Comprehensive rate coefficients for electron collision induced transitions in hydrogen

Energy-changing electron-hydrogen atom collisions are crucial to regulating the energy balance in astrophysical and laboratory plasmas and relevant to the formation of stellar atmospheres, recombination in H-II clouds, primordial recombination, three-body recombination and heating in ultracold and fusion plasmas. Computational modeling of electron-hydrogen collision has been attempted through quantum mechanical scattering state-to-state calculations of transitions involving low-lying energy levels in hydrogen (with principal quantum number n < 7) and at large principal quantum numbers using classical trajectory techniques. Analytical expressions are proposed which interpolates the current quantum mechanical and classical trajectory results for electron-hydrogen scattering in the entire range of energy levels, for nearly all temperature range of interest in astrophysical environments. An asymptotic expression for the Born cross-section is interpolated with a modified expression derived previously for electron-hydrogen scattering in the Rydberg regime using classical trajectory Monte Carlo simulations. The derived formula is compared to existing numerical data for transitions involving low principal quantum numbers, and the dependence of the deviations upon temperature is discussed.Comment: To appear in The Astrophysical Journa

Structure and Spectroscopy of Ground and Excited States of LiYb

Multireference configuration interaction and coupled cluster calculations have been carried out to determine the potential energy curves for the ground and low-lying excited states of the LiYb molecule. The scalar relativistic effects have been included by means of the Douglas–Kroll Hamiltonian and effective core potential and the spin-orbit couplings have been evaluated by the full microscopic Breit–Pauli operator. The LiYb permanent dipole moment, static dipole polarizability, and Franck–Condon factors have been determined. Perturbations of the vibrational spectrum due to nonadiabatic interactions are discussed.Astronom

Non-Maxwellian rate coefficients for electron and ion collisions in Rydberg plasmas: implications for excitation and ionization

Scattering phenomena between charged particles and highly excited Rydberg atoms are of critical importance in many processes in plasma physics and astrophysics. While a Maxwell-Boltzmann (MB) energy distribution for the charged particles is often assumed for calculations of collisional rate coefficients, in this contribution we relax this assumption and use two different energy distributions, a bimodal MB distribution and a $\kappa$-distribution. Both variants share a high-energy tails occurring with higher probability than the corresponding MB distribution. The high energy tail may significantly affect rate coefficients for various processes. We focus the analysis to specific situations by showing the dependence of the rate coefficients on the principal quantum number of hydrogen atoms in n-changing collisions with electrons in the excitation and ionization channels and in a temperature range relevant to the divertor region of a tokamak device. We finally discuss the implications for diagnostics of laboratory plasmas.Comment: 14 pages, 4 figures, Journal of Plasma Physics collection on 'Laboratory and Astrophysical Plasmas: New Perspectives

Collision-Induced Spin Exchange of Alkali-Metal Atoms With 3^3He: An Ab Initio Study

We present a rigorous quantum study of spin-exchange transitions in collisions of the alkali-metal atoms with $^3$He in the presence of an external magnetic field. Using accurate ab initio interaction potentials, we obtain refined estimates for the Fermi contact interaction constants for complexes of Na, K, and Rb atoms with $^3$He. Ab initio calculations show that the Fermi contact interaction in Li-$^3$He varies more slowly with inter-nuclear distance than predicted by the atomic model [R. M. Herman, Phys. Rev. 37, A1062 (1965)]. The calculated spin-exchange rate constants for Na, K, and Rb atoms in a gas of $^3$He are in good agreement with experimental data. Our calculations demonstrate that at a temperature of 0.5 K, collision-induced spin exchange of the alkali-metal atoms occurs at a very slow rate of ~10$^−22$ cm$^3$/s, suggesting potential applications in cryogenic cooling, precision spectroscopy, and quantum optics.Astronom

