Rate constants for electron exchange in collisions between thermal-energy, spin-polarized electrons and 0& and NO have been measured using a Aowing-helium afterglow apparatus. The measured rate constants, -10 ' cm' sec ', are substantially smaller than those for electron exchange in collisions with hydrogen or alkali-metal atoms.Collisions of spin-polarized electrons with molecular targets such as 02 or NO that are not spin singlets can lead to a degradation in electron polarization via electron exchange reactions of the typẽ s =o~s =+1 M= -1 2 M=O s s e (1)+NO[Ms = --, ' I~e ( J, )+NOIMs=+ -, ' I,where Ms denotes the spin-projection quantum number for the molecule. Such exchange reactions are frequently studied, at electron energies greater than a few electron volts, using beam techniques, ' but few experimental data are available at thermal collision energies. ' In the present work a flowing-helium afterglow apparatus has been used to investigate electron exchange in thermalenergy collisions with Oz and NO. The measured rate constants, -10 ' cm sec ', are substantially smaller than those for spin exchange in collisions with hydrogen or alkali-metal atoms. ' The present apparatus is shown schematically in Fig. 1. ' Briefly, a microwave discharge is used to generate He(2 S) metastable atoms in a flowing-helium afterglow.The 2 S atoms are optically pumped to preferentially populate either the Mz(Ms)=+1 or -1 magnetic sublevels. CO2 is then introduced into the flow tube resulting in the production of polarized electrons through Penning ionization.These electrons are rapidly thermalized by collisions and are then allowed to interact with either 02 or NO introduced downstream. The degradation in polarization that results from electron-exchange reactions is determined by extracting electrons from the flow tube through a differentially pumped aperture and measuring their polarization using a Mott polarimeter.Rate constants k(02) and k(NO) for exchange are derived from measurements of the dependence of the extracted electron polarization on the 02 or NO density p in the flow tube and on the reaction time t. The density p is governed by the flow rate Q of the injected target gas and is given by p=Q/ot, 3 where o" is the average bulk gas-flow velocity in the flow tube and 2 its crosssectional area. The reaction time t is determined by the reaction length L and mean electron axial flow velocity v, and is given by t =L /U, .

The performance of the Rice source of spin polarized electrons, which is based on an optically pumped flowing helium afterglow, has been substantially improved. He(23S) metastable atoms contained in the afterglow are optically pumped using 1.08 μm 23S1↔23P1 radiation from an LNA laser. Spin conservation in subsequent chemi-ionization reactions with CO2 results in the production of free polarized electrons that are extracted from the afterglow. At low currents, ≲1 μA, polarizations of 80%–90% are achieved. This decreases to ∼75% at 10 μA and to ∼50% near 100 μA. The polarization can be simply reversed (P→−P). The energy spread in the extracted beam is <0.4 eV, and the beam emittance is <4 mrad cm−1 at 270 eV. This source is suitable for use in a wide variety of applications, and is particularly attractive for use with the new generation of high-duty factor electron accelerators that are currently being developed.

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We investigate the validity of the Boltzmann equation to predict the reflection and transmission coefficients for an intensity modulated laser beam passing through a microscopic medium consisting of discrete scatterers. For a one-dimensional model system we demonstrate that the Boltzmann equation works remarkably well for small modulation frequencies, even to describe a medium comprised of only 10 scatterers. Discrepancies can be found only if the modulation wavelength of the laser intensity is commensurate with the spacing between the scatterers and if the medium is sufficiently ordered.

Spin-labeling techniques, specifically the use of electron-spin-polarized He(2 3S) metastable atoms coupled with energy-resolved spin analysis of the product electrons, are used to investigate the dynamics of Penning ionization in collisions between He(2 3S) atoms and H2O, SO2, NO, and NO2. The data complement earlier studies of the reaction dynamics based on analysis of the energy distribution of the product electrons and confirm that ionization can occur via a number of different reaction channels. For example, the present results show that in collisions with targets having a positive electron affinity, ionization via the ionic channel is important and that effects due to spin-orbit coupling must be considered. The data also provide evidence that exchange may be important in collisions with open-shell targets.