Photon Echoes Stimulated from Long-Lived Ordered Populations in Multi-Level Systems. The Effect of Intersystem Crossing and Optical Branching

It is shown that the three pulse stimulated photon echo (3PSE) may be used to determine the yield for intersystem crossing in a guest molecule doped into a host lattice. An exact expression is derived for the intensity of the 3PSE for the case when all three spin sublevels participate in the optical pumping cycle. Yields for intersystem crossing in the systems naphthalene in durene (45%) and pentacene in naphthalene (15%) were obtained from the stimulated photon echo decay. In addition, the 3PSE formation is discussed in pseudo two-level systems where the level structure is caused by magnetic interactions. As an example, the 3PSE decay of the system triphenylmethyl in triphenylamine is analyzed.