“…In the presence of metastable triplet levels the molecule can exhibit light and dark periods in its fluorescence ("blinking") [5,6], just as ions in the V or Λ configuration [7,8,9,10,11,12,13,14]. Spectroscopy of single molecules is of fundamental importance both for basic quantum mechanical aspects like antibunching, quantum jumps or the dynamical Stark effect [5,15,16], as well as for applications in chemical and biophysics, cf. e.g.…”