Coherence effects in electron-hydrogen scattering

Karl Blum, Hans Kleinpoppen

The angular distribution and polarisation of hydrogenic line radiation excited by electron impact is considered for the case of coincidence experiments. The theory of coherent excitation of hydrogenic states with opposite parity is developed. A set of multipole parameters is defined which characterise the excited states and which enable the formulation of Fano and Macek to be generalised.

Arend Niehaus

Robert V. Krotkov

This paper is a theoretical discussion of a "coherence effect" in the excitation of hydrogen atoms to the $n=3$ state by electron bombardment. By coherence is meant that the density matrix describing the atom after the collision has off-diagonal matrix elements in the field-free basis. The differential cross section for producing Balmer-$\ensuremath{\alpha}$ photons at 90\ifmmode^\circ\else\textdegree\fi{} to the electron beam is calculated as a function of an applied longitudinal electric field. The off-diagonal elements show up as a dependence of the observed light on the directions of the electric field. Theoretical results based on the Born approximation are compared with an experiment at 200 and 500 eV. It is pointed out that this method for measuring relative phases of scattering amplitudes can be generally applied and is not restricted to $n=3$ excitation of hydrogen.

A. K. Ghatak, Heinrich Hora, Runwen Wang +1 more

J. Lienemann, Andreas Schüller, D. Blauth +5 more

The coherence for diffraction effects during grazing scattering of fast hydrogen and helium atoms from a LiF(001) surface with energies up to some keV is investigated via the coincident detection of two-dimensional angular distributions for scattered projectiles with their energy loss. For keV H atoms, we identify electronic excitations of the target surface as the dominant mechanism for decoherence, whereas for He atoms this contribution is small. The suppression of electronic excitations owing to the band gap of insulators plays an essential role for preserving quantum coherence and thus for the application of fast atom diffraction as a surface analytical tool.

Pascal Degiovanni, Clément Cabart, Benjamin Roussel +4 more