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Using the most accurate potential curves available for the $a{}^{3}{\ensuremath{\sum}}_{u}^{+}$ and $X{}^{1}{\ensuremath{\sum}}_{g}^{+}$ states of lithium molecules, we compute the positions of shape resonances. We also calculate the deexcitation probability from a bound level ${v}^{\ensuremath{'}}$ with rotational number ${N}^{\ensuremath{'}}$ to the continuum of the lower state, and conclude that a shape resonance should be measurable for the transition $2{}^{3}{\ensuremath{\prod}}_{g}\ensuremath{\rightarrow}a{}^{3}{\ensuremath{\sum}}_{u}^{+}$. Such a shape resonance has been identified from the spectra of the transition from the ${v}^{\ensuremath{'}}{=2,N}^{\ensuremath{'}}=4$ level of $2{}^{3}{\ensuremath{\prod}}_{g}$ into the ${N}^{\ensuremath{''}}=4$ continuum of $a{}^{3}{\ensuremath{\sum}}_{u}^{+}$. Its position is in good agreement with the theoretical prediction.

Vibrational relaxation of trapped molecules due to collisions with cold atoms is investigated using the results of quantum-mechanical scattering calculations. Trap loss is analyzed using an exactly solvable kinetic model that includes direct collisional quenching and an indirect process of vibrational predissociation. At low atom density, the relaxation is due primarily to collisional quenching. At high atom density, the relaxation involves additional time scales due to the formation and decay of van der Waals complexes. It is shown that the most weakly bound state of the van der Waals complex for a given diatomic vibrational level controls the relaxation at all atom densities. Possible experiments using trapped molecules are discussed.

We describe a method for measuring the forward-scattering amplitude for electron collisions with atoms or molecules. Our scheme uses a gas cell in one arm of an electron interferometer and measures the resulting attenuation and phase shift of the electron matter wave. The complex index of refraction of the gas is determined along with the forward-scattering amplitude. Calculations of the scattering of electrons by atoms are performed using a self-energy potential obtained by treating the atom as an inhomogeneous electron gas. The results indicate that the proposed experiments are feasible. {copyright} {ital 1999} {ital The American Physical Society}

We interpret measurements of photoassociative spectroscopy in a gas of lithium atoms at ultralow temperatures in which photons are absorbed into high vibrational levels of excited electronic singlet and triplet states of ${\mathrm{Li}}_{2}.$ The excited vibrational levels decay by spontaneous emission into the vibrational continuum and the discrete vibrational levels of the ground singlet and lowest triplet electronic states. Spontaneous emission into discrete levels produces molecules that are no longer trapped, and decay into the vibrational continuum can produce atoms with sufficient kinetic energy to escape the trap. We present values of the efficiencies of trap loss by radiative excitation as a function of the trap depth for individual excited vibrational levels of the singlet and triplet states of the ${}^{6}{\mathrm{Li}}_{2}$ and ${}^{7}{\mathrm{Li}}_{2}$ molecules.

We have determined the scattering lengths for collisions between the different isotopes of two potassium atoms in singlet and triplet molecular states of ${\mathrm{K}}_{2}$. We show that for the two bosonic species ${}^{39}\mathrm{K}$ and ${}^{41}\mathrm{K}$ the scattering lengths are positive, hence, leading to stable condensates. The fermionic isotope ${}^{40}\mathrm{K}$ also has positive scattering lengths, but the triplet value is nearly zero, hence, a quasi-non-interacting fermionic gas. We also give results for the interspecies collisions. We conclude that double condensates of ${}^{39}\mathrm{K}$ and ${}^{41}\mathrm{K}$ are possible for purely spin-polarized atoms, and sympathetic cooling between the bosonic ${}^{39}\mathrm{K}$ and the fermionic ${}^{40}\mathrm{K}$ will be efficient due to low spin-flip rates and a sizable interspecies scattering length.

