Collisions Of Alkali Atoms Research Articles

Frequency shifts of the magnetic resonance of Rb and K atoms in the K-Rb tandem magnetometer

Collisions of alkali atoms in the ground state are accompanied by the exchange of electron polarization between the colliding atoms, which lead to the polarization transfer between particles. As a result of this process, the electronic orientation is transferred from the spin oriented atoms to unpolarized atoms, along with the polarization transfer, the magnetic resonance frequency shifts of polarized atoms occur too. In this work the magnetic resonance frequency shifts of arising from spin exchange collisions in a mixture of alkali potassium and rubidium atoms at the optical orientation of atoms are calculated. The situation when optical pumping by resonant polarized radiation is carried out only for atoms of the one kind is considered. The spin exchange process between alkali atoms was described in the formalism of the complex cross section for spin exchange process. The real and imaginary parts of the complex cross section were calculated using the interaction potentials of alkali atom dimers. The calculation of the frequency shifts is carried out on the basis of data on the interaction potentials for the following pairs of alkali atoms K-K, K-Rb and Rb-Rb. Maxwell averaging of the cross sections over the velocities was done and the temperature dependences of the magnetic resonance frequency shifts for K and Rb atoms were plotted.

Non-negligible collisions of alkali atoms with background gas in buffer-gas-free cells coated with paraffin

We measured the rate of velocity-changing collisions (VCCs) between alkali atoms and background gas in buffer-gas-free anti-relaxation-coated cells. The average VCC rate in paraffin-coated rubidium vapor cells prepared in this work was $1 \times 10^{6}$ s$^{-1}$, which corresponds to $\sim 1$ mm in the mean free path of rubidium atoms. This short mean free path indicates that the background gas is not negligible in the sense that alkali atoms do not travel freely between the cell walls. In addition, we found that a heating process known as "ripening" increases the VCC rate, and also confirmed that ripening improves the anti-relaxation performance of the coatings.

Effect of Polarization Potential on Proton–Sodium Inelastic Scattering

Inelastic collisions of protons with sodium atoms are treated for the first time within the framework of the coupled static and frozen core approximations. The method is used for calculating partial and total cross-sections with the assumption that only two channels (elastic and hydrogen formation in 2s-state) are open. The calculations are carried out, in each case, for seven values of the total angular momentum l(0 ≤ l ≤ 6). In the second channel, the effect of “switching on” the polarization potential of hydrogen (in 2s-state) on the coupled static cross-sections is investigated. Although the resulting hydrogen (in 2s-state) formation cross-sections are smaller than the elastic ones, their large values should draw the attention of experimental and theoretical physicists to the field of proton–alkali atom collisions. Our results for the total hydrogen (2s-state) formation cross-sections and those determined by previous authors are in reasonable agreement.

Magnetic resonance frequency shifts upon spin-exchange collisions of alkali atoms

Magnetic resonance frequency shifts caused by spin-exchange collisions between 87Rb and 133Cs atoms are calculated, and temperature dependences of magnetic resonance frequency shifts are drawn for two hyperfine states (F = 1, 2) of 87Rb atoms. The obtained dependences are compared with experimental data.

Sub-Shot-Noise Magnetometry with a Correlated Spin-Relaxation Dominated Alkali-Metal Vapor

Spin noise sets fundamental limits to the precision of measurements using spin-polarized atomic vapors, such as performed with sensitive atomic magnetometers. Spin squeezing offers the possibility to extend the measurement precision beyond the standard quantum limit of uncorrelated atoms. Contrary to current understanding, we show that, even in the presence of spin relaxation, spin squeezing can lead to a significant reduction of spin noise, and hence an increase in magnetometric sensitivity, for a long measurement time. This is the case when correlated spin relaxation due to binary alkali-atom collisions dominates independently acting decoherence processes, a situation realized in thermal high atom-density magnetometers and clocks.

Negative-ion formation and mutual neutralization in atom–atom and ion–ion collisions below keV energies

Dynamics and cross-section values for ion-pair formation and its reverse process, mutual neutralization in atom–atom and ion–ion collisions are reviewed for hydrogen–hydrogen atom collisions and hydrogen–alkali atom collisions. For ion-pair formation process, the initial state shows a weak polarization potential, while the final ion-pair state arises from a strong Coulomb potential. Hence, the final state shows a series of avoided crossings with many intermediate states which correspond to electronic excited states of one of pairs. Therefore, the ladder climbing mechanism is the driving dynamics for ion-pair formation. This fact, that is, many sates are involved in dynamics, makes theoretical treatments more complex and cumbersome for an accurate description. We overview the current experimental and theoretical status of these processes.

