Spin-exchange Cross Section Research Articles

Complex spin-exchange cross sections in collisions of rubidium isotopes in the ground state

Complex spin-exchange cross sections are calculated on the basis of the data on the singlet (X 1Σ+) and triplet (a 3Σ+) potentials describing the interaction of isotopes of Rb alkali-metal atoms in the ground state. The cross sections calculated in this study are compared with the cross-section data obtained earlier with the model interaction potentials. The cross-section values allow one to calculate the processes of polarization transfer and the relaxation times, as well as the magnetic resonance frequency shifts caused by the Rb‒Rb spin-exchange collisions.

Hyperfine-changing transitions in 3He II and other one-electron ions by electron scattering

SynopsisWe present short-range phase shifts and spin-exchange cross sections for elastic electron scattering from selected one-electron ions from He+ to Fe25+. The results are explained in a simple physical picture based on quantum defect theory.

Spin exchange upon collision of two cesium atoms in the ground state

The spin exchange cross sections and the magnetic resonance frequency shift upon collision of two cesium atoms in the ground state are calculated. The calculations are carried out on the basis of the interaction potentials for the singlet (X−1Σ+) and triplet (a3Σ+) states of the Cs2 dimer, reported in the literature in recent years. The results are compared with earlier data.

HYPERFINE-CHANGING TRANSITIONS IN3He II AND OTHER ONE-ELECTRON IONS BY ELECTRON SCATTERING

We consider the spin-exchange (SE) cross section in electron scattering from $^3$He\,{\scriptsize II}, which drives the hyperfine-changing \hbox{3.46 cm} (8.665 GHz) line transition. Both the analytical quantum defect method --- applicable at very low energies --- and accurate R-matrix techniques for electron-He$^+$ scattering are employed to obtain SE cross sections. The quantum defect theory is also applied to electron collisions with other one-electron ions in order to demonstrate the utility of the method and derive scaling relations. At very low energies, the hyperfine-changing cross sections due to e$-$He$^+$ scattering are much larger in magnitude than for electron collisions with neutral hydrogen, hinting at large rate constants for equilibration. Specifically, we obtain rate coefficients of $K(10\,{\rm K}) = 1.10 \times 10^{-6}\,\rm cm^3/s$ and $K(100\,{\rm K}) = 3.49\times 10^{-7}\,\rm cm^3/s$.

Optimized production of hyperpolarized 129Xe at 2 bars for in vivo lung magnetic resonance imaging

In this work, the production rate of a spin-exchange optical pumping 129Xe gas polarizer was optimized for routine generation of hyperpolarized 129Xe for in vivo lung MRI. This system uses a narrow (∼ 0.1 nm linewidth), tuneable external cavity laser (operating at ∼25 W) for SEOP of 3% gas mixtures of Xe inside a mid-pressure (2 bars) cell of 491 cm3 volume. Under this regime, theoretical and experimentally measured 129Xe polarizations were calculated to be 24% and 12%, respectively, for a gas flow rate of 300 sccm and a cell temperature of 373 K. The photon efficiency was evaluated, yielding theoretical and experimental values of 0.039 and 0.046, respectively. The theoretical efficiency was calculated from spin-exchange and spin-destruction cross sections and the experimental photon efficiency was measured under flow for a gas-cell residency time equal to an empirically determined spin-exchange time of 45 s. In addition, details of the Xe freeze-out process were analyzed with a model of polarization decay during Xe accumulation in the frozen phase, where a T1 of 87 ± 2 min was observed. To demonstrate the system's application, in vivo lung magnetic resonance images (signal-to-noise ratio ∼ 50 from a voxel of 15 mm× 4 mm× 4 mm) were acquired using modest volumes (<400 ml) of isotopically enriched (86% 129Xe) Xe gas polarized to >10%. Despite the experimental polarization being a factor of 2 lower than the predicted polarization for typical operating parameters, the system is close to the theoretical photon efficiency and the system has so far produced polarized gas for more than 100 in vivo 129Xe lung imaging studies.

Spin-exchange processes in a single-chamber Cs-K tandem magnetometer

The dependences of real and imaginary parts of the complex spin-exchange cross section on collision energy are calculated for the systems K-Cs and K-K.

Interaction of antihydrogen with ordinary atoms and solid surfaces

The characteristic features of cold atom-antiatom collisions and antiatom-surface interactions are discussed and illustrated by the results for hydrogen-antihydrogen scattering and for quantum reflection of ultracold antihydrogen from a metallic surface. We discuss in some detail the case of spin-exchange in ultracold \(\bar{H}-H\) collisions, exposing the interplay of Coulombic, strong and dispersive forces, and demonstrating the sensitivity of the spin-exchange cross sections to hypothetical violations of Charge-Parity-Time (CPT) symmetry.

