Electron exchange in collisions withO2and NO

Abstract

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, .

Spin-exchange effects in elastic electron scattering from linear triatomic radicals

“…2,3 In contrast, such effects were found to be very small for elastic electron collisions with O 2 and NO. These observations were in agreement with the results of Ratliff et al 4 in their measurement of average SF cross sections for scattering of electrons at thermal energies from O 2 and NO molecules.…”

“…For elastic e --NCN collisions, the sharp resonance-like feature located at about 3.3 eV incident energy is identified as due to the occurrence of a 4 Π u shape resonance whereas that located at about 4.7 eV is due to a 2 Π u shape resonance. Similarly, the physical origin of the broad resonance centered at around 2.2 eV, in the ICS for the e --CNN is identified as due to the 4 Π scattering channel and that located at 3.5 eV as due to the 2 Π symmetry. The shift in the resonance positions due to different spin couplings is somehow expected since the resonances are not originated from the same state.…”

“…Moreover, the resonance features seen in the spin-specific ICSs for elastic e --CNN scattering, shown in Figure 1(b), are much broader. Therefore, the unique resonance feature seen in the SF ICSs for this target is due to the overlap of the spin-exchange effects associated to the resonances in both 4 Π and 2 Π scattering channels. In Figures 6 and 7, we present the angular distribution of the SPFs in the (1-10) eV energy range calculated for e --NCN and e --CNN collisions, respectively.…”

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

“…Polarized electrons were used by Hanne and Kessler 8,9 who measured the change of the polarization of scattered electrons in electron-impact excitation of Hg, but only for forward scattering. Recently, Ratliff et al 10 used a similar method to measure spin-exchange cross sections for elastic scattering of polarized electrons from O2 and NO molecules at thermal energies. Exchange effects have also been observed in experiments where polarized electrons and polarized targets are used simultaneously if the spin-parallel-antiparallel asymmetries of scattered electrons, 11 " 14 ions, 1115 " 18 or photons are measured.…”

Spin-Exchange-Induced Circularly Polarized Molecular Fluorescence

“…In contrast, P 0 =P does not vary significantly from unity for elastic scattering from O 2 and NO targets with any combination of the scattering angles and energies studied, except near 100 with O 2 where P 0 =P was closer to 0:90 0:05 over a broad range of energies. In a separate experiment, Ratliff et al [11] determined the rate constants for spin-exchange in collisions of O 2 and NO molecules with the thermal polarized electrons of a flowing afterglow. The values they extracted were more than an order of magnitude below those calculated for alkali atoms and H.…”