We study long-range diagonal adiabatic corrections to the interaction potentials of the ground state of homonuclear alkali-metal dimers. We show that in the long-range limit the diagonal adiabatic matrix elements may be expressed as a series of inverse powers of the internuclear distance. Each of the nonzero coefficients of this expansion is a mass-dependent correction term to the dispersion coefficients. They are computed by using a complex integral representation, that transforms the molecular problem into an atomic one. Numerical results are presented. {copyright} {ital 1998} {ital The American Physical Society}

Long-range interaction potentials between two and three sodium atoms, between a sodium atom and a perfectly conducting wall, and for a sodium atom between two perfectly conducting walls, are calculated with an electric-dipole oscillator strength distribution constructed from combinations of experimental and theoretical energy levels, oscillator strengths, and photoionization cross section data, constrained by accurate values of oscillator strength sum rules. The leading dispersion coefficients for Na-Na and Na-Na-Na and the Lennard-Jones coefficient for the Na-wall system are determined. For Na-Na and for the Na-wall systems, the retarded (Casimir) potentials are also calculated. {copyright} {ital 1997} {ital The American Physical Society}

The triplet s-wave scattering length of {sup 6}Li is determined using two-photon photoassociative spectroscopy of the diatomic a{sup 3}{Sigma}{sub u}{sup +} state of {sup 6}Li{sub 2}. The measured binding energy of the highest-lying bound state, combined with knowledge of the potential, determines the s-wave scattering length to be ({minus}2160{plus_minus}250)a{sub 0}, where a{sub 0} is the Bohr radius. This extraordinarily large scattering length signifies a near-threshold resonance. A complete table of singlet and triplet scattering lengths for collisions involving {sup 6}Li and {sup 7}Li determined from this and our previous spectroscopic investigations is given. {copyright} {ital 1997} {ital The American Physical Society}

The dispersion coefficients $C_6$, $C_8$, and $C_{10}$ for the interactions between H, He, and Li are calculated using variational wave functions in Hylleraas basis sets with multiple exponential scale factors. With these highly correlated wave functions, significant improvements are made upon previous calculations and our results provide definitive values for these coefficients., RevTeX, 24 pages

The retarded dipole-dipole dispersion interaction potential in helium is evaluated from a set of very accurate effective dipole transition frequencies and oscillator strengths already obtained from a variational calculation. The asymptotic form changes from the inverse sixth to the inverse seventh power of the nuclear separation as the atoms move apart. Simple representations of the potential are given for use in scattering and structure calculations. © 1995 The American Physical Society.

We present results for the photoionization of ground and excited [ital S] states of two-electron systems by x-ray photons. The method uses the asymptotic formulation of Dalgarno and Stewart [Proc. Phys. Soc. London 76, 49 (1960)] with highly correlated Frankowski-Pekeris--type wave functions for [ital N] [sup 1]S and [ital N] [sup 3]S initial states. The calculations show a smaller ratio of double-to-single photoionization [ital R] for triplet states due to the spatial antisymmetry of the wave function near the nucleus. We find an asymmetric population of photoion satellite states and provide expressions for the asymptotic single and total photoionization, and [ital R] as a function of nuclear charge. We also make predictions for the differential double photoionization cross sections in the extreme energy sharing case when one electron is ejected with zero energy.

An analytical approximation of the asymptotic exchange energy of two interacting hydrogen atoms is obtained. This approximation depends only on functions of the internuclear distance R, which remain bounded as R→∞ and is derived using the Herring-Holstein surface-integral technique. It is found that, for large R, the exchange energy is O(R3exp(−2R)) in contrast to earlier approximations of O(R2.5exp(−2R)). Our result is similar to the classic Heitler-London expression without the unphysical term O(R3ln(R)exp(−2R)).

