Hydrogen migration and C–C bond breaking in 1,3-butadiene in intense laser fields studied by coincidence momentum imaging Huailiang Xu a , Tomoya Okino a , Katsunori Nakai a , Kaoru Yamanouchi a, * , Stefan Roither b , Xinhua Xie b , Daniil Kartashov b , Markus Schöffler c , Andrius Baltuska b , Markus Kitzler b a Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan b Photonics Institute, Vienna University of Technology, Gusshausstrasse 27, A-1040 Vienna, Austria c Institut für Kernphysik, J.W. Goethe Universität Frankfurt, Max-von-Laue-Strasse 1, 60438 Frankfurt am Main, Germany article info Article history: Received 23 September 2009 In final form 3 November 2009 Available online xxxx abstract Two-body dissociation processes of H 2 C@CH–CH@CH 2þ 2 induced by an intense laser field were investi- gated by the coincidence momentum imaging method. Four dissociation pathways, C 4 H 2þ 6 ? CH 2 + + C 3 H þ 4 ,C 4 H 2þ 6 ? CH 3 + +C 3 H þ 3 ,C 4 H 2þ 6 ? C 2 H þ 3 +C 2 H þ 3 and C 4 H 2þ 6 ? C 2 H þ 2 +C 2 H þ 4 , were identified. The exis- tence of the second and fourth pathways can be regarded as evidences of the chemical bond rearrange- ment processes associated with hydrogen migration in the intense laser field. It was found that the hydrogen atom bonded originally to one of the two central carbon atoms migrates preferentially to its neighboring terminal carbon atom site. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction During the last several years, ultrafast hydrogen migration pro- cesses within a molecule in intense laser fields have been investi- gated extensively, not only because of the findings that hydrogen atoms (or protons) move extremely rapidly within a molecule, but also their potential applicability for controlling chemical bond breaking processes [1–9]. The hydrogen migration process, in which hydrogen atom(s) or proton(s) migrate from one site to an- other within a molecule, can induce large scale deformation of molecular skeletal structure and rearrangement of chemical bonds [10–12]. Such processes induced by intense laser fields have been demonstrated so far in various hydrocarbon molecules. For exam- ple, in the case of methanol (CH 3 OH), the observation of singly charged OH þ 2 , OH þ 3 , and CH þ 4 , generated through two-body Cou- lomb explosion of CH 3 OH 2+ , confirmed that hydrogen atom(s) or proton(s) migrate between the methyl group and the hydroxyl group induced by an intense laser field (10 14 W/cm 2 , 800 nm, 60 fs) [13]. In the present study, we investigate hydrogen migration pro- cesses in 1,3-butadiene (CH 2 @CH–CH@CH 2 ) induced by an ultra- short intense laser field by detecting singly charged fragment ions generated through two-body fragmentation processes of dou- bly charged 1,3-butadiene using the coincidence momentum imag- ing (CMI) method [1]. The simple conjugated diene, 1,3-butadiene, is one of the simplest hydrocarbon molecules containing both sin- gle and double C–C bonds and could offer a possibility of studying the effect of the changes in the bond-order associated with the migration of hydrogen atoms (and/or protons