Abstract
We have investigated nuclear dynamics in bound and dissociating acetylene molecular ions in a time-resolved reaction microscopy experiment with a pair of few-cycle pulses. Vibrating bound acetylene cations or dissociating dications are produced by the first pulse. The second pulse probes the nuclear dynamics by ionization to higher charge states and Coulomb explosion of the molecule. For the bound cations, we observed vibrations in acetylene (HCCH) and its isomer vinylidene (CCHH) along the CC-bond with a periodicity of around 26 fs. For dissociating dication molecules, a clear indication of enhanced ionization is found to occur along the CH- and CC-bonds after 10 fs to 40 fs. The time-dependent ionization processes are simulated using semi-classical on-the-fly dynamics revealing the underling mechanisms.
Highlights
The ionization of molecules with intense laser pulses typically results in the vibrational and electronic excitation of the molecule
The process of double ionization via enhanced ionization (EI) can be understood for the diatomic case as follows:4 First, a diatomic potential well, with an inter-nuclear distance of r being the equilibrium distance Req, is tilted by a strong laser field such that an electron can tunnel through the outer barrier into the continuum, cf., Fig. 1(a)
We focus on the observation of fragments from the quadruply charged instead of a triple charged molecule, since it enables a better probe for the dissociative dynamics on the dication excited state due to its steeper potential and stronger energy difference as a function of bond length
Summary
The ionization of molecules with intense laser pulses typically results in the vibrational and electronic excitation of the molecule. The process of double ionization via EI can be understood for the diatomic case as follows (see Fig. 1): First, a diatomic potential well, with an inter-nuclear distance of r being the equilibrium distance Req, is tilted by a strong laser field such that an electron can tunnel through the outer barrier into the continuum, cf., Fig. 1(a). When the inter-nuclear distance increases further, the inner barrier becomes larger and enhanced ionization ceases, cf., Fig. 1(c). This simple picture, while illustrative, may be incomplete for the case of polyatomic molecules where multiple bonds are present. In 1995, Zuo and Bandrauk theoretically predicted enhanced ionization in H2þ ions for a charge resonance between two strongly coupled states As this process depends on the internuclear distance, which may change following the first laser-molecule interaction, its temporal evolution is of particular interest. Wu et al verified that in the process of EI the potentially high-lying nucleus is ionized using circularly polarized pulses. Theoretical calculations were able to reproduce EI both in simple hydrogen and in more complex acetylene showing the importance of two or more interacting states where charge-resonance enhanced ionization (CREI) can occur
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