Fig. 2. Spin system and experimental setup.

(A) Monoclinic crystal structure of pentacene-doped p-terphenyl. (B) Simplified Jablonski diagram depicting the origin of pentacene triplet spins and the ∣X⟩ ↔ ∣Z⟩ maser transition at zero field. The population distributions on the three sublevels ∣X⟩, ∣Y⟩, and ∣Z⟩ of pentacene’s lowest triplet state T₁ are labeled and indicated by the blue circles. ISC, intersystem crossing; IC, internal conversion. (C) Experimental setup for line shape measurements and sensing an external magnetic field. B₁, the microwave field generated in the strontium titanate (STO) resonator; B₀, the static magnetic field applied for sensing.