The effects of Coulomb interaction screening on electron-hydrogen atom excitation in the $n=2$ threshold region are investigated by using the $R$-matrix method with pseudostates. The interaction screening lifts the $l$ degeneracy of $n=2$ Coulomb energy level, producing two distinct thresholds for $2s$ and $2p$ states. The phenomenon of transformation of ${}^{1,3}P$ and ${}^{1}D$ Feshbach resonances into shape-type resonances is observed when they pass across the $2s$ and $2p$ threshold, respectively, as the interaction screening increases. It is shown that this resonance transformation leads to dramatic effects in the $1s\to 2s$ and $1s\to 2p$ excitation collision strengths in the $n=2$ threshold collision energy region.

• Received 2 November 2009

DOI:https://doi.org/10.1103/PhysRevLett.104.023203