Many Particle Fragmentation Dynamics of Polyatomic Hydrocarbon Molecules Li Zhang 1 , S. Roither 1 , D. Kartashov 1 , M. Schöffler 2 , Xinhua Xie 1 , M. Lezius 3 , R. Dörner 2 , A. Baltuska 1 , and M. Kitzler 1 1 Photonics Institute, Vienna University of Technology, Gusshausstrasse 27/387, A-1040, Vienna, Austria 2 Institute for Nuclear Physics, Johann Wolfgang Goethe University, Max-von-Laue-Strasse, D-60438, Frankfurt/Main, Germany 3 Max-Planck-Institute for Quantum Optics, Hans-Kopfermnn-Strasse 1, D-85748 Garching, Germany E-mail: li.zhang@tuwien.ac.at , markus.kitzler@tuwien.ac.at Abstract: Laser induced Coulomb explosion of 1,3-Butadiene is investated by the COLTRIMS technique. The main fragmentation channels are identified and their momentum distributions are characterized. Remarkably high proton energies are measured and their origin is investigated. Keywords: molecular fragmentation dynamics, laser induced Coulomb explosion imaging 1. INTRODUCTION Bond-breaking and bond-creation in chemical reactions are preceded and ultimately governed by the intra-molecular dynamics of the electrons. While many concepts of the interaction of strong, short laser pulses with molecules can be studied using the smallest molecule, H 2 , it has been shown that the laser-driven electrons’ dynamics in the more chemically (and biologically) relevant polyatomic molecules can lead to remarkable fragmentation behaviour, e.g. [1-3]. Fig. 1: Structure of 1,3-Butadiene. The two dominant fragmentation channels of doubly charged Butadiene, A and B, are indicated. The hydrogen migration, that is identified from our data, is depicted by arrows Laser induced molecular fragmentation via Coulomb explosion has been studied by coincidence momentum imaging methods, e.g. [4-6], which allow to selectively investigate specific breakup channels. Such studies revealed that for many molecules the Coulomb explosion is accompanied by remarkably fast hydrogen and/or proton migration, e.g. [4,5]. Here we investigate laser-induced fragmentation of a highly polarisable polyatomic hydrocarbon molecule, 1,3-Butadiene, C 4 H 6 . see Fig. 1. In the coincidence analysis of the resulting fragments we focus on two types of fragmentation pathways: (i) Coulomb explosion of doubly charged Butadiene into 2 fragments, and (ii) Coulomb explosion of higher charge states into several fragments featuring the ejection of protons with remarkably high kinetic energies. For route (i) we identify and characterize 2 dominant fragmentation pathways, one of them involving hydrogen migration prior to the Coulomb explosion. Route (ii) is motivated by the appearance of a very broad proton band in the time-of-flight spectrum of Butadiene. For larger molecules (C 14 H 10 ) it has been shown [1] that the protons during Coulomb explosion can acquire remarkably high kinetic energies. This was attributed to a nonadiabatic charge localization effect, whereby the electrons are pushed to one side of the molecule by the strong laser field, which exposes the outgoing proton to large electric charge for time periods much longer than a laser cycle [1]. By using our coincidence data in combination with search algorithms, a technique that was also applied in [6], we unambiguously identify those multi-particle fragmentation channels which result in high energy protons. From the momentum vectors of all involved fragments we reconstruct their relative angles and kinetic energies. 2. EXPERIMENT We use cold target recoil-ion momentum spectroscopy (COLTRIMS) to record the three-dimensional momentum distributions of all resulting fragments from a single molecule in coincidence. Additionally we record the emitted electrons within a solid angle of 4 From the fragments’ momentum vectors we can unambiguously identify different fragmentation pathways. Linearly and circularly polarized 27fs (FWHM) laser pulses from a Ti:sapphire laser system with peak intensities of 2×10 14 and 5×10 14 W/cm 2 , respectively, are applied onto a cold, thin jet of 1,3-Butadiene molecules. From the time-of-flight and the position of impact on the position