Transition Probability for Class of Two Level System Time Dependent   Hamiltonians
  Transition probabilities for a class of two level systems described by explicitly time dependent Hamiltonians are considered. Provided only that the approach to the infinite time limit is non-trivial falling at least as fast as 1/t for large t, the transition probability takes a particularly simple form depending only on the value of Hamiltonian parameters in this limit. 
S.T. Love
quant-ph