Weak-coupling radiationless transitions (internal conversion or inter system crossing) are studied assuming separability and symmetry over N identical modes. Franck-Condon factors control the branching ratios between exciting just one of the equivalent modes, or equally distributing the available energy. The dominant process can be predicted by an exact quantum mechanical solution if the wavefunctions are known (Gaussian initial distributions and accepting Morse or Poeschl-Teller oscillators, for example); or more generally by a Wiper phase space surface-jumping analysis based on a classical limit of the Wiper function, using only the donor distribution and the acceptor potential surface. (C) 2002 Elsevier Science B.V. All rights reserved.