Spin-exchange Cross Section Research Articles

Unexpected reduction of the spin-exchange cross-section for fast $\mathsf{^3He^+}$ ions incident on Rb atoms

The spin-exchange cross-section, σse, was measured for a 6.33 keV/amu 3He+ ion incident on a polarized Rb atom. The result is σse = −0.26+0.27 × 10−15 cm2, which is unexpectedly an order of magnitude smaller than the theoretical value σse = 5.9 × 10−15 cm2 evaluated by the semiclassical impact parameter method assuming formation of a single molecular state.

Accurate3Hepolarimetry using the Rb Zeeman frequency shift due to theRb−3Hespin-exchange collisions

We describe a method of ${}^{3}\mathrm{He}$ polarimetry relying on the polarization-dependent frequency shift of the Rb Zeeman resonance. Our method is ideally suited for on-line measurements of the ${}^{3}\mathrm{He}$ polarization produced by spin-exchange optical pumping. To calibrate the frequency shift we performed an accurate measurement of the imaginary part of the $\mathrm{Rb}{\ensuremath{-}}^{3}\mathrm{He}$ spin-exchange cross section in the temperature range typical for spin-exchange optical pumping of ${}^{3}\mathrm{He}.$ We also present a detailed study of possible systematic errors in the frequency shift polarimetry.

Optical pumping magnetic resonance in high magnetic fields: Measurement of high field spin exchange cross sections

The polarization of 129Xe gas by spin exchange with optically pumped Rb vapor is examined in high magnetic field. Measurements of the Rb–Xe velocity averaged spin exchange cross section 〈 σ v̄〉 and the 129Xe relaxation time in the absence of Rb, T 1 Xe, are discussed. Comparison of these high field results to low field literature values for 〈 σ v̄〉 show that this parameter is essentially field independent. T 1 Xe is also found to be significantly longer in high field. On the basis of these observations, it is shown that production of large amounts of spin polarized 129Xe gas can be made more efficient by high field operation.

Atomic hydrogen-deuterium mixtures at 1 kelvin: Recombination rates, spin-exchange cross sections, and solvation energies

A new technique has been applied to the study of atomic hydrogen and deuterium mixtures confined by liquid helium coated walls. The method uses standard low-field hyperfine magnetic resonance of the H atoms at 1420 MHz, but takes advantage of the dramatically different spin-exchange broadening for H-H and H-D collisions to simultaneously monitor the H and D densities. This provides a powerful means for studying the interaction of D with itself and with liquid helium, something otherwise difficult to achieve, and it also makes possible the study of spin-exchange and recombination interactions between H and D. A wide range of experimental results are presented, including the rate constants for H-D and D-D recombination, the spin-exchange broadening cross sections for H-D and D-D collisions, the H-D spin-exchange frequency shift cross section and an improved value for the H-4He buffer gas shift. Finally, a detailed study of the solvation of D into liquid4He has yielded an improved value for the salvation energy, a useful lower bound for the effective mass for D in liquid4He, and evidence for the reaction D + HD → D2 + H on the surface under the liquid4He film.

Observation of exchange in collisions of polarized electrons with atoms and molecules

Exchange collisions of electrons with Hg and Na atoms and O2 and NO molecules have been studied by means of polarized electrons for energies between 2.5 and 15 eV and scattering angles up to 110 degrees . While significant exchange effects have been observed for collisions of electrons with Hg and Na atoms, differential spin exchange cross sections for elastic collisions from the open shell molecules O2 and NO are much smaller. This is in agreement with theoretical calculations that predict significant exchange effects only for oriented molecules. A small but significant spin exchange structure for e-O2 collisions between 8 and 15 eV at theta =100 degrees is attributed to a shallow 4 Sigma u- resonance.

Polarization-dependent frequency shifts from Rb-3He collisions.