Study of the K-H${_2}$ quasi-molecular line satellite in the potassium resonance line

Context. The optical spectra of L and T-type dwarfs exhibit a continuum dominated by the far wings of the absorption profiles of the Na 3s-3p and K 4s-4p doublet perturbed by molecular hydrogen and helium. The presence of a broad absorption feature in the blue wing of the K 4s-4p doublet may be attributed to the K-H 2 quasi-molecular line satellite as we predicted in a previous work. Aims. The position of the predicted line satellite was not in good agreement with the observed feature. This was a motivation to compute new interaction potentials and corresponding profiles. Methods. New molecular potentials for K-H 2 were computed using a valence pseudopotential with Gaussian orbitals on potassium, for an H 2 molecule assumed to be in its ground state. A quasi-full three-electron multireference calculation agrees very well with available spectroscopic data. The potentials and radiative dipole transition moments were input data for a unified spectral line shape evaluation of the complete K resonance line profile. Results. We show that these improved potentials now predict a line satellite at the position that is observed in late brown dwarf spectra. The existence, shape and strength of this feature is found to depend strongly on temperature, as well on the abundance ratio of potassium and sodium. Conclusions. The structure of the continuum in brown dwarf spectra from 500 to 1000 nm is determined by radiative collisions of alkali atoms with H 2 molecules. We conclude that the sensitivity of the spectrum to temperature, pressure and abundances is a tool for determining basic parameters of brown dwarf atmospheres, provided that the physics underlying its formation is described well.

Dynamics of spin–orbit recoupling in collisions of alkali atoms with noble-gas atoms using atomic core potentials

Spin–orbit recoupling in alkali-rare-gas atom colliding pairs is developed within a first principles description of the quantum dynamics introducing l-dependent pseudopotentials and including two and three-body polarization terms and the spin–orbit interatomic potential. The treatment combines an eikonal (or short wavelength) approximation for the nuclear motion and time-dependent molecular orbitals to provide interatomic potentials, their nonadiabatic couplings, and state populations during interactions. The time evolution of recoupling of angular momenta in alkali-rare-gas atom thermal collisions is described and computational aspects of spin–orbit coupling in terms of pseudopotentials are presented. Results for potentials arising from the spin–orbit splitting of the P2 states of Li and Na are presented along with cross sections for fine-structure transitions at 400 K and 450 K. The agreement with experiment and other theories is very good.

Dynamics of electronic excitation in collisions of alkali atoms with noble-gas atoms using atomic core potentials

The electronic excitation of the valence electron of alkali atoms in collisions with noble gas atoms has been studied with a procedure that combines l-dependent pseudopotentials including two and three-body polarization terms and a first principles description of the dynamics based on the eikonal approximation of atomic motions and time-dependent molecular orbitals. We discuss the effects of the atomic basis set size on the calculations. We present results for interaction potentials and couplings, the time-dependence of atomic populations, and state-to-state integral cross sections in the keV range. Our results for Li–He, Li–Ne, Na–He, and Na–Ne are in very good agreement with other theoretical and experimental data.

Fully adiabatic interpretation of fine-structure changing transitions in collisions of alkali atoms in 2P electronic states with noble gas

The authors present in detail a fully adiabatic description of Na(32P)-He and Na(32P)-Ar collisions. The study reveals the presence in the long-range region of a strong non-adiabatic parity-dependent coupling. This parity-dependent coupling makes an important contribution to the intramultiplet transition cross section, is the origin of the dissimilar energy dependence of the Na-He and Na-Ar cross sections, and contributes to the sigma +/ sigma - asymmetry observed in the scattering of oriented alkali 2P atoms with noble gases.

Resonant charge transfer in symmetric alkali-ion-alkali-atom collisions.

Resonant charge transfer in alkali-ion--alkali-atom collisions is investigated by using the molecular-orbital expansion method incorporating the use of electron translation factors. Molecular wave functions and eigenenergies are obtained by the pseudopotential method. Molecular properties, ${R}_{e}$, ${D}_{e}$, and ${\ensuremath{\omega}}_{e}$, obtained in the present calculation are in good accord with other recent theoretical results, as well as spectroscopic measurements. Three-state close-coupling calculations reproduce the positions of the maxima and minima in the oscillatory structure seen experimentally in the resonant-charge-transfer cross sections for the ${\mathrm{Li}}_{2}$${\mathrm{}}^{+}$ and ${\mathrm{Cs}}_{2}$${\mathrm{}}^{+}$ systems. The magnitude of the total cross sections and their velocity dependence are in agreement with experimental measurements.

