New energy shifts of nS1/2 atomic states due to a specific boundary

New energy shifts have been calculated for the nS1/2 states of the atom placed at the center between two parallel, perfectly conducting plates. They are inversely proportional to the distance, b, from two plates. Our numerical calculations with b = 1 μm give the energy shifts, -1.478 x 10-8 eV (-3.574 MHz) and -1.189 x 10-8 eV (-2.876 MHz), to the 2S1/2 and 4S1/2 states of He+. The shifts in the 1S ground states of the hydrogen atom and He+ have also been estimated as 1.455 x 10-8 eV (3.518 MHz) and 5.841 x 10-8 eV (14.124 MHz) for b = 1 μm. (author)