Absolute, apparent cross sections for 295-410 nm band emission from naphthalene have been measured over an electron-impact energy range from threshold to 300 eV. Numerous processes, including electron exchange, are observed to contribute to the excitation in the near threshold region. At higher energies direct excitation through optically allowed channels is preferred. The electron-impact-induced fluorescence was found to be unpolarized (<0.5%) in the near-threshold region but showed small negative polarizations of a few per cent at impact energies greater than 25 eV. The radiation decay occurs to excited vibrational levels of the ground electronic state which is expected to lead to strong IR emission.

Differential and integral cross section data for electron-impact excitation of the 21p level in He have been critically reviewed. Experimental and theoretical results have been compared and a set of differential cross sections at 20 ° scattering angle in the 25 to 500 eV impact energy range has been deduced based on all available information. It is proposed that this set of data represents the most accurate inelastic differential cross sections available at the present time and could be used as a secondary standard for normalization of cross sections.

A preliminary study of the electron-impact excitation of thermally evaporated coronene at 550° C was carried out using electron-energy-loss spectroscopy. Measurements of the energy-loss spectra of coronene at high (100 eV) and low (5–20 eV) impact energies are presented. One of the high-energy spectra was converted to an apparent generalized oscillator strength spectrum and compared to the photoabsorption spectrum of coronene. Observations concerning vibrational excitation of coronene by electron impact are also presented and discussed.

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, .

A URV compatible Mott polarization analyzer is described that employs electron accelerating voltages of -20 kV. The efficiency of the analyzer, ~ 3 X 10--5 , is competitive with those provided by other polarimeters. The present analyzer is considerably simpler and more compact than earlier designs arld can be used to undertake energy-and angle-resolved polarization measurements with input beam currents below _10--14 A.