diagram of the ion in the Paul trap creating the regions where the ion is either cooled or heated due to the energy quanta exchanged with the time-dependent potential. Our calculation indicates that micromotion constitutes an important source of heating limiting the final temperature of ions to values higher than 100$\mu$K for parameters of present experiments.   

Laser wavelength effect on charge transfer and excitation processes in laser-assisted collisions of Li$^{+} + $H

Total, $n=2$, and $3$ charge transfer and $n = 2$ target excitation probabilities for collision of Li$^+$ with ground state atomic hydrogen are calculated numerically, in the impact energy collision range 0.25-5 keV. The total wave function at the end of the dynamics of the collision is obtained by solving the time-dependent Schr\"odinger equation by means the finite-difference method. We use a pseudo-potential method to model the electronic structure of the Li$^+$ ion. The $n = 2$, and $3$ charge transfer and $n = 2$ target excitation probabilities are obtained by projecting the stationary states of Lithium and Hydrogen neutral atoms to the total wave function of the collision, respectively; the stationary states of Li and H are obtained numerically. To assess the validity of our method, our numerical results have been compared with those obtained experimentally and by other theoretical methods found in the literature. We study the laser-assited collision by using a short (3 fs at FWHM) and intense (3.15 $\times^{12}$ W/cm$^2$) Gaussian laser pulse. We consider a wavelength range between 400 - 1000 nm in steps of 100 nm. Finally, we analyze the laser assisted collision by a qualitatively way with a two level approach.   

Analysis of the Absolute Cross Section of Charge Transfer Collisions in H$+$H$_{2}^{+}$ and Isotopic Systems using Merged-Beams Technique

We are reporting the absolute charge transfer cross sections for H$+$H$_{2}^{+}$ and isotopic systems, which were measured with the Oak Ridge National Laboratory Multicharged Ion Research Facility from keV/u collision energies where the collision is considered ``ro-vibrationally frozen'' to meV/u energies where collision times are long enough to sample vibrational and rotational modes. The charge transfer of these systems involve the most fundamental ion-molecule two-electron system (H-H$_{2})^{+}$. This temporary complex, formed during charge transfer collisions of H$+$H$_{2}^{+}$, proceeds through dynamically coupled electronic, vibrational, and rotational degrees of freedom (J. Phys. Conf. Ser.\textbf{194} 012043(2009)). The measurements reported here are compared to the existing high energy theory and low energy state-to-state calculations (NIM B \textbf{235} 362 (2005), PRA \textbf{67} 022708 (2003), and PRA \textbf{66,} 042717(2002)).