Photoisomerisation quantum yield and non-linear cross-sections with femtosecond excitation of the photoactive yellow protein

Abstract

The quantum yield of photoisomerisation of the photoactive yellow protein (PYP) strongly depends on peak power and wavelength with femtosecond optical excitation. Using systematic power titrations and addition of second order dispersion resulting in 140, 300 and 600 fs pulse durations, the one and multi-photon cross-sections at 400, 450 and 490 nm have been assessed from transient absorption spectroscopy and additionally the Z-scan technique. Applying a target model that incorporates photoselection theory, estimates for the cross-sections for stimulated emission and absorption of the first excited state, the amount of ultrafast internal conversion and the underlying species associated dynamics have been determined. The final quantum yields for photoisomerisation were found to be 0.06, 0.14–0.19 and 0.02 for excitation wavelengths 400, 450 and 490 nm and found to increase with increasing pulse durations. Transient absorption measurements and Z-scan measurements at 450 nm, coinciding with the maximum wavelength of the ground state absorption, indicate that the photochemical quantum yield is intrinsically limited by an ultrafast internal conversion reaction as well as by stimulated emission cross-section. With excitation at 400 nm photoisomerisation quantum yield is further significantly limited by competing multi-photon excitation into excited state absorption at 385 nm previously proposed to result in photoionisation. With excitation at 490 nm the photoisomerisation quantum yield is predominantly limited further by the significantly higher stimulated emission cross-section compared to ground state cross-section as well as multi-photon processes. In addition to photoionisation, a second product of multi-photon excitation is identified and characterised by an induced absorption at 500 nm and a time constant of 2 ps for relaxation. With power densities up to 138 GW cm⁻² the measurements have not provided indication for coherent multi-photon absorption of PYP. In the saturation regime with 450 nm excitation, the limit for the photoisomerisation quantum yield was found to be 0.14–0.19 and the excited state absorption cross-section 6.1 × 10⁻¹⁷ cm² or 0.36 times the ground state cross-section of 1.68 × 10⁻¹⁶ cm² per molecule. This places a fundamental restriction on the maximum populations and sample penetration that may be achieved for instance in femtosecond pump–probe experiments with molecular crystals of PYP.