Maximum angular momentum transfer in electron-atom collisions

The electron–atom collision process can be fully described by determining complex scattering amplitudes, which depend on collision energy and scattering angle. In the case of electronic excitation from the S to P atomic state, such data can be obtained using the electron-photon coincidence technique. The results of such experiments are usually presented as Electron Impact Coherence Parameters (EICP), which describe the angular distribution of the electron charge cloud of an excited P state. They are the alignment angle of the electron charge cloud (γ), its shape parameter (P_L), angular momentum transfer (L_perp) and coherence parameter (P⁺)[1].



Our experimental EICP data analysis for zinc and cadmium atoms allowed us to find some helical structures in gamma function in energy and scattering angle domains. They correspond to the special conditions in which only one of two scattering channels is available. Therefore the electron charge cloud is rotationally symmetric (P_L=0), the alignment angle parameter is undefined, and the angular momentum transfer reaches the maximum value (+/-1). Due to their local and narrow character, such structures can be an exact tool for verifying theoretical models with experimental results.