Electron capture and excitation in O[sup +]([sup 4][ital S],[sup 2][ital D],[sup 2][ital P])+He collisions above 100 eV are studied theoretically by using a semiclassical molecular representation and electron capture in He[sup +]+O([sup 3][ital P]) collisions; excitation and deexcitation in O[sup +]([sup 4][ital S])+He[leftrightarrow]O[sup +]([sup 2][ital D])+He collisions at lower energies are studied by using a fully-quantum-mechanical molecular representation. At higher energies, nonadiabatic couplings are the driving forces that cause transitions. At collision energies below 10 eV, transitions are driven by spin-orbit couplings. At kilo-electron-volt energies, the contribution from metastable O[sup +]([sup 2][ital D],[sup 2][ital P]) ions to electron capture is much larger than that from the ground O[sup +]([sup 4][ital S]) ions. At energies below 1 eV, the cross section for electron capture in He[sup +]+O collisions is very small, with a magnitude of less than 10[sup [minus]20] cm[sup 2]. The cross sections for the excitation-deexcitation of metastable O[sup +] ions are larger, with values near 10[sup [minus]18] cm[sup 2].

A fully quantal calculation of the excitation cross sections for the fine-structure levels of ground-state atomic oxygen, in collisions with oxygen atoms at low energies, is presented. The results are compared with the cross sections obtained in a previous calculation.

The retarded long-range potentials for the dispersion interaction arising from induced multipole moments of two ground-state alkali-metal atoms are evalulated as functions of the separation R for Li, Na, K, Rb, and Cs. Accurate atomic properties, determined using published model potentials, are utilized. Values for the potentials are given over a wide range of R. Results for H are also given. For small R the results reproduce the accepted values for the unretarded dispersion dipole-dipole (van der Waals), dipole-quadrupole, quadrupole-quadrupole, and dipole-octupole coefficients, while for R\ensuremath{\rightarrow}\ensuremath{\infty} the Casimir-Polder potentials, due to retardation or effects due to the finite speed of light, are obtained. The overlap between the retardation potentials and terms arising from the Breit-Pauli Hamiltonian is explored.

Nonradiative and radiative electron captures in collisions of ${\mathrm{He}}^{+}$ ions with carbon atoms at energies below 1 keV are studied theoretically by using a molecular-orbital expansion method to calculate the adiabatic potential-energy curves and the nonadiabatic coupling matrix elements of ${\mathrm{CHe}}^{+}$. The corresponding scattering equations are solved. For nonradiative transitions, electron capture into channels separating to He+${\mathrm{C}}^{+}$${(}^{2}$D) are found to be dominant below 10 eV. However, as the energy increases, the channels separating to He+${\mathrm{C}}^{+}$${(}^{2}$P) take over because of the positive asymptotic energy defect. At 100 eV, the nonradiative capture cross section is 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ ${\mathrm{cm}}^{2}$. The radiative electron-capture cross section is found to be very small, with a magnitude nowhere greater than ${10}^{\mathrm{\ensuremath{-}}22}$ ${\mathrm{cm}}^{2}$.

A molecular theory of collision-induced fine-structure-level transitions in complex atoms is presented. The theory is applied to calculate the excitation cross sections for the fine-structure levels in the $^{3}$P ground state of atomic oxygen. We discuss and compare the theory presented here with previous molecular theories.

We calculate the excitation rates of the 5 [sup 2][ital D][sub 3/2] and 5 [sup 2][ital D][sub 5/2] states of rubidium through the simultaneous absorption of two photons of different frequencies by the ground 5 [sup 2][ital S][sub 1/2] state. We include spin-orbit effects in the contribution of the 5[ital P] and 6[ital P] states, situated between the initial and final states, which give rise to the resonance structure in the two-photon spectrum. We indicate how our results can be used for the case in which the transition takes place between states with specified total angular momenta [ital F], for both [sup 85]Rb and [sup 87]Rb. For the case of counterpropagating photons with the same frequency, our results are compared with existing experimental data.

The molecular states that control the charge transfer of protons with chlorine atoms and of chlorine ions with hydrogen atoms are identified, and estimates are made of the strengths of the spin-orbit interactions that drive the processes. The cross sections are calculated by solving numerically the coupled equations of scattering theory and the resulting rate coefficients are presented. The rate coefficient for the charge transfer of protons with chlorine atoms at 70 K is 1.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and at 300 K is 4.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$.

