Photothermal lensing signal enhancement by the transient absorption of photoexcited states in liquid solutions

A two-color double-pulse excited photothermal lensing measurement was performed to investigate a signal enhancement owing to transient absorption by the excited states of solute molecules in liquid solutions. A colinear, mode-mismatched configuration with a continuous-wave probe laser at 633 nm was adapted with a focused pump pulse (6.9 ns, 355 nm) and a sequentially incident softly focused second-pump pulse (7.2 ns, 532 nm). The signal-intensity dependence on the energies of the pump and the second-pump pulses was measured for 1.0 μM solutions of 2,2'-p-phenylenebis(5-phenyloxazole) in 1-butanol and anthracene in ethanol. A signal enhancement was observed for both solutions. The signal intensity under certain conditions was one order higher than that expected from ground-state absorption. A discussion was made on determination of the absorption coefficient of the transient species and on the absorption saturation of the ground and excited states.