Spin-exchange Cross Section Research Articles

Spin exchange collision cross sections for fast hydrogen atoms incident on hydrogen or alkali atoms

Spin exchange cross sections have been calculated for hydrogen atoms with energy in the keV range incident on hydrogen or alkali-atom targets. The cross sections generally decrease with increasing energy, but also show oscillations as a function of the energy. For a hydrogen atom with an energy of 5 keV incident on a Na atom target, the total spin exchange cross sections is 1.2*10-15 cm2. The effect of spin exchange collisions on an optically pumped polarised H- ion source is analysed.

Spin-exchange cross sections for hydrogen-atom-alkali-metal-atom collisions.

The pseudopotential molecular-structure method has been used to calculate the ${X}^{1}$\ensuremath{\Sigma} and ${a}^{3}$\ensuremath{\Sigma} interaction potentials for the alkali-metal-atom--hydrogen-atom systems. These potentials were then used in a quantum-mechanical calculation to determine the spin-exchange cross sections in the energy range from 2.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ eV to 2.5 eV. The cross sections follow the general form ${Q}^{1/2}$=a-b lnv. However, abundant structure on the cross sections is present due to orbiting resonances induced by the deeply bound well of the ${X}^{1}$\ensuremath{\Sigma} molecular state. The spin-exchange cross sections range from 17\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}16}$ to 25\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}16}$ ${\mathrm{cm}}^{2}$ at room temperature for H(F=1) transferring to H(F=0) in collision with unpolarized lithium and cesium, respectively.

Temperature dependence of muonium spin exchange with O2 in the range 88 K to 478 K

Our group at TRIUMF reported earlier a study of the spin exchange reactions of Mu with O2 and NO in the range 295 K to 478 K. We have extended the measurement with O2 to a low temperature region down to 88 K. From 135 K to 296 K, the spin depolarization rate constantkd(T) was found to vary according to the relative velocity of the colliding species,T1/2, which indicates that the spin exchange cross section of Mu-O2 is temperature independent in this temperature range. The value ofkd(T) at 296 K is in good agreement with our earlier study. However, it was found that below 105 K and above 400 K,kd(T) tends to have stronger temperature dependences (Tn, withn>1/2). This deviation fromT1/2-behavior can be attributed to the velocity (energy) dependence of the spin exchange cross section.

Efficiency of Spin Exchange between Rubidium Spins andXe129Nuclei in a Gas

By directly observing the nuclear polarization of $^{129}\mathrm{Xe}$, the efficiency $\ensuremath{\eta}$ of spin exchange between optically pumped Rb spins and $^{129}\mathrm{Xe}$ nuclei has been measured. It is found that $\frac{1}{\ensuremath{\eta}}=23\ifmmode\pm\else\textpm\fi{}4$ rubidium ${D}_{1}$ resonance-line photons are required to polarize a $^{129}\mathrm{Xe}$ nucleus when long-lived van der Waals molecules are unimportant. The binary spin-exchange cross section deduced from our measurements is ${\ensuremath{\sigma}}_{\mathrm{ex}}=(7.3\ifmmode\pm\else\textpm\fi{}1.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}$ ${\mathrm{cm}}^{2}$.

A temperature dependent study of the spin exchange reactions of muonium with O2 and NO in the range 295 to 478 K

The temperature dependence of the gas phase spin exchange reaction rates for muonium collisions with O2 and NO has been measured over the temperature range 295 to 478 K. In both cases the bimolecular rate constant k(T) was found to vary essentially as T1/2, leading to the following temperature independent spin exchange cross sections: σSE(Mu+NO) = (10.5±0.6)×10−16 cm2 and σSE(Mu+O2) = (9.1±0.7)×10−16 cm2. These results are in quantitative agreement with the H+O2 and H+NO results reported by Gordon et al., demonstrating that there is no isotope effect in spin exchange in this temperature range.

Measurement of the spin-exchange cross section in the collision of H atoms withO2and NO by means of stored atomic-beam spectroscopy

The spin-exchange cross section sigma for the collision of hydrogen atoms with O/sub 2/ and NO have been measured by means of stored atomic-beam spectroscopy. Average values at room temperature are sigma (H-O/sub 2/)10=(plus-or-minus1) x 10/sup -16/ cm/sup 2/ and sigma (H-NO)13=(plus-or-minus1) x 10/sup -16/ cm/sup 2/. A comparison between our data and pulsed-maser measurements is presented. The development of a passive hydrogen maser based on our bolometric detection technique is proposed.

Spin-exchange scattering cross sections for hydrogen atoms at low temperatures

Calculations are presented of the low energy (0 < E ≤ 10−2 eV) spin exchange and frequency shift cross sections for (H,H) scattering, and of their thermal averages for 0 < T < 10 K. In particular, the behaviour of the cross sections in this low energy limit and the role of resonances due to orbiting collisions is studied in detail. A comparison is made with recent nmr measurements on gaseous atomic hydrogen at liquid helium temperatures. The results of this work suggest further useful experiments at low temperatures.

