Role of He excited configurations in the neutralization of He<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mrow/><mml:mo>+</mml:mo></mml:msup></mml:math>ions colliding with a HOPG surface

Abstract

The very high neutral fractions measured in He${}^{+}$ scattered by graphitelike surfaces, at intermediate incoming energies (1 keV $&lt;{E}_{\mathrm{in}}&lt;$ 6 keV), cannot be explained only by the resonance of the He ionization level with the valence band states of the surface. Excited configurations $(1s2s)$ and $(1s2p)$ appear as possible resonant neutralization channels together with the ground state one $(1{s}^{2})$. We develop, in this work, a time-dependent quantum-mechanical calculation of the charge-transfer process in He${}^{+}$/HOPG collision, where the resonant neutralization to the ground and first excited states of He is taken into account. We use an Anderson Hamiltonian projected on the electronic configurations of the projectile atom which are energetically favorable for the charge-exchange process. Thus, an exhaustive analysis of different possible approximations to the neutralization of He${}^{+}$ is performed: the typical neutralization to the ground state by either neglecting or not the electron spin and finally the one including excited configurations. Our results reproduce the observed experimental trends only when excited configurations $(1s2s)$ and $(1s2p)$ are involved in the charge exchange between the ion and the surface.