We present measurements of the frequency shift of the Rb electron-paramagnetic-resonance (EPR) line in the presence of nuclear-polarized $^{3}\mathrm{He}$ gas for the temperature range of 30 to 85 \ifmmode^\circ\else\textdegree\fi{}C. The frequency shift is due to the Fermi-contact interaction between the Rb valence electron and the polarized $^{3}\mathrm{He}$ nucleus. Expressions for both the frequency shift and the spin-exchange cross section are derived in terms of the strength of this contact interaction. From these expressions and the measured frequency shift, we estimate the Rb${\mathrm{\ensuremath{-}}}^{3}$He spin-exchange cross section. The Rb EPR frequency shift, which is 53 kHz for a 100% polarized 10-amagat $^{3}\mathrm{He}$ sample at 50 \ifmmode^\circ\else\textdegree\fi{}C, can be used to determine the absolute polarization of nuclear polarized $^{3}\mathrm{He}$ targets. From these measurements, one can also predict the shift of the $^{3}\mathrm{He}$ NMR line due to a polarized Rb vapor.

NMR study of low-pressure 129Xe gas

The enhanced 129Xe gas (15 Torr) NMR signal has been observed on a Varian XL-200 NMR spectrometer with spin-exchange optical pumping. An amplification factor of the nuclear spin polarization of gaseous 129Xe is of the order 10 3 times the Boltzmann polarization at 60°C. The binary spin-exchange cross section for Cs 129Xe has been measured by observing the nuclear spin relaxation of 129Xe in the presence of Cs vapor. The measured cross section is (1.7 ± 0.2) × 10 −20 cm 2. The effect of diffusion of 129Xe atoms on NMR signals is also discussed.

Calculations for designing a 3He polarizer

As first stages in a project to optimize the design of a 3He polarizer, lower limits for the quality factor are defined and its dependnce on different parameters is considered. The relation for the 3He relaxation is estimated from the literature and further measurements of alkali- 3He spin exchange cross-section are recommended.

Rb-129Xe spin-exchange rates due to binary and three-body collisions at high Xe pressures.

We have studied the spin relaxation of $^{129}\mathrm{Xe}$ nuclei due to collisions with Rb atoms at Xe pressures of 245--1817 Torr. Our results can be characterized by two parameters, the Rb${\mathrm{\ensuremath{-}}}^{129}$Xe velocity-averaged binary spin-exchange cross section 〈\ensuremath{\sigma}v〉 and a rate ${\ensuremath{\gamma}}_{\mathit{M}}$ that characterizes spin relaxation due to van der Waals molecules. Our results complement earlier studies performed at Xe pressures of about 1 Torr and ${\mathrm{N}}_{2}$ pressures of 10--100 Torr. This work is useful for predicting spin-exchange rates between polarized Rb atoms and $^{129}\mathrm{Xe}$ nuclei.

Hyperfine resonance of atomic deuterium at 1 K

Atomic-deuterium gas has been studied at temperatures just above 1 K. A short rf discharge was used to dissociate solid D2 on the surface of a sealed pyrex bulb lined with a saturated helium film. Pulsed magnetic resonance on the β-δ transition at its minimum frequency (309 MHz in a magnetic field of 3.9 mT) was used to observe the resulting D. The free induction decays were shortened by spin-exchange collisions with impurity H atoms present in the sample. The H density, inferred using calculated spin-exchange cross sections, was typically higher than the D density. In addition, the samples of D were short-lived, decaying exponentially in time with a strongly temperature-dependent lifetime. We were able to show that recombination with the H impurity was not the cause of the sample decay, and we propose that the decay is due to the thermally activated process wherein D atoms penetrate the liquid helium film that coats the cell walls. Analysis of the lifetime data yields the value 13.6 (6) K for the rest energy of a D atom dissolved in liquid helium, the first measurement of this quantity for any hydrogen isotope.

Study of exchange in collisions of polarized electrons with atoms and molecules.

Exchange collisions of electrons with Na and Hg atoms and ${\mathrm{O}}_{2}$ and NO molecules have been studied by means of polarized electrons for energies between 4 and 15 eV and scattering angles ranging from 0\ifmmode^\circ\else\textdegree\fi{} to 110\ifmmode^\circ\else\textdegree\fi{}. While significant exchange effects have been observed for collisions of electrons with Na and Hg atoms, in agreement with theoretical predictions, differential spin-exchange cross sections for elastic collisions from the open-shell molecules ${\mathrm{O}}_{2}$ and NO are much smaller. This was not anticipated before. A satisfactory explanation of this different behavior is still missing.

