Аннотации:
We experimentally investigate the ionizationmechanism behind the formation of remarkably high charge states
observed in the laser-pulse-induced fragmentation of different hydrocarbon molecules by Roither et al. [Phys.
Rev. Lett. 106, 163001 (2011)], who suggested enhanced ionization occurring at multiple C-H bonds as the
underlying ionization mechanism. Using multiparticle coincidence momentum imaging we measure the yield of
multiply charged fragmenting ethylene and acetylene molecules at several intensities and pulse durations ranging
from the few-cycle regime to 25 fs. We observe, at constant intensity, a strong increase of the proton energy
with increasing laser pulse duration. It is shown that this is caused by a strong increase in the yield of highly
charged parent molecular ions with pulse duration. Based on experimental evidence we explain this increase by
the necessary population of precursor states in the parent ion that feature fast C-H stretch dynamics to the critical
internuclear distance, where efficient ionization via enhanced ionization takes place. For increasing pulse duration
these precursor ionic states are more efficiently populated, which leads in turn to a higher enhanced-ionization
probability for longer pulses. Our work provides experimental evidence for the existence of a multiple-bond
version of enhanced ionization in polyatomic molecules