Measurement of theRb87-Xe129spin-exchange cross section

The $^{87}\mathrm{Rb}$-$^{129}\mathrm{Xe}$ spin-exchange cross section has been measured in ${\mathrm{N}}_{2}$ buffered cells as a function of the ${\mathrm{N}}_{2}$ density using an alkali vapor magnetometer technique. The strong dependence of the spin-exchange cross section on the ${\mathrm{N}}_{2}$ density indicates a significant role for the Rb-Xe van der Waals molecule in the spin-exchange process. Our data have allowed us to estimate the Rb-Xe molecular formation rate, which is in reasonable agreement with previous measurements. The extrapolated binary spin-exchange cross-section term is found to be ${\ensuremath{\sigma}}_{\mathrm{ex}}{\overline{V}}^{2}=1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ ${\mathrm{cm}}^{4}$ ${\mathrm{sec}}^{\ensuremath{-}2}$.

A comparison of Mu-H and H-H electron spin exchange cross sections

The spin exchange cross sections for H-H and Mu-H collisions are calculated and compared. At low temperatures ( or approximately=400K), the thermally averaged cross sections are comparable and it is shown that the ratio of the Mu-H and H-H cross sections is given approximately by ( mu Mu-H/ mu H-H)1(p-1)/ where mu is the reduced mass and the difference between the triplet and singlet potentials is approximately proportional to r-p.

Effects of coherently excited Zeeman transitions on a hydrogen maser hyperfine transition

The effects of transitions excited within the F = 1 multiplet of the ground state hydrogen atoms while radiating on the ΔmF = 0 transition are analyzed theoretically. The results are valid for low transition rates and unrestricted maser power. The predictions are: (a) an important modification of the well-known spin-exchange frequency shift of a hydrogen maser, due to the imaginary part of the spin-exchange cross section, (b) a frequency shift proportional to the asymmetry parameter between the 2-Zeeman frequencies in the upper multiplet F = 1, and (c) a frequency shift proportional to the polarization of the atoms in the storage bulb.Also shown is a displacement of the zero crossing of the maser detuning vs. the low frequency coherent transition frequency with respect to the dip of the maser absorption. This effect is visible for a maser tuned by spin-exchange and operated at a relatively high magnetic field and nonuniform Zeeman populations.An experimental verification of the theory is presented. A new method for tuning the hydrogen maser is suggested, and a proposal is made to measure the collisional frequency shift proportional to the average duration of hydrogen atom spin-exchange collisions.

A new technique for measuring the spin-exchange cross-section between different atoms

A technique for measuring the spin-exchange between unlike atoms is proposed which makes use of a tunable dye laser; it has been tested in the measurement of the Sodium-Rubidium cross-section yielding a value of 2.6×10−14 cm2. Suggestions for improving on its accuracy are given.

Discussion of Temperature Dependence of Wall and Spin Exchange Effects in the Hydrogen Maser

Measurements have been performed of wall effects (atomic loss rate, wall relaxation and wall shift for a FEP 120 coating) and spin exchange effects (spin exchange cross-section, spin exchange frequency shift) in a hydrogen maser for various temperatures of the wall and the atomic hydrogen gas in the range 77-363 K.These experimental results allow a discussion of the phenomena involved in atom-wall collisions. It is shown that the FEP wall behaviour is different according to the temperature range. When the temperature is smaller than 202 K, the wall behaves like a perfect solid and the H atoms are perturbed during weak physical adsorption. When the temperature is greater than 202 K, the wall is no longer a perfect solid: the adsorption sites are modified, or new sites appear, the effects of this change being stronger and stronger when the temperature rises. The phenomena responsible for the loss of atoms on the wall are not modified at 202 K. For temperatures above 296 K, new phenomena appear, which introduce strong positive wall shift and atomic loss, when the temperature rises.These changes of the wall behaviour are connected with second order transitions affecting FEP.The experimental results relative to spin exchange effects are in good agreement with theoretical calculations by Allison: at room temperature the experimental values for the spin exchange cross section is σ = 23.1 ± 2.8 Å2 and for the spin exchange frequency shift is λ = 2.5 ± 2.5 Å2 (the theoretical values are respectively σ = 23.5 ± 2.4 Å2 and λ = 4.5 ± 0.45 Å2). We observe small variations of σ in the considered temperature range and a change of the sign of λ at a temperature around 205 K, as predicted by the theoretical calculations.

Temperature dependence of the hydrogen-hydrogen spin-exchange cross section

Spin-exchange relaxation rates are measured, as a function of the atomic hydrogen density, in the storage bulb of a hydrogen maser of special design. The values of the hydrogen-hydrogen spin-exchange cross section in the temperature range 77-363 K result. These experimental values are in very good agreement with the result of a theoretical calculation by Allison. For ambient temperature, the experimental value is $\ensuremath{\sigma}=23.1\ifmmode\pm\else\textpm\fi{}2.8$ ${\mathrm{\AA{}}}^{2}$ (the theoretical value is 23.5 \ifmmode\pm\else\textpm\fi{} 2.4 ${\mathrm{\AA{}}}^{2}$) and it varies slowly with temperature.

