Dependence of photoionization quantum yield of indole and tryptophan in water on excitation wavelength

Excitation wavelength dependence (λex = 220–290 nm) of photoionization quantum yields (Φion) of indole and tryptophan in water were directly obtained through nanosecond time-resolved transient absorption measurements. Monophotonic photoionization occurred efficiently in this wavelength range. We conclude that photoionization proceeds through electron transfer from the neutral excited state to water by comparing the Φion versus λex dependences with the ground-state spectra. Mechanism of the electron transfer process is discussed.