Instanton Induced Tunneling Amplitude at Excited States with the LSZ   Method
  Quantum tunneling between degenerate ground states through the central barrier of a potential is extended to excited states with the instanton method. This extension is achieved with the help of an LSZ reduction technique as in field theory and may be of importance in the study of macroscopic quantum phenomena in magnetic systems. 
J.-G. Zhou, J.-Q. Liang, J. Burzlaff, H.J.W. Mueller-Kirsten