  We investigate the interaction of metastable 2S hydrogen atoms with a
perfectly conducting wall, including parity-breaking S-P mixing terms (with
full account of retardation). The neighboring 2P_1/2 and 2P_3/2 levels are
found to have a profound effect on the transition from the short-range,
nonrelativistic regime, to the retarded form of the Casimir-Polder interaction.
The corresponding P state admixtures to the metastable 2S state are calculated.
We find the long-range asymptotics of the retarded Casimir-Polder potentials
and mixing amplitudes, for general excited states, including a fully quantum
electrodynamic treatment of the dipole-quadrupole mixing term. The decay width
of the metastable 2S state is roughly doubled even at a comparatively large
distance of 918 atomic units (Bohr radii) from the perfect conductor. The
magnitude of the calculated effects is compared to the unexplained Sokolov
effect.
