Orientation and alignment of collisionally excited atomic states

Abstract

The orientation and alignment parameters of atomic excited states of hydrogen and helium atoms formed in collisions with electrons, positrons, protons, and antiprotons are examined. For the orientation parameter 〈${L}_{y}$〉 (defined to be the expectation value of the electronic angular momentum perpendicular to the collision plane), it is found that the signs of 〈${L}_{y}$〉 for proton and positron impact are negative, consistent with the prediction of the classical model with a repulsive force. For electron and antiproton impact, except for electron scattering at large angles, the signs of 〈${L}_{y}$〉 are positive, consistent with the classical model with an attractive force. A simple semiempirical scaling of 〈${L}_{y}$〉 for electron-impact excitations to 2p states was found. The orientation of 2p states formed in electron capture in proton--hydrogen-atom collisions and in positron--hydrogen-atom collisions are also studied and the angular dependences of 〈${L}_{y}$〉 are found to show similar behavior. We also examine the orientation parameters for excitation and capture to 3d states and the alignment angles for excitation and capture to 2p states. It is found that in the latter cases these parameters do not show similar simple dependences with scattering angles.