Spin exchange during collisions of potassium atoms: Complex cross sections

Singlet (X1Σ+) and triplet (a3Σ+) potentials of interaction of two potassium atoms residing in the ground state (4s2S1/2) are presented. Based on the given interaction potentials, the complex cross sections of spin exchange \(q = \bar q + i\overline{\overline q}\) for the system under investigation are calculated. Obtained dependences of the real and imaginary parts of the spin-exchange cross section on temperature allow one to obtain information both on the broadening of a magnetic resonance line of K atoms and on the frequency shift of the magnetic resonance during collision.

Electronic spin-flipping collisions of hydrogen atoms

We present a unified multichannel approach to calculate electron spin-exchange and spin-flipping transition cross sections for collisions of H with H, H with T, and T with T. We use the theory to calculate the hyperfine quenching cross sections for collision energies that range from $1$ mK to thermal temperatures. We show that spin-flipping transitions are induced by the splitting of the $b {}^{3}{\ensuremath{\Sigma}}_{u}$ Born-Oppenheimer potential via the long-range magnetic interactions among electrons. We find that the spin-flipping cross sections in the tritium dimer are about a magnitude larger than that predicted by mass scaling the H-H cross sections. For the former, we show that the spin-exchange cross sections are several magnitudes larger, at cold temperatures, than that of the hydrogen system. We compare the results of the multichannel approach with those obtained using approximate methods such as the degenerate internal-state, the elastic, and Born approximations and discuss their respective range of validity.

Intermediate-coupling phenomena in electron-impact excitation of Hg atoms

In my talk the influence of the intermediate-coupling scheme of Hg on various electron scattering observables such as spin exchange cross sections, spin up-down asymmetries or spin-resolved circular polarization is discussed with special emphasis to recent experimental results from our group and recent theoretical data.

Chemiionization and spin exchange in collisions of triplet metastable atoms in a mixture of helium isotopes

Cross sections of spin exchange and chemiionization in collisions of two triplet metastable helium atoms are calculated for different isotopes of helium. The data on the spin exchange cross sections for the systems studied are reported for the first time.

RCNP polarized He3 ion source

We have developed a polarized He3 ion source named “SEPIS” (spin-exchange polarized ion source) at RCNP, and Department of Physics, Osaka Univeristy. The SEPIS uses a large spin-exchange cross section, σse, and a small electron capture cross section,σec, for the He+3+Rb system theoretically expected at low He+3 incident energies. The validity of SEPIS was experimentally proven by observing the He+3 nuclear polarization as a function of the incident He+3 energy.

Spin exchange cross sections for collisions of metastable helium atoms with lithium atoms in the ground state

The spin exchange and chemi-ionization cross sections for the metastable helium atom-lithium atom in the ground state system have been calculated. Using data on the interaction potentials obtained in this study, the spin exchange cross sections are determined for the first time for the He(23 S 1)-Li(22 S 1/2) system in the interval of collision energies from 5 × 10−3 to 16 eV.

Chemi-ionization and spin exchange upon collisions of metastable helium atoms with potassium atoms in the ground state

The spin exchange and chemi-ionization cross sections have been calculated for the metastable helium atom-potassium atom system in the ground state. The data on the spin exchange cross section in the system He(23S1)-K(42S1/2) have been obtained for the first time. The cross sections were calculated for the collision energies ranging from 2 × 10−4 to 60 × 10−4 au The chemi-ionization cross sections obtained have been compared with the data in the literature.

Electron capture cross-sections between 3He + ion and rubidium vapor in the energy range 1.0–19 keV

Electron capture cross-sections have been measured for 3He + ions in Rb vapor. The deduced data were used for evaluating the performance of the proposed polarized 3He ion source (SEPIS) based on the spin-exchange collisions. Measurements of the 3He + beam components after passing through the target were made as a function of the Rb target thickness at energies ranging from 1.0 to 19 keV. The Rb vapor thickness was varied by changing the Rb cell temperature and the thicknesses were calibrated by observing the Faraday rotation angles of a probe laser ( λ = 782.0 nm ) penetrating the Rb vapor under the presence of a magnetic field ( ∼ 0.26 T ) . Assuming that the double electron capture and stripping of 3He 0 atoms are neglected, the electron capture cross-sections σ c were deduced. It was found that the observed energy dependence of σ c significantly deviated from that of the He + 4 + Cs system which was measured in the past. Lastly, the observed energy dependence of σ c was compared with the theoretical calculation assuming the semi-classical close-coupling method based on the molecular-orbital expansion. It was found that the calculated results, as a whole, reproduced the experimental results except for the data collected at 1 keV. This qualitative reproduction partly supports the appropriateness of the above theory which predicts the large spin-exchange cross-sections at low energies.