Knowledge of the long-range interaction between atoms and molecules is of fundamental importance for low-energy and low-temperature collisions. The electronic interaction between the charge distributions of two ground-state alkali-metal atoms can be expanded in inverse powers of R, the internuclear distance. The coefficients ${\mathit{C}}_{6}$, ${\mathit{C}}_{8}$, and ${\mathit{C}}_{10}$ of, respectively, the ${\mathit{R}}^{\mathrm{\ensuremath{-}}6}$, ${\mathit{R}}^{\mathrm{\ensuremath{-}}8}$, and ${\mathit{R}}^{\mathrm{\ensuremath{-}}10}$ terms are calculated by integrating the products of the dynamic electric multipole polarizabilities of the individual atoms at imaginary frequencies, which are in turn obtained by solving two coupled inhomogeneous differential equations. Precise one-electron model potentials are developed to represent the motion of the valence electron in the field of the closed alkali-metal positive-ion core. The numerical results for the static multipole polarizabilities for the alkali-metal atoms and the coefficients ${\mathit{C}}_{6}$, ${\mathit{C}}_{8}$, and ${\mathit{C}}_{10}$ for homonuclear and heteronuclear alkali-metal diatoms are compared with other calculations.

We present a theoretical study of the isotope effect arising from the replacement of H by T in the charge-transfer collision H(n=2) + He +(1s) at low energy. Using a quasimolecular approach and a time-dependent wave-packet method, we compute the cross sections for the reaction including the effects of the nonadiabatic radial and rotational couplings. For H(2s) + He+(1s) collisions, we find a strong isotope effect at energies below 1 eV/amu for both singlet and triplet states. We find a much smaller isotopic dependence of the cross section for H(2p) + He +(1s) collisions in triplet states, and no isotope effect in singlet states. We explain the isotope effect on the basis of the potential energy curves and the nonadiabatic couplings, and we evaluate the importance of the isotope effect on the charge-transfer rate coefficients. © 2011 American Physical Society., SCOPUS: ar.j, info:eu-repo/semantics/published

The potential-energy curves of the low-lying 2 Σ + and 2 Π states of the cation NHe 2+ are calculated and it is shown that the lowest state of each symmetry has a potential well inside a repulsive Coulomb barrier that contains several long-lived vibrational levels. Estimates are made of the cross sections of the charge-transfer processes N 2+ +He→N + +He + which occur through transitions at avoided crossings of the molecular states

A rigorous quantum theory of atomic collisions in the presence of radio frequency (rf) magnetic fields is developed and applied to elucidate the effects of combined dc and rf magnetic fields on elastic scattering in ultracold collisions of Rb atoms. We show that rf fields can be used to induce Feshbach resonances, which can be tuned by varying the amplitude and frequency of the rf field. The rf-induced Feshbach resonances occur also in collisions of atoms in low-field-seeking states at moderate rf field strengths easily available in atom chip experiments, which opens up the world of tunable interactions to magnetically trappable atomic quantum gases., 10 pages, 4 figures; minor typos corrected, journal reference added

An algebraic perturbation theory is presented for efficient calculations of localized states and hence of exchange energies, which are the differences between low-lying states of the valence electron of a molecule, formed by the collision of an ion ${Y}^{+}$ with an atom $X$. For the case of a homonuclear molecule these are the gerade and ungerade states and the exchange energy is an exponentially decreasing function of the internuclear distance. For such homonuclear systems the theory is used in conjunction with the Herring-Holstein technique to give accurate exchange energies for a range of intermolecular separations $R$. Since the perturbation parameter is essentially $1/R$, this method is suitable for large $R$. In particular, exchange energies are calculated for ${{X}_{2}}^{+}$ systems, where $X$ is H, Li, Na, K, Rb, or Cs.