Large electron polarization of H-like20Ne-ions traversing gd foils at very high velocities

It is shown that transient magnetic fields in Gd-host for20Ne ions at a mean velocity of 12.5v0(v0=c/137), considerably beyond the Bohr velocity of 1s electrons of Ne ions (v1s=10v0), are dominated by the Fermi contact field of these electrons. The first excited 2+-state of2Ne was used as probe. The derived value for the mean degree of polarization p1s=0.32(13), is surprisingly large, though still consistent with values obtained at lower velocities. Present calculations of spin exchange cross sections severely underestimate the observed polarization at this high velocity.

A Precise Measurement of the Cesium Spin-Exchange Relaxation Cross Section

A measurement of the Cs spin-exchange cross section ( σex = 1.40 ± 0.10 × 10−14 cm2) is presented. It is a result of an experiment on the hyperfine relaxation of an optically pumped cesium vapor in the presence of nitrogen buffer gas in experimental conditions which ensure the absence of quadrupole contributions and the minimizing of the influence of high diffusion orders on the relaxation signals.

The optically pumped polarized proton ion source at KEK

An optically pumped polarized proton ion source has been developed and used for the 12 GeV synchrotron. For charge-change injection into the ring, nuclear-polarized H − ions are needed. A beam intensity of 180 μA with 43% polarization has been attained so far in a pulse mode operation of 100 μs duration. In the source, 5 keV protons out of an ECR source capture polarized electrons from sodium atoms which are optically pumped. A 500 W pulsed dye laser was developed to polarize the sodium atoms. Their target thickness and polarization is measured by the Faraday rotation of linearly polarized light out of a probe laser. Polarization of hydrogen is determined by H − current out of the charge-exchange region. The source delivers typically 60 μA beams with about 60% polarization for routine operation. The spin-exchange reaction cross section between sodium and hydrogen atoms is investigated for further development of the polarized source.

Muonium depolarization by electron spin exchange with O2 gas in the temperature range 90-500 K.

The thermally averaged depolarization cross sections, \ensuremath{\sigma}${\ifmmode\bar\else\textasciimacron\fi{}}_{D}$(T), for Mu+${\mathrm{O}}_{2}$ electron spin exchange, have been measured by the muon spin relaxation (\ensuremath{\mu}SR) technique in a ${\mathrm{N}}_{2}$ moderator at total pressures near 1 atm over the temperature range 88--500 K. These values are related to the thermal spin-flip cross sections of interest (\ensuremath{\sigma}${\ifmmode\bar\else\textasciimacron\fi{}}_{\mathrm{SF}}$) by a simple numerical factor. At temperatures \ensuremath{\gtrsim}120 K, \ensuremath{\sigma}${\ifmmode\bar\else\textasciimacron\fi{}}_{D}$(T) is essentially temperature independent [with \ensuremath{\sigma}${\ifmmode\bar\else\textasciimacron\fi{}}_{\mathrm{SF}=(5.7\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\mathrm{\ensuremath{-}}16}}$ ${\mathrm{cm}}^{2}$], though exhibiting a slight tendency to increase with temperature. At lower temperatures, \ensuremath{\sigma}${\ifmmode\bar\else\textasciimacron\fi{}}_{\mathrm{SF}(\mathrm{T}}$) decreases noticeably. Comparison with the only currently available theoretical calculations of Mu(H)+${\mathrm{O}}_{2}$ spin-flip cross sections by Aquilanti, Grossi, and Lagan\`a [Hyperfine Interact. 8, 347 (1981)] on the potential-energy surface of Farantos et al. [Mol. Phys. 34, 947 (1977)] gives poor agreement with the data, particularly in their temperature dependence. The present results for \ensuremath{\sigma}${\ifmmode\bar\else\textasciimacron\fi{}}_{\mathrm{SF}(\mathrm{T}}$) for Mu+${\mathrm{O}}_{2}$ qualitatively exhibit the same trend with temperature as found by Desaintfuscien and Audoin [Phys. Rev. A 13, 2070 (1976)] for H-H spin exchange over a comparable temperature range, but the H-H cross sections are, surprisingly, about four times larger. Comparisons with the experimental H+${\mathrm{O}}_{2}$ spin-exchange cross sections of Anderle et al. [Phys. Rev. A 23, 34 (1981)] and of Gordon et al. [JETP Lett. 17, 395 (1973)], indicate a significant isotope effect, with \ensuremath{\sigma}${\ifmmode\bar\else\textasciimacron\fi{}}_{\mathrm{SF}(\mathrm{H})\mathit{\ensuremath{\gtrsim}}1.5\mathit{\ensuremath{\sigma}}{\ifmmode\bar\else\textasciimacron\fi{}}_{\mathrm{SF}}}$(Mu). While this effect can be qualitatively understood in terms of the differing numbers of partial waves involved, detailed theoretical calculations on more recent potential-energy surfaces for ${\mathrm{HO}}_{2}$ are called for.