Spin-exchange frequency shift in alkali-metal-vapor cell frequency standards

In this paper we calculate the effect of spin-exchange collisions in alkali-metal vapors. In the framework of the high-energy approximation, we evaluate the spin-exchange cross sections related to the line broadening and to the frequency shift of the ground state hyperfine transition. We do the calculation for the four isotopes, $^{23}\mathrm{Na}$, $^{39}\mathrm{K}$, $^{87}\mathrm{Rb}$, and $^{133}\mathrm{Cs}$. The results are used in particular to evaluate the spin-exchange frequency shift in Rb vapor cell frequency standards used in many applications. It turns out that, due to possible fluctuations in the atomic density, spin exchange may affect significantly the medium and long term frequency stability of the frequency standard.

Analysis of structures in the cross sections for elastic scattering and spin exchange in low-energyH++Hcollisions

Elastic scattering and spin exchange cross sections in ${\mathrm{H}}^{+}+\mathrm{H}$ collisions are computed using accurate adiabatic potential curves for the center-of-mass energy range ${10}^{\ensuremath{-}4}lEl100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Both cross sections show considerable structure which necessitates computation on a fine energy grid to resolve them. We analyze the structures using a comparison equation method to find the poles of the scattering matrix in the complex energy plane. We show that many features of the cross sections can be correlated with these poles and tabulate the positions of the most important poles. Finding stationary phases in the partial wave cross sections, we also explain in all details the glory oscillations of the elastic cross section that extends below $100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ down to the lowest energies.

Radiative charge-transfer lifetime of the excited state of(NaCa)+

New experiments were proposed recently to investigate the regime of cold atomic and molecular ion-atom collision processes in a special hybrid neutral-atom--ion trap under high vacuum conditions. The collisional cooling of laser pre-cooled Ca$^+$ ions by ultracold Na atoms is being studied. Modeling this process requires knowledge of the radiative lifetime of the excited singlet A$^1\Sigma^+$ state of the (NaCa)$^+$ molecular system. We calculate the rate coefficient for radiative charge transfer using a semiclassical approach. The dipole radial matrix elements between the ground and the excited states, and the potential curves were calculated using Complete Active Space Self-Consistent field and M\"oller-Plesset second order perturbation theory (CASSCF/MP2) with an extended Gaussian basis, 6-311+G(3df). The semiclassical charge transfer rate coefficient was averaged over a thermal Maxwellian distribution. In addition we also present elastic collision cross sections and the spin-exchange cross section. The rate coefficient for charge transfer was found to be $2.3\times 10^{-16}$ cm$^3$/sec, while those for the elastic and spin-exchange cross sections were found to be several orders of magnitude higher ($1.1\times 10^{-8}$ cm$^3$/sec and $2.3\times 10^{-9}$ cm$^3$/sec, respectively). This confirms our assumption that the milli-Kelvin regime of collisional cooling of calcium ions by sodium atoms is favorable with the respect to low loss of calcium ions due to the charge transfer.

Dynamics of two overlapping spin ensembles interacting by spin exchange.

We describe linear and nonlinear dynamics of spin-polarized K and 3He ensembles interacting by spin exchange. The interactions are dominated by the imaginary part of the spin-exchange cross section and each spin species is primarily affected by the average magnetization of the other. Operating in a very low magnetic field we demonstrate novel dynamics when the electron and nuclear spin precession frequencies are nearly matched. We observe transverse damping as well as a dynamic instability of the 3He spins interacting with polarized K vapor. We also demonstrate operation as a self-compensating comagnetometer, useful for tests of CPT violation and other precision measurements.

Spin-exchange cross section for a3He+ion incident on a Rb atom

Spin-exchange cross sections between a ${}^{3}{\mathrm{He}}^{+}$ ion and a Rb atom were calculated in a ${}^{3}{\mathrm{He}}^{+}$ impact-energy range from 0.01 to 10 keV/amu by the semiclassical close-coupling method based on the molecular-orbital expansion. The previous calculation in which only two states of the ${}^{3}{\mathrm{He}}^{+}$ $(1s{ }^{2}S)+\mathrm{Rb}(5s{ }^{2}S)$ system were taken into account overestimated the experimental result at 6.33 keV/amu by orders of magnitudes, whereas the present calculation, which allowed the target excitation and electron transfer, showed a rough agreement with the experimental result. In view of the application, the spin-exchange cross sections larger than ${10}^{\ensuremath{-}14} {\mathrm{cm}}^{2},$ predicted at ${}^{3}{\mathrm{He}}^{+}$ impact energies lower than 0.3 keV/amu will hopefully provide a powerful tool for producing a highly polarized ${}^{3}\mathrm{He}$ ion beam.