Instanton - like transitions at high energies in (1+1) dimensional   scalar models. II.Classically allowed induced vacuum decay
  We consider classical Minkowskian solutions to the field equation in the (1+1) dimensional scalar model with the exponential interaction that describe the unsuppressed false vacuum decay induced by $n$ initial particles. We find that there is a critical value of $n$ below which there are no such solutions, i.e., the vacuum decay is always suppressed. For the number of initial particles larger than this value the vacuum decay is unsuppressed at high enough energies. 
V.A.Rubakov and D.T.Son