Polarization measurement of atomic hydrogen beam spin-exchanged with optically oriented sodium atoms

The spin-exchange reaction between hydrogen atoms and optically oriented sodium atoms was used to produce a polarized atomic hydrogen beam. The electron-spin polarization of the atomic hydrogen beam, which underwent the spin-exchange reaction with the optically oriented sodium atoms, was measured. A beam polarization of −(8.0 ± 0.6)% was obtained when the thickness and polarization of the sodium target were (5.78 ± 0.23) × 10 13 atoms/cm 2 and −(39.6 ± 1.6)%, respectively. The value of the spin-exchange cross section in the forward scattering direction, whose scattering angle in the laboratory system was less than 1.0°, was obtained from the experimental results as Δσ ex = ∫ dσ dω dω θ lab =0 →1.0° =(3.39±0.34)× −15 cm 2. This value is almost seven times larger than the theoretical value calculated from the NaH potential. The potential was computed quantum mechanically in the space of the appropriate wave functions of the hydrogen and the sodium atoms.

Spin-exchange cross section for electron excitation of Na 3S-3P determined by a novel spectroscopic technique.

An experimental technique is described which enables determination of the partial cross sections for electron excitation of atoms as a function of changes in spin and orbital angular momentum. This method provides a good signal-to-noise ratio in the energy region near threshold, and could be used to study long- or short-lived excited states of many atomic systems. Measurements for Na 3s-3p, near-threshold excitation are reported here. The results are generally in good agreement with the close-coupling calculations of D. L. Moores and D. W. Norcross (J. Phys. B 5, 1482 (1972)) for the largest ..delta..m/sub S/ and ..delta..m/sub L/ components of the cross section, but not for the component with chemically bond..delta..m/sub S/chemically bond = 1 and chemically bond..delta..m/sub L/chemically bond = 1, indicating that the exchange interaction is larger than previously recognized.

Analytic solution of the stochastic-Liouville model of spin exchange

An analytic solution is obtained for the stochastic-Liouville model of spin exchange between a pair of radicals undergoing isotropic diffusion in solution and interacting via an exchange interaction that decays exponentialy with radical separation. The resulting spin exchange cross section is the sum of a ‘‘strong encounter’’ term that is approximately equal to the biomolecular reaction cross section and a ‘‘grazing encounter’’ term that is due to encounters in the tail of the exchange interaction. The latter term may range from negligible to twice the former term for plausible values of the diffusion rate and the range of the exchange interaction. It is shown that the theory combined with experimental data on the spin-exchange contribution to spin–lattice relaxation can provide an estimate of the range of the exchange interaction between two radicals.

Relaxation rates for optically pumped Na vapor on silicone surfaces

The relaxation rate for polarized Na atoms colliding with a silicone surface has been studied as a function of duration of exposure to Na vapor, wall temperature, Na density and magnetic field strength. We find that such surfaces have relaxation times corresponding to over 100 wall collisions even after weeks of exposure to Na vapor. Our measurements allow a determination of the amplitude and correlation time of the depolarizing interaction on the surface and yield a value of 1.6±0.5× 10 −14 cm 2 for the Na-Na spin exchange cross section.

Potential Scattering and Spin-Exchange Cross Sections in Atomic Hydrogen at Very Low Energies

Potential scattering and spin-exchange scattering cross sections in the energy range from 10 -3 cm -1 to 100 cm -1 have been calculated numerically for hydrogen atoms in collision based on the most accurate Kolos-Woniewicz adiabatic potentials available at present. The scattering lengths for the singlet and triplet states are calculated to be a 0 s =0.595 a.u. and a 0 t =1.34 a.u., respectively. The quasi-bound state with ν=14 and l =4 is found to be at E =1.675 cm -1 above the dissociation limit. The thermally averaged spin-exchange cross section has the low temperature limit \barσ ex (0 K)=0.982 A 2 with a minimum \barσ ex (2.5 K)=0.329 A 2 .