Many-body quantum chaos in stroboscopically-driven cold atoms

Seeking signatures of quantum chaos in experimentally realizable many-body systems is of vigorous interest. In such systems, the spectral form factor (SFF), defined as the Fourier transform of two-level spectral correlation function, is known to exhibit random matrix theory (RMT) behaviors, namely a 'ramp' followed by a 'plateau' in sufficiently late time. Recently, a generic early-time deviation from the RMT behavior, which we call the 'bump', has been shown to exist in random quantum circuits and spin chains as toy models for many-body quantum chaotic systems. Here we demonstrate the existence of the 'bump-ramp-plateau' behavior in the SFF for a number of paradigmatic, stroboscopically-driven cold atom models of interacting bosons in optical lattices and spinor condensates. We find that the scaling of the many-body Thouless time $t_{\text{Th}}$ -- the time of the onset of the (RMT) ramp behavior -- and the increase of the bump amplitude in atom number are significantly slower in (effectively 0D) chaotic spinor gases than in 1D optical lattices, demonstrating the role of locality in many-body quantum chaos. Moreover, $t_{\text{Th}}$ scaling and the bump amplitude are more sensitive to variations in atom number than the system size regardless of the hyperfine structure, the symmetry classes, or the choice of the driving protocol. We obtain scaling functions of SFF which suggest power-law behavior for the bump regime in quantum chaotic cold-atom systems. Finally, we propose an interference measurement protocol to probe SFF in the laboratory.Comment: 10 pages, 7 figures, supplementary materia

Collisions of Trapped Molecules With Slow Beams

We present a theoretical study of molecular-trap loss induced by collisions with slow atomic beams based on an explicit analysis of collision kinematics in the laboratory frame and a rigorous quantum description of atom-molecule scattering in external fields. The theory is applied to elucidate the effects of nonuniform magnetic and optical trapping fields on low-temperature collisions of OH $(J=\frac{3}{2},M_J=\frac{3}{2},f)$ molecules with $^{4}$He atoms. Our calculations quantify the extent to which both elastic and inelastic cross sections are suppressed by external trapping fields, clarify the role of small-angle scattering in trap loss, and may benefit future experiments on collisional cooling of molecules in electromagnetic traps. The calculated cross sections for trap loss in $^{4}$He + OH collisions are consistent with recent experimental observations at low beam energies [ B. C. Sawyer $et al.$ Phys. Rev. Lett. 101 203203 (2008)], demonstrating the importance of including the effects of nonuniform trapping fields in theoretical simulations of cold collision experiments with trapped molecules and slow atomic beams.Astronom

Collision-induced spin exchange of alkali-metal atoms withH3e: Anab initiostudy

We present a rigorous quantum study of spin-exchange transitions in collisions of the alkali-metal atoms with H3e in the presence of an external magnetic field. Using accurate ab initio interaction potentials, we obtain refined estimates for the Fermi contact interaction constants for complexes of Na, K, and Rb atoms with H3e . Ab initio calculations show that the Fermi contact interaction in Li-H3e varies more slowly with internuclear distance than predicted by the atomic model [R. M. Herman, Phys. Rev. 37, A1062 (1965)]. The calculated spin-exchange rate constants for Na, K, and Rb atoms in a gas of H3e are in good agreement with experimental data. Our calculations demonstrate that at a temperature of 0.5 K, collision-induced spin exchange of the alkali-metal atoms occurs at a very slow rate of ˜10-22cm3/s , suggesting potential applications in cryogenic cooling, precision spectroscopy, and quantum optics.Astronom

Collision-Induced Spin Depolarization of Alkali-metal Atoms in Cold 3^3He Gas

We present a joint experimental and theoretical study of spin depolarization in collisions of alkali-metal atoms with $^3$He in a magnetic field. A rigorous quantum theory for spin-changing transitions is developed and applied to calculate the spin exchange and spin relaxation rates of Li and K atoms in cryogenic $^3$He gas. Magnetic trapping experiments provide upper bounds to the spin exchange rates for Li-$^3$He and K-$^3$He, which are in agreement with the present theory. Our calculations demonstrate that the alkali-metal atoms have extremely slow spin depolarization rates, suggesting a number of potential applications in precision spectroscopy and quantum optics.Physic