Electron-potassium spin-exchange cross section

A spin-exchange optical-pumping experiment to measure the electron-potassium spin-exchange cross section in a He buffer gas is reported. Electrons are polarized by spin-exchange collisions with optically pumped K atoms. The value of the spin exchange cross section at thermal energy is measured to be (6.8+or-1)*10-14 cm2. The electron-spin resonance linewidths are measured over the temperature range 85-120 degrees C. They fit the theoretical temperature dependence Delta nu (T) varies as P(K)T-32/, indicating that the width of the free electron resonance is caused mainly by spin-exchange collisions.

Prototype polarized-electron source through electron-hydrogen spin exchange with teflon containment of hydrogen and a longitudinal magnetic trap

A prototype polarized-electron source has been constructed and results are reported. It is based on electron-hydrogen spin-exchange collisions using a longitudinal-geometry electron trap and teflon coatings to contain the state-selected atomic-hydrogen target. This scaled-down prototype is compatible with an electron linac and produces an extracted pulse of about 107 electrons/μs with about 60% polarization and 50–60 Hz repetition rate. Construction of a more intense (1010 or more electrons per pulse) full-scale device based upon this technique is also discussed. The average measured total electron-hydrogen spin-exchange cross section for 5–10 eV electrons is estimated to be in the range (0.5–1.55) × 10−15 cm2.

Orientation of diamagnetic Cs+ions by charge-exchange collisions with optically pumped caesium

A charge-exchange optical pumping experiment to study collisions between Cs atoms and Cs+ ions in a He buffer gas is reported. Cs+ ions are polarized by charge-exchange collisions with optically pumped Cs atoms. From measurements of the line-widths of the Cs+ ion resonance and electron spin resonance, the charge- exchange cross section sigma ce(Cs+-Cs) and electron spin-exchange cross section sigma se(e-Cs)are found to be sigma ce(Cs*-Cs)=(4.5+or-0.5)*10-14 cm2 and sigma se(e-Cs)=(2.6+or-0.4)*10-14 cm2 respectively.

Stimulated Emission in the Hydrogen Maser and the H Spin-Exchange Cross Section

Experiments are described in which the phenomenon of stimulated emission with a delayed maximum of power is observed in a hydrogen maser and is studied in some detail. The results are coupled to measurements of relaxation rates made through a pulse technique in which the hydrogen beam is modulated. A solution of the rate equations describing the phenomenon for a two-level model is obtained on an analog computer and is compared with the experimental data. From the comparison, we conclude that the phenomenon is explained by the assumed theoretical model. It is found also that the published values of the spin-exchange cross section of hydrogen are consistent with our experimental data.

Variational Calculations of Electron—Alkali-Metal-Atom Scattering: Elastic Scattering by Li, Na, and K

Elastic total and spin-exchange cross sections are computed for electron scattering by Li, Na, and K, for energies below the first excitation threshold. The method used is the variational form of solution of continuum Bethe-Goldstone equations. Results are in substantial agreement with close-coupling calculations and with recent experimental data.

Temperature Dependence of the Electron-Rubidium Spin-Exchange Cross Section

A spin-exchange optical-pumping experiment to study the temperature dependence of the electron-rubidium spin-exchange cross section is reported. In this experiment, electrons were polarized in a weak magnetic field by spin-exchange collisions with optically pumped Rb atoms. The electron-Rb collisions were the principal source of the electron-spin-resonance linewidth, and they also produced a shift in the resonance frequency. The magnitudes of the linewidth and frequency shift are functions of the electron-Rb-scattering phase shifts. The electron linewidth at fixed Rb density was essentially constant over the temperature range 200-840 \ifmmode^\circ\else\textdegree\fi{}K indicating the spin-exchange cross section is well approximated by a $\frac{1}{v}$ velocity dependence. The ratio of the frequency shift to linewidth was 0.075\ifmmode\pm\else\textpm\fi{}0.005 at 300 \ifmmode^\circ\else\textdegree\fi{}K with an estimated Rb polarization of (20\ifmmode\pm\else\textpm\fi{}10)%, and the ratio was 0.027\ifmmode\pm\else\textpm\fi{}0.007 at 600 \ifmmode^\circ\else\textdegree\fi{}K with a Rb polarization of (10\ifmmode\pm\else\textpm\fi{}5)%.

Spin-Relaxation Effects on the EPR Spectrum of Gaseous Hydrogen Atoms.

A theoretical analysis is presented of the relaxation processes which lead to the magnetization of gaseous hydrogen atoms in a strong magnetic field. Experiments are described which demonstrate that the spin-lattice relaxation time in a pure H2 carrier can be of the order of 10 ms. From the effects of traces of O2 on the hydrogen atom EPR spectrum, a H-O2 spin-exchange collision cross section of (8.0 ± 2.5) × 10-16 cm2 was derived.