Transitions made by the free valence electrons of the molecules are responsible for the production of scintillation light in organic crystals. These electrons are associated with the whole molecule rather than any particular atom and occupy the so-called -molecular orbitals. The ground state S is a singlet state above which are the excited singlet states (S,  the lowest triplet state (T), and its excited levels (T,  A fine structure corresponding to molecular vibrational modes is associated with each of those electron levels. The energy spacing between electron levels is ≈1 eV; the spacing between the vibrational levels is about 1/10 of that for electron levels.
An incoming particle can excite either an electron level or a vibrational level. The singlet excitations immediately decay ( state without the emission of radiation (internal degradation). The S state then decays to the ground state S (typically to one of the vibrational levels above S) by emitting a scintillation photon. This is the prompt component or fluorescence. The transparency of the scintillator to the emitted photon is due to the fact that the energy of the photon is less than that required for an S → S transition (the transition is usually being to a vibrational level above S).
When one of the triplet states gets excited, it immediately decays to the T state with no emission of radiation (internal degradation). Since the T → S transition is very improbable, the T state instead decays by interacting with another T molecule:
and leaves one of the molecules in the S state, which then decays to S with the release of a scintillation photon. Since the T-T interaction takes time, the scintillation light is delayed: this is the slow or delayed component (corresponding to delayed fluorescence). Sometimes, a direct T → S transition occurs (also delayed), and corresponds to the phenomenon of phosphorescence. Note that the observational difference between delayed-fluorescence and phosphorescence is the difference in the wavelengths of the emitted optical photon in an S → S transition versus a T → S transition.
Organic scintillators can be dissolved in an organic solvent to form either a liquid or plastic scintillator. The scintillation process is the same as described for organic crystals (above); what differs is the mechanism of energy absorption: energy is first absorbed by the solvent, then passed onto the scintillation solute (the details of the transfer are not clearly understood).