We examine the effect of theoretically varying the collision-system reduced mass in collisions of He with vibrationally excited molecular hydrogen and observe zero-energy resonances for select atomic ``hydrogen'' masses less than 1 u or a ``helium'' mass of $1.95$ u. Complex scattering lengths, state-to-state vibrational quenching cross sections, and a low-energy elastic scattering resonance are all studied as a function of collision-system reduced mass. Experimental observations of these phenomena in the cold and ultracold regimes for collisions of $^{3}\mathrm{He}$ and $^{4}\mathrm{He}$ with ${\mathrm{H}}_{2}$, HD, HT, and DT should be feasible in the near future.

{ital Ab} {ital initio} calculations using the Born-Oppenheimer approximation are described of the dipole moments of the {ital i} {sup 3}{Pi}{sub {ital g}--}{ital b} {sup 3}{Sigma}{sub {ital u}}{sup +}, {ital c} {sup 3}{Pi}{sub {ital u}--}{ital a} {sup 3}{Sigma}{sub {ital g}}{sup +}, and {ital i} {sup 3}{Pi}{sub {ital g}--}{ital c} {sup 3}{Pi}{sub {ital u}} transitions of molecular hydrogen. Tables are provided of the dipole transition probabilities and Franck-Condon factors. Franck-Condon factors are presented for transitions originating in the lowest vibrational level of the ground {ital X} {sup 1}{Sigma}{sub {ital g}}{sup +} state.

We present a combined experimental and theoretical study of angular momentum depolarization in cold collisions of {sup 2}P atoms in the presence of an external magnetic field. We show that collision-induced Zeeman relaxation of Ga({sup 2}P{sub 1/2}) and In({sup 2}P{sub 1/2}) atoms in cold {sup 4}He gas is dramatically suppressed compared to atoms in {sup 2}P{sub 3/2} states. Using rigorous quantum-scattering calculations based on ab initio interaction potentials, we demonstrate that Zeeman transitions in collisions of atoms in {sup 2}P{sub 1/2} electronic states occur via couplings to the {sup 2}P{sub 3/2} state induced by the anisotropy of the interaction potential. Our results suggest the feasibility of sympathetic cooling and magnetic trapping of {sup 2}P{sub 1/2}-state atoms, such as halogens, thereby opening up exciting areas of research in precision spectroscopy and cold-controlled chemistry.

We calculate the cross sections for scattering of a neutral ytterbium atom with its positive ion at energies up to 30 eV. We identify peaks in the forward direction as arising from elastic collisions and those in the backward direction as from charge transfer. We show that the total cross section follows a semiclassical expression over a large range of energies and that the resonant charge transfer may be characterized by four energy regimes: an $s$-wave regime at ultralow energies, a modified Langevin regime at low energy, a Langevin regime at medium energy, and an ``exchange'' regime at high energy. We find large variations between the different isotopes for the two lowest-energy regimes and very little variation for the two highest-energy regimes. Our results are consistent with recent measurements.

We present a rigorous quantum study of spin-exchange transitions in collisions of the alkali-metal atoms with $^{3}\text{H}\text{e}$ 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 $^{3}\text{H}\text{e}$. Ab initio calculations show that the Fermi contact interaction in $\text{Li-}^{3}\text{H}\text{e}$ 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 $^{3}\text{H}\text{e}$ 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 $\ensuremath{\sim}{10}^{\ensuremath{-}22}\text{ }{\text{cm}}^{3}/\text{s}$, suggesting potential applications in cryogenic cooling, precision spectroscopy, and quantum optics.

In the artificial-channel method, an {ital N}-channel Hamiltonian describing the multichannel decay of a system is embedded in an ({ital N}+1)-channel Hamiltonian. The resonance energies and partial widths of the original {ital N}-channel physical problem are related simply to the parameters in the Lorentzian forms of the inelastic-scattering probabilities of the ({ital N}+1)-channel problem, the artificial-channel method permitting a straightforward study of a half-collision process with algorithms developed for full scattering processes. The utility of the method is demonstrated numerically for a model problem of Child and Lefebvre (Mol. Phys. 34, 379 (1977)).

