Polarization study of the extreme-ultraviolet emission from helium following electron impact

Abstract

Experimental results are presented on the degree of linear polarization of the extreme-ultraviolet emission of neutral and ionized helium following electron-impact excitation and ionization excitation of helium. The polarization of the photon emission from the decay of He (1snp) 1 P o states with wavelengths of 517 to 584 Å has been extended to electron-impact energies of 1500 eV, and compared with theory for electron-impact excitation of neutral helium. In addition, the polarization of a number of the He ϩ decays with wavelengths between 256 and 1640 Å have been measured. Particular attention has been paid to the (2p) 2 P o˜( 1s) 2 S and (3p) 2 P o˜( 1s) 2 S decays with wavelengths of 304 and 256 Å, respectively. These have been measured from threshold ͑66 and 73 eV, respectively͒ to 1500 eV using a characterized molybdenum/silicon multilayer mirror polarimeter whose reflection and polarization characteristics have been optimized at 304 Å He ϩ (2p) 2 P o results are compared with a recent threshold alignment measurement and distorted-wave Born-approximation calculation for the (e Ϫ ,2e Ϫ ) reaction of ionization excitation. Very good agreement with the threshold theoretical predictions is obtained. These results indicate that near threshold, partial waves with LϾ0 contribute substantially to the two-electron wave function of the escaping electrons. Measurements on the unresolved multiplets at 1215 and 1640 Å, made using a more conventional reflection-type polarization analyzer, indicate that relative fine-structure cross sections are quite different than predicted by some theories.

Development of x-ray and extreme ultraviolet (EUV) optical devices for diagnostics and instrumentation for various surface applications

“…4. More details about the experimental set-up are given by Merabet et al 24 The photon source in this study was a di †erentially pumped gas cell operated at a helium target pressure of \1 mTorr. An electron beam enters the cell through a 1 mm entrance aperture and the EUV photon emission is observed through a 1.3 mm emission window.…”

Probing Molecular Dynamics at Attosecond Resolution with Femtosecond Laser Pulses

“…Since D + 2 is aligned perpendicular to the electron beam direction, we use the 2p 1 excitation cross section for the σ u state and the 2p 0 excitation cross section for the π u state, but each rescaled with their respective excitation energies. It is interesting to note that electron impact excitation to 2p 0 is five to three times larger than excitation to 2p 1 in the energy region of interest (the branching ratio for data from Bray [14] for H and from the experimental data of [15] for He are essentially identical). Thus, different from I and II, we found that excitation to π u state is not negligible.…”

<title>Spectropolarimetric measurements of the extreme-ultraviolet emission from helium following e-, H<formula>^{<roman>+</roman>}</formula>, H2<formula>^{<roman>+</roman>}</formula>, andH<formula><inf><roman>3</roman></inf></formula><formula>^{<roman>+</roman>}</formula> charged particle impact</title>

“…The only VUV polarization data reported previously was by McConkey et al (1988) 17 for the n=4 to n=2 transition. An angular distribution of the He + (2p) 2 P o alignment induced by electron impact was reported by Götz et al 18 -workers [12][13][14] .…”

“…here I || and I ⊥ are the intensities of radiation with electric field vectors parallel and perpendicular with respect to the incident projectile beam direction [12][13][14] when measured at an angle of 90 degrees. The determination of P is of interest for three reasons: (1) the measurement of P does not involve an absolute calibration of the radiation intensity emitted, a procedure which usually gives rise to considerable uncertainty; (2) in most cases the anisotropy of the radiation must be taken into account when performing absolute and relative differential cross sections measurements; (3) The degree of linear polarization P contains more detailed information about the collision process because it is directly related to the sublevel cross sections.…”