Аннотация
Summary form only given. Here we present the results of a combined experimental and numerical study on strong-field ionization of acetylene performed with the aim of identifying the mechanism behind the reported surprisingly large multielectron ionization probabilities of polyatomic molecules [3,4,5]. Using coincidence momentum imaging techniques and time-dependent density functional simulations, we show that the strong ionization enhancement measured for molecules aligned parallel to the laser polarization [see Fig. 1(a,b)] is due to the combined action of (i) a significant energy up-shift of the most relevant valence orbitals as the C-H distance increases [see Fig. 1(c)], and (ii) a concomitant strong increase in the coupling between multiple molecular orbitals. Both processes lead to a strong effective reduction of the binding energy and an enhancement of the emission probability for lowervalence orbitals, making their ionization contributions even the dominant ones. For molecules aligned perpendicularly to the laser polarization direction this mechanism is inhibited because of a dominant emission of π electrons at low charge states, which in turn suppresses the C-H stretch motion and therewith prohibits entering the regime of the enhanced ionization.