Anomalous behavior of metastable hydrogen atoms as they pass through a metal slit
Sokolov Yu. L., Yakovlev V. P., Pal'chikov V. G., Lin D. N.
Experiments on metastable 2s,/2 hydrogen atoms (with an energy — 20keV) have shown that as they pass through a metal slit at a large distance from the walls of the slit (e.g., at a distance 106 times the first Bohr radius) they go into a superposition state (2s-2p).