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

[Display omitted] •Photofragmentation of gas-phase protonated peptides by 15fs intense laser pulses.•Transition from multiphoton excitation to multiphoton ionization.•Two-pulse pump-probe scheme reveals resonant photofragmentation at Δt∼750fs.•Broadness of pump-probe maximum hints at a large number of transient resonances. We have combined a tandem mass spectrometer with a 780nm fs-laser system to study photoionization and photofragmentation of trapped protonated leucine enkephalin cations for laser intensities between 2×1013W/cm2 and 1×1014W/cm2 and pulse durations of 15fs. 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 750fs. The particularly flat appearance of the pump-probe curves suggests that not a single resonance, but a broad distribution of resonances is involved.