Exciton trapping with coherent emission of phonons at impurities in molecular crystals

The process of exciton trapping at impurities with deep trap depths is studied in molecular crystals assuming that the excess exciton energy can be emitted in the form of coherent phonons. We represent the state of emitted phonons by coherent states and derive rates of exciton trapping for isotopic impurities. The calculation is done with acoustic as well as optical phonons. In naphthalene crystals with impurities of trap depth 350 cm-1 rates are calculated in terms of the coherent state parameter |α|. For 1.1 ≤ |α| ≤1.8 it is found that the rate of acoustic phonon emission is 1016 s-1 and of optical phonon emission is 1018 s-1. Such high rates suggest that a laser-like new source of intense and coherent phonon energy can be designed