Role of proton dynamics in efficient photoionization of hydrocarbon molecules

Abstract

We experimentally investigate the ionization mechanism 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.

Time-dependent density-functional study of the alignment-dependent ionization of acetylene and ethylene by strong laser pulses

“…Our choice of the two molecules is motivated, on the one hand, by previous experimental [30,31] and theoretical [38][39][40] studies on these molecules and, on the other hand, by the different complexities and structures of them. The acetylene molecule is one step from the diatomic molecules towards more complicated structures.…”

“…We have studied the response to nine different laser fields (see Table I) with intensities and pulse durations typical in experiments [31].…”

“…The two laser pulses that we used exhibit an intensity and duration typical in experiments [31]. Three simple geometric alignments were considered [see Figs.…”

“…Finally, one of the most important mechanisms in molecular ionization is enhanced ionization [27][28][29], where highly efficient ionization occurs at critical nuclear separations. Recently, experimental evidence has been found that this process may also take place at many bonds in parallel in a polyatomic molecule [30,31].…”

“…First, the C-H bonds expand to a critical separation R c . Then, the increased C-H bond length leads to highly efficient ionization of the molecule [30,31]. After the enhanced ionization, the molecule often undergoes fragmentation, ejecting the hydrogen ions.…”