Influence of surface electric fields on the alignment of the hydrogen-2p state after beam-foil-excitation

Abstract

In contrast to other atomic systems, foil excited hydrogen or hydrogen-like atoms show a negative alignment for various energy ranges. We suggest that this particular behaviour results from the degeneracy of different L states in hydrogenic systems and an observable influence of a surface electric field modifying the initial alignment via Stark effect. L IntroductionThe beam-foil interaction process produces a difference in the magnetic sublevel populations of the excited states. This population asymmetry can be observed by the radiation emitted in transitions from excited states. If a foil is oriented perpendicular to the beam axis, axial and reflection symmetry with respect to this axis hold and only linear polarization is measured. From these measurements one determines the alignment (Po-o -P~a)/(Vao + Pal) of an excited p state, where the density matrix element %M,~ is proportional to the cross section for the formation of a state [nLML).As a particular result the alignment of the beam-foil excited states of hydrogen [1][2][3] and of hydrogen-like ions [4] is negative over a wide energy range in contrast to non-hydrogenic systems, for which always positive alignment [5] is found. We use the assumption of Eck [6] and Lombardi [7] that a strong electric field exists at the surface of a carbon foil in order to explain the particular behaviour ofhydrogenic systems. Their energy levels are strongly affected by an electric field, which may cause the alignment to change sign as will be shown below. II. Theoretical ModelIn the present work we discuss the alignment of a hydrogen-2p state as determined from the polarization * This work is part of the research program of the "Sonderforschungsbereich 161" supported by the Deutsche Forschungsgemeinschaft of the Lyman-e radiation emitted perpendicular to the beam. We refer to the following geometry: the beam and the detector axis coincide with the z and y direction, respectively. The foil is located in the x-y-plane. An ensemble of excited ions or atoms leaves the foil at a time t o. Postulating, as is usually done, that the excitation process results from pure electrostatic interactions, the initial density matrix of the excited states factorizes into an isotropic spin and an anisotropic angular momentum matrix (nLML[a(to) In' I2M~).(The angular momentum basis is characterized by n, the principal quantum number, L, the angular momentum, and ML, the corresponding magnetic quantum number with respect to the z direction as quantization axis). We recall that the hermiticity of a(to)From symmetry of reflection at the x-z-plane follows (lb) and axial symmetry about the beam direction means (lc)The angular momentum density matrix of the foilexcited hydrogen n=2 state at time t= t o is given by 0340-2193/78/0285/0003/$01.00

Etude par battements quantiques de l'interaction d'un faisceau d'ions 4He+ et d'une feuille de carbone

“…Dans le but d'expliquer les phases des cohérences par l'existence d'un champ électrique intense à la surface de la feuille, nous avons fait différentes simulations sur ordinateur. Nous avons en particulier simulé, en référence à Kupfer et Winter [ 19], un champ de 101 V/cm sur 6 A. Les résultats s'éloignent beaucoup des données expérimentales. Il nous paraît donc que les cohérences ne sont pas créées uniquement par un champ électrique intense au voisinage de la feuille.…”

“…Nous trouvons pour les niveaux 4P, 4D, 4F Contrairement donc à ce qui se passe dans l'hydrogène neutre excité par la même méthode à la même vitesse [15,19], on constate que l'alignement est négatif dans He+. En mesurant la polarisation Ijj -1 I-L Berry et al [16] trouvent pour le niveau 1>+ 1 z n = 5 de He II à E = 600 keV une valeur positive, ce qui correspond (comme dans notre cas) à un alignement négatif.…”