Two-photon decays of highly excited states in hydrogen
  The relativistic and nonrelativistic approaches for the calculations of the
two-photon decay rates of highly excited states in hydrogen are compared. The
dependence on the principal quantum number (n) of the ns, nd, and np initial
states is investigated up to n = 100 for the nonresonant emissions. For the ns
states together with the main E1E1 channel the contributions of higher
multipoles (M1M1, E2E2, E1M2) are considered. For the np states the E1M1 and
E1E2 channels are evaluated. Moreover, the simple analytical formula for the
E1M1 decay is derived in the nonrelativistic limit.