We present a joint experimental and theoretical study of spin depolarization in collisions of alkali-metal atoms with $^{3}\mathrm{He}$ 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 $^{3}\mathrm{He}$ gas. Magnetic trapping experiments provide upper bounds to the spin exchange rates for $\mathrm{Li}\text{\ensuremath{-}}^{3}\mathrm{He}$ and $\mathrm{K}\text{\ensuremath{-}}^{3}\mathrm{He}$, 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.

Previous calculations of low-temperature cross sections for collisions between spin-polarized metastable hydrogen atoms are improved to include nonadiabatic radial and angular coupling at large interatomic separations. The electrostatic dipole-quadrupole interaction produces nonadiabatic radial coupling between (2s,2p) and (2p,2p) states, while the Coriolis interaction produces nonadiabatic angular coupling. Both of these long-range contributions are handled in a space-fixed atomic gauge that is particularly convenient for a spin-polarized system. The improved theoretical results are compared with an existing experiment.

Quantum-mechanical scattering calculations of Feshbach resonances arising from van der Waals molecule formation are used to determine vibrational and rotational predissociation lifetimes. A multichannel effective range theory is used to establish the relationship between predissociation and the zero-temperature limit of collisional quenching for resonances lying close to the thresholds for dissociation. The elastic scattering Feshbach resonances may be measurable in ultracold atom-molecule experiments. {copyright} {ital 1998} {ital The American Physical Society}

A numerical investigation of the doubly excited states of H2 converging to the H(n=2)+H(n′=2) limit was performed. Special emphasis was put on the accurate description of the range of intermediate internuclear distances in order to correctly connect the short range with the asymptotic van der Waals regime where perturbation theory is applicable. The present nonperturbative calculation extends to internuclear separations R=200a0 and is sufficiently accurate to achieve a connection between the two extreme regimes without any need for an interpolation procedure. The high precision of the ab initio results revealed a long range dipole-quadrupole interaction that had been omitted in two earlier calculations. In addition to revised first-order perturbation theory results the leading second-order term varying as R−6 was obtained. The impact of the present findings for cold H(n=2) collisions is briefly discussed.

We have investigated collisions of $\mathrm{Ca}(4{s}^{2})$ with ${\mathrm{H}}^{+}$ in the energy range of $200\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{u}\ensuremath{-}10\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{u}$ using the semiclassical molecular-orbital close-coupling (MOCC) method with 18 coupled molecular states ($11\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}^{+}$ and seven $^{1}\ensuremath{\Pi}^{+}$ states) to determine charge-transfer cross sections. Except for the incoming channel $6\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}^{+}$, the molecular states all correspond to charge-transfer channels. Inclusion of ${\mathrm{Ca}}^{2+}\text{\ensuremath{-}}{\mathrm{H}}^{\ensuremath{-}}$ is crucial in the configuration-interaction calculation for generating the molecular wave functions and potentials. Because of the Coulomb attraction, the state separating to ${\mathrm{Ca}}^{2+}\text{\ensuremath{-}}{\mathrm{H}}^{\ensuremath{-}}$ creates many avoided crossings, even though at infinite separation it lies energetically above all other states that we included. Because of the avoided crossings between the incoming channel $6\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}^{+}$ and the energetically close charge-transfer channel $7\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}^{+}$ the charge-transfer interaction occurs at long range. This makes calculations of charge-transfer cross sections by the MOCC method very challenging. The total charge-transfer cross sections increase monotonically from $3.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$ at $200\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{u}$ to $4.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$ at $10\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{u}$. Charge transfer occurs mostly to the excited ${\mathrm{Ca}}^{+}(5p)$ state in the entire energy range, which is the sum of the charge transfer to $7\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}^{+}$ and $4\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}^{+}$. It accounts for $\ensuremath{\sim}47%$ of the total charge transfer cross sections at $200\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{u}$. However, as the energy increases, transfer to ${\mathrm{Ca}}^{+}(4d)$ increases, and at $10\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{u}$ the charge-transfer cross sections for ${\mathrm{Ca}}^{+}(5p)$ and ${\mathrm{Ca}}^{+}(4d)$ become comparable, each giving $\ensuremath{\sim}38%$ of the total cross section.

