<title>Investigation of excitation and ionization excitation of He following electron and proton impact using EUV polarimetry</title>

Abstract

A detailed investigation of excitation of He (1s<SUP>2</SUP>) <SUP>1</SUP>S to He (1snp) <SUP>1</SUP>P degree(s) (n equals 2 - 5) states and ionization-excitation of He (1s<SUP>2</SUP>) <SUP>1</SUP>S to He (2p) <SUP>2</SUP>P degree(s) and He (3p) <SUP>2</SUP>P degree(s) states in e<SUP>-</SUP> + He and H<SUP>+</SUP> + He collision systems is presented for a wide range of projectile velocities (2.2 a.u. &lt; v &lt; 6.9 a.u.). Specifically new experimental data are presented on measurements of the degree of linear polarization for excitation and excitation-ionization of He following proton impact in the extreme ultraviolet (EUV) wavelengths. These measurements have been performed using a characterized molybdenum/silicon multilayer mirror polarimeter (MLM) whose polarization characteristics have been optimized for EUV emission of He and He<SUP>+</SUP>. Furthermore, the proton experimental results are compared with theoretical polarization data using the first Born approximation and 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 also given in order to elucidate differences in the collision mechanisms of two electron targets. It is important to note that these results are relevant for astrophysical diagnostics such as solar flares.