M. Bailey

Characteristics of a multilayer mirror polarimeter for measurements at extreme ultraviolet wavelengths

We have developed and tested a multilayer mirror (MLM) polarimeter optimized for λ=304 A using electron impact on helium. Specifically we present the first experimental results of the polarization of the extreme ultraviolet Lyman-α emission from He+ [He II (2p)2P°→(1s)2S] as a function of the electron impact energy for impact energies ranging from 150 to 980 eV. A major challenge to performing this measurement is the suppression of the dominant He I (1snp)1P°→(1s2)1S radiation originating from the excitation of helium. Excellent spectral selectivity has been achieved with a VYNS copolymer filter whose transmission ratio of He II to He I emission has been measured to be greater than 1200:1. Additionally, we have measured the instrumental polarization using two identical MLMs in a crossed polarizer configuration.

Ionization-excitation magnetic sublevel cross sections for He+ (2p)2P0 states following fast electron and proton impact

The first experimental magnetic substate scattering-angle-integrated cross sections following ionization–excitation of He(1s 2 ) 1 St o He þ (2p) 2 P 0 in the e � þ He collision system have been determined using a combination of total cross sections and polarization fraction measurements in the extreme ultraviolet range. The derived magnetic sublevel cross sections, r0 and r1,for ML ¼ 0, � 1 have been studied over a wide range of electron velocities (2–8.5 a.u.). These results are compared with previous cross sections for equi-velocity proton impact. In addition,our experimental data are compared with earlier theoretical predictions,our recent second-Born calculations fully including off-energy shell terms, and new R-matrix results. We have extended our polarization measurements for H þ þ He collisions and found excellent agreement between theory and experiment at nearly all impact energies. However,the present second-Born and Rmatrix results deviate slightly from the experimental electron data. 2003 Elsevier Science B.V. All rights reserved.

Scattering angle-integrated (total) and magnetic sublevel cross-sections and degree of linear polarization for electron and proton induced excitation [HeI (n=2–5)] of helium

Experimental scattering-angle-integrated (total) cross-sections �, (scattering) angle-integrated magnetic sublevel cross-sections �ML , and degree of linear polarization data have been measured in the extreme ultraviolet (EUV) wavelength region following decay of HeI ð1snpÞ 1 P 0 ðn ¼ 2–5Þ states induced by electron and proton impact on a neutral helium target. These measurements are compared with a first Born approach as well as more sophisticated theoretical calculations. Specifically, theoretical values for electron impact include convergent close-coupling (CCC) and R-matrix with pseudo states (RMPS) methods in addition to first Born (Born 1) approximation while proton induced excitation cross-sections are compared with atomic-orbital close-coupling (AOCC) and first Born predictions.

Non-statistical magnetic substate populations following excitation of helium by electron and proton impact

The first experimental magnetic substrate scattering angle-integrated cross sections following excitation of He (1s2)1S to He (1snp)1Po (n=2–5) in e−+He and H++He collision systems have been determined using our differential cross sections and polarization fraction data in the extreme ultraviolet (EUV) range. The derived magnetic sublevel integrated cross sections, σ0 and σ1, for ML=0,±1 have been studied over a wide range of projectile velocities: 1.4 to 8.5 a.u. and 1.4 to 7.5 a.u. for electron and proton impact on helium, respectively. In addition, the electron and proton collision data are compared with theoretical predictions using our improved first Born approximation (IFB), convergent close coupling (CCC) calculations for electron impact, and our recent atomic orbital close coupling (AOCC) calculations for protons. We have found that the electron results match very well the CCC predictions at all energies. However, the experimental proton data slightly deviate from the AOCC approach at intermedia...

Polarization study of the extreme ultraviolet (EUV) emission from helium following electron and proton impact

A detailed investigation of excitation of He (1s2)1S to HeI (1 snp)1Po (n=2–5) states and ionization-excitation of He (1s2)1S to HeII (2p)2Po and HeII (3p)2Po states following electron and proton impact on He is presented for a wide range of projectile velocities (2.2 a.u.<v<6.9 a.u.). Specifically, new experimental data are presented on measurements of the degree of linear polarization for excitation and ionization-excitation of He following proton impact in the extreme ultraviolet (EUV) wavelength region. Furthermore, the proton experimental results are compared with theoretical polarization data using the first Born approximation and our recent atomic orbital close coupling (AOCC) calculations for the excitation process. A comprehensive comparison of experimental data for negatively and positively charged projectiles at equal impact velocities is given in order to elucidate differences in the collision mechanisms of two electron targets.