We perform fully quantum mechanical calculations in the binary approximation of the emission and absorption profiles of the sodium $3s$-$3p$ and potassium $4s$-$4p$ resonance lines under the influence of a helium perturbing gas. We use carefully constructed potential energy surfaces and transition dipole moments to compute the emission and absorption coefficients at temperatures from 158 to 3000 K. Contributions from quasi-bound states are included. The resulting red and blue wing profiles agree well with previous theoretical calculations and with experimental measurements., 16 figures

Photoassociation of ultracold lithium atoms into bound vibrational levels of the $A^{1}\ensuremath{\Sigma}_{u}^{+}$ excited state is used to probe the $X^{1}\ensuremath{\Sigma}_{g}^{+}$ ground-state interaction potential of $^{6}\mathrm{Li}_{2}$ and $^{7}\mathrm{Li}_{2}$. It had been predicted that the $s$-wave photoassociation signal strength would pass through a minimum as a function of vibrational level for positive $s$-wave scattering length. We report the observation of this novel effect, and use the location of the minimum to precisely determine the singlet $s$-wave scattering length for both isotopes. The sensitivity of this technique is demonstrated by distinguishing the minima for collisions involving $^{7}\mathrm{Li}$ atoms in different hyperfine states.

The scalar and tensor dynamic polarizabilities of the first excited P1/2,3/2 states of the alkali-metal atoms Na, K, and Rb are calculated using relativistic many-body perturbation theory. Comparison with experimental measurements of the static polarizabilities suggests that the errors in the theoretical predictions do not exceed 1%. The dynamic polarizabilities at imaginary frequencies are employed to calculate the long-range van der Waals coefficients for the interaction of ground-state helium atoms with the excited alkali-metal atoms to a probable error of less than 2%.

We consider the question of competition between leptonic and hadronic annihilation in matter-antimatter interaction. The rate of direct positron-electron annihilation in cold hydrogen-antihydrogen collisions has been calculated. The presence of leptonic annihilation introduces an absorptive, imaginary component to the hydrogen-antihydrogen scattering length; this component is 1.4x10{sup -4} a.u. for the singlet state of the leptonic spins, and 1.2x10{sup -7} a.u. for the triplet state. Leptonic annihilation is shown to be about 3 orders of magnitude slower than proton-antiproton annihilation.

Ab initio calculations of cross sections and rate constants for elastic scattering and Zeeman relaxation in collisions of $\mathrm{NH}(X{}^{3}{\ensuremath{\Sigma}}^{\ensuremath{-}})$ molecules with ${}^{3}\mathrm{He}$ atoms in a magnetic field are presented and the mechanism for Zeeman transitions in ground-state ${}^{3}\ensuremath{\Sigma}$ molecules is established. The potential surface for He-NH interaction is obtained using all-electron coupled-cluster calculations with single, double, and noniterative triple excitations in a basis set augmented by bond functions. The ratio of the rates for elastic energy transfer and Zeeman relaxation in NH-${}^{3}\mathrm{He}$ collisions exceeds ${10}^{5}$ at temperatures between $0.5$ and 1 K.

A method for quantum-mechanical calculations of cross sections for the Zeeman transitions in collisions of {sup 3}P atoms with structureless targets in a magnetic field is presented and applied to the study of magnetic and electronic relaxation in oxygen-helium and carbon-helium collisions at cold and ultracold temperatures. The rate constants for collisionally induced transitions between Zeeman levels in ground-state oxygen have large magnitudes in a 1 T field. It is shown that magnetic fields induce the forbidden {sup 3}P{sub 1} {yields} {sup 3}P{sub 0} transition in ultracold collisions of carbon with helium. The cross section vanishes at zero energy for field-free collisions, but becomes infinitely large in a finite magnetic field, varying with velocity v and magnetic field B as B{sup 2}/v.

