Femtosecond laser induced ionization and dissociation of gas-phase protonated leucine enkephalin

We have combined a tandem mass spectrometer with a 780 nm fs-laser system to study photoionization and photofragmentation of trapped protonated leucine enkephalin cations for laser intensities between 2 × 1013 W/cm2 and 1 × 1014 W/cm2 and pulse durations of 15 fs. In this intensity range, the transition from multiphoton ionization and excitation to tunneling ionization is expected to occur. The observed partial ion yield curves as a function of laser intensity exhibit a power-law dependence, indicating multiphoton absorption to be the dominating mechanism. Pump-probe studies were performed to investigate the time-evolution of the multiphoton ionization process. The partial ion yields of almost all fragmentation channels show a broad but distinct maximum at a delay-time of approximately 750 fs. The particularly flat appearance of the pump-probe curves suggests that not a single resonance, but a broad distribution of resonances is involved.