Figure 1: Maser process in pentacene.
From: Enhanced magnetic Purcell effect in room-temperature masers
Jablonski diagram showing electronic state cycle responsible for generating population-inverted spin-polarized triplet-state sublevels in pentacene:p-terphenyl. Optical excitation of pentacene molecules from the ground state to the singlet states, S0→S1, S2, S3, followed by spin-orbit coupling mediated spin-selective intersystem crossing from the excited singlet states into the T2 triplet manifold. The T2 triplet states decay non-radiatively down to the T1 triplet state while preserving the spin-polarized populations. Spin-lattice relaxation can occur between the triplet sublevels with rate γXZ. The triplet sublevels ultimately decay back to the S0 ground state with rates kX,Y,Z, thus completing a cycle. X→Z transitions due to stimulated emission lead to amplification of the microwave phonons in the maser cavity. Singlet excitation wavelengths were taken from optical extinction data21.