It is shown that spin-flipping transitions in low energy collisions of {sup 2}{sigma} diatomic molecules in the lowest rotational N=0 level with structureless atoms proceed through coupling to the rotationally excited N>0 levels, and are determined by the spin-rotation interaction in the rotationally excited molecule. We carry out calculations for collisions of CaH and {sup 3}He. For the spin-flip rate coefficient corresponding to an interaction potential that does not generate a shape resonance, we obtain 1.20x10{sup -17} cm{sup 3} s{sup -1} at a temperature of 0.4 K consistent with the measured value of 10{sup -17{+-}}{sup 1} cm{sup 3} s{sup -1}. We make predictions as to which molecules should have the least probability of spin-flipping in collisions with atoms.

It is shown that the velocity dependence of cross sections for projection-changing transitions in collisions of paramagnetic atoms or molecules with structureless targets near threshold has the form ${v}^{2\ensuremath{\Delta}m}$ when $\ensuremath{\Delta}m$ is even and ${v}^{2(\ensuremath{\Delta}m+1)}$ when $\ensuremath{\Delta}m$ is odd, where $\ensuremath{\Delta}m$ is the change in the projection quantum number and $v$ is the relative velocity.

We calculate the cross sections that characterize the frequency shift and broadening of the resonance magnetic hyperfine transition in atomic hydrogen due to collisions with hydrogen atoms in a masing cavity. We consider collision energies that correspond to the temperature range, $10\mathrm{mK}lTl1000\mathrm{K},$ and introduce and apply a fully quantal, multichannel collision theory for the entire temperature range under consideration. Our results for the spin-exchange induced cross sections at room temperature, are in harmony with previous calculated values and experimental measurements. For the hyperfine-induced cross sections, our predicted values are in fair agreement with measurements at cryogenic temperatures with the exception of the hyperfine frequency shift at 0.5 K and 323 K for which we obtain a different sign. A study of possible resonance structures shows that they cannot be invoked to resolve the discrepancy and nor can any plausible modification of the ${X}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ and ${b}^{3}{\ensuremath{\Sigma}}_{u}^{+}$ interaction potentials.

Cross sections and rate coefficients for collisions of two spin-polarized metastable hydrogen atoms are calculated at low energies and temperatures using complex scaling and Feshbach projection methods for the determination of the molecular potential-energy curves. The inclusion of the Lamb shift and fine-structure splitting of the excited $n=2$ atoms leads to an attractive van der Waals potential for two ${2s}_{1/2}$ atoms with an exceptionally large coefficient. The quenching of the metastable atoms is dominated at temperatures below 20 mK by ionization. The calculated values for the total loss rate coefficient at 87 \ensuremath{\mu}K and 230 \ensuremath{\mu}K are within a factor of 2 or 3 of the error bars of recent measurements of metastable loss rate coefficients.

Absolute measurements of cross sections for photoionization of a statistical admixture of ${\mathrm{Ne}}^{+}$ in the ${}^{2}{P}_{3/2}^{o}$ ground state and the ${}^{2}{P}_{1/2}^{o}$ metastable state are reported in the energy range 40--71 eV at photon energy resolutions ranging from 22 meV to 2 meV. The experiments were performed using synchrotron radiation from an undulator beamline of the Advanced Light Source with a newly developed ion-photon-beam endstation. The data are characterized by multiple Rydberg series of autoionizing resonances superimposed upon a direct photoionization background cross section where some of the observed resonance line shapes show evidence of interference between the direct and indirect photoionization channels. The resonance features are assigned spectroscopically, and their energies and quantum defects are tabulated. The experimental photoionization cross sections are in satisfactory agreement with the predictions from theoretical calculations performed in intermediate coupling using the semirelativistic Breit-Pauli approximation with ten states. The resonances nearest to the ionization thresholds exhibit anomalous behavior with respect to their positions and strengths due to the presence of interloping resonances associated with higher-lying ionic states causing disruption of the regular Rydberg spectral pattern.