Systematic theoretical investigation of charge exchange inHe+-alkali-atom collisions

A comprehensive systematic study of near-resonant charge exchange into ${\mathrm{He}}^{*}(n=2)$ levels in ${\mathrm{He}}^{+}$ -alkali-atom collisions is reported. The relevant quasimolecular properties of the ${(\mathrm{H}\mathrm{e}\ensuremath{-}\mathrm{a}\mathrm{l}\mathrm{k}\mathrm{a}\mathrm{l}\mathrm{i})}^{+}$ systems are obtained within the one-electron projected-valence-bond method of Kubach and Sidis. The detailed investigation of the considered systems enables one to assess the relevance of the two-state Demkov model for near-resonant charge exchange and the role of rotational coupling. The present multistate close-coupling treatment considerably improves upon the previous simplified theory of Olson and Smith and nicely reproduces the measured relative abundance of the specific ${\mathrm{He}}^{*}(2^{3,1}S,2^{3,1}P)$ states formed by charge exchange. The calculated summed total charge-exchange cross sections generally agree with experiment for ${\mathrm{He}}^{+}$-Cs, -Rb, and -K, whereas problems remain for ${\mathrm{He}}^{+}$-Na.

Model for fast, nonadiabatic collisions between alkali atoms and diatomic molecules

Equations for collisions involving two potential surfaces are presented in the impact parameter approximation. In this approximation, a rectilinear classical trajectory is assumed for the translational motion, leading to a time-dependent Schroedinger’s equation for the remaining degrees of freedom. Model potentials are considered for collisions of alkali atoms with diatomic molecules that lead to a particularly simple form of the final equations. Using the Magnus approximation, these equations are solved for parameters chosen to model the process Cs+O2→Cs++O2−, and total cross sections for ion-pair formation are obtained as a function of energy. The results exhibit oscillations that correspond qualitatively to those seen in recent measurements. In addition, the model predicts that the oscillations will become less pronounced as the initial vibrational level of O2 is increased.

Comparison of ion pair formation in the systems Ar*+I2 and K+I2

A simple model that has been used extensively by Los and co-workers to treat ion pair formation in collisions of alkali atoms with diatomic molecules is extended to include continuum coupling via a competing Penning ionization channel. This extended model is then used to calculate the differential cross sections for ion pair formation for the system Ar*+I2 over the energy range 28–154 eV and to compare with a previous treatment of K+I2. In the absence of significant competition from continuum processes, Ar* is expected to behave in a manner similar to K, since the active electron is an unpaired 4s electron in both cases. We perform model calculations for Ar*+I2 to investigate the effects of varying the potential curves and charge exchange matrix elements and of including a continuum coupling function Γ (R). Comparison with previous calculations for K+I2 suggests increased repulsion on the Ar*–I2 surfaces relative to those of K–I2. The competing mechanisms of excitation transfer and Penning ionization may have a small effect upon the ion pair angular distributions.

Reactive and nonreactive modes of electronic excitation and molecular dissociation in hyperthermal collisions of alkali atoms with alkali halides

A molecular beam study of translational-to-electronic energy transfer in collisions of hyperthermal K or Rb atoms with thermal alkali halide molecules is reported. Data are presented for the chemiluminescence cross sections for the Tb–NaI system. (AIP)

Fragmentation of negative ions formed in collisions of alkali atoms and halogen molecules

Total cross sections have been measured for the production of negative halogen ions in collisions between alkali atoms and homonuclear halogen molecules at relative energies from threshold up to about 20 eV. By means of a mass spectrometer the cross sections for negative parent-ion and fragment-ion production could be measured separately. These measurements confirm the previously measured adiabatic electron affinities of the homonuclear halogen molecules. The fragment- and parent-ion cross sections are about equally large. It was found that the general behaviour of the fragment-ion cross sections is characteristically dependent on the alkali collision partner: the potassium cross sections peak sharply at energies just above threshold; the sodium cross sections, however, have a smooth maximum at higher energies. This could be explained qualitatively by a simple model.

Excitation cross sections for collisions of alkali atoms with molecules: Thresholds and energy dependence from 1 to 10 eV

Sputtered Na and K beams have been used to study collisional excitation of alkali atoms by molecules. Na *(3 2P)- and K *(4 2P) was detected by its radiative decay. There are some indications that excitation of other states may play an important role at higher energies.

Total cross sections for charge transfer between Alkali Atoms and Halogen Molecules

Ionic products have been observed in collisions of alkali atoms and halogen molecules in the eV region. The energy dependence of the total cross sections for ion-production has been measured for K, Na and Li on Br 2 and a discussion of the results is given based on the Landau Zener formula.

Der Regenbogeneffekt und Interferenzerscheinungen bei molekularen St��en

Crossed beam measurements of differential elastic scattering cross sections for collisions of alkali atoms with mercury and xenon atoms as well as with different molecules are presented. Special attention is given to the rainbow effect including supernumary rainbows and to interference effects connected with rainbow scattering.