Excited-state absorption in red perylimide dye in solution

We measured the gain spectrum of red perylimide dye (RPD) dissolved in cyclohexane. A frequency-doubled Nd: YAG laser at 1 Hz repetition rate was used for pumping. A probe pulsed laser beam was obtained using a series of laser-grade dyes. Gain is observed in the region 550-660 nm and loss in the region 660-730 nm. The loss is interpreted as absorption from the first singlet excited state S₁ to a higher singlet excited state S_n. The cross section σ_1n is calculated as 1.3 × 10^-16 cm² / molecule in the region 550-660 nm and 5 × 10^-17 cm² / molecule in the region 650-710 nm. This process competes with the stimulated emission to a lower state, which establishes the mechanism for the dye-laser action.