Field ionization and Coulomb explosion of small hydrocarbon molecules driven by intense laser pulses are studied in a combined theoretical and experimental framework. The spectra of ejected protons calculated by the time-dependent density functional approach are in good agreement with the experimental data. The results of the simulations give detailed insight into the correlated electron and nuclear dynamics and complement the experiment with a time-dependent physical picture. It is demonstrated that the Coulomb explosion in the studied molecular systems is a sudden, all-at-once fragmentation where the ionization step is followed by a simultaneous ejection of the charged fragments. [7], and Coulomb explosion [8,9]. Highly energetic dissociation of a molecule or solid due to multiple ionization, known as the Coulomb explosion, is a particularly interesting process because it exposes the key physical mechanisms associated with the electron and nuclear dynamics and ionization [9][10][11][12][13]. Once the laser strips electrons from the molecule, the remaining positively charged structure can explode, creating a molecular plasma cloud. Coulomb explosion can be used to generate bright keV x-ray photons [14,15], highly energetic electrons [16], and for imaging [17][18][19].The strong-field ionization and fragmentation of hydrocarbon molecules is a prototypical example of the Coulomb explosion of polyatomic molecules, and it has been the subject of several experiments [9,11,[20][21][22][23]]. An important quantity measured in these experiments is the kinetic energy distribution of the protons ejected during the fragmentation. Proton energies in excess of 30 eV at only very moderate peak intensities of the driving laser pulses have been reported for both large [11] and small hydrocarbon molecules [9]. The proton kinetic cutoff energies depend sensitively on the laser intensity and saturate at intensities that depend on the molecular species [9,11]. Earlier, this behavior was attributed to the creation of a long-lived charge localization state [11]. A more recent experiment [9], in contrast, suggested that the high kinetic proton energies originate from Coulomb explosions from a high molecular charge state. It was suggested [9] that a multibond version of the enhanced ionization process [24,25] is responsible for reaching the observed high charge states, from which the protons are ejected simultaneously in a concerted Coulomb explosion process resulting in complete molecular fragmentations. A recent theoretical study [26] using a one-dimensional model of acetylene, C 2 H 2 , confirms the proposed ionization mechanism leading to the high charge states, but it did not investigate the fragmentation dynamics. While the experimental approaches allow the study of important aspects of the Coulomb explosion of molecules by analyzing the properties of the resulting fragments, they do not provide a complete, dynamical description of the internal mechanism taking place during the fragmentation.In this work, the intense laser puls...

Abstract. Strong-field control of acetylene fragmentation by fully determined fewcycle laser pulses is demonstrated. The control mechanism is shown to be based on electron recollision and inelastic ionization from inner-valence molecular orbitals.Chemistry is usually perceived as breaking and making molecular bonds. These processes that typically occur on the timescale of tens of femtoseconds (fs) to nanoseconds are preceded and ultimately governed by the much faster intra-molecular motion of electrons that proceeds on the sub-femtosecond timescale [1,2]. This timescale matches the light oscillations of laser pulses carried at frequencies in the visible and near-infrared. As the electric field of strong laser pulses is capable of exerting forces onto the electrons that are comparable to those of the binding forces, it becomes possible to drive the intra-molecular electron density by light. Deterministic steering, however, requires fully controlled laser electric fields, for example few-cycle pulses with a locked carrier-envelope (CE) offset frequency. Using such pulses it has been shown that the localization of an electron during dissociation of D + 2 [3, 4] and CO + [5], as well as the directionality of multiple dissociative channels of CO [6] can be controlled.In this submission we extend this scheme from simple diatomics studied so far to chemically more relevant polyatomic molecules. Particularly interesting for applications are hydrocarbon molecules. It has been shown previously that dissociation of these molecules is accompanied by extraordinarily rich internal electronic dynamics [7,8] that are still far from being understood.We study the fragmentation of acetylene, C 2 H 2 , subject to fully determined few-cycle laser electric fields. In our experiments we generate 4.5 fs laser pulses by spectral broadening of ≈ 25 fs laser pulses from a Titanium-Sapphire laser amplifier system in a 1 m long hollow-core glass capillary filled with argon and subsequent recompression by several bounces from chirped mirrors. The pulses, with a spectrum centered around 750 nm are directed into an ultrahigh vacuum chamber where they are focused onto a cold supersonic jet of randomly aligned acetylene molecules. The three-dimensional momenta of the resulting ionic fragments from a single molecule are recorded as described previously [9]. As at maximum only one molecule interacts with one laser pulse it becomes possible to measure the duration and the carrier-envelope (CE) phase of each of the few-cycle pulses delivered at the repetition rate of 5 kHz on a single-shot basis [10][11][12] instead of actively stabilizing it, leading to improved accuracy. The peak intensity of the laser pulses on target was 1.5×10 14 W/cm 2 , as determined from a separate calibration measurement using single ionization of argon atoms in circularly polarized light [13]. a

Abstract. Field ionization of hydrocarbon molecules to high charge states is studied as a function of laser pulse duration, peak intensity and molecular alignment. Results are in agreement with the recently proposed mechanism of multi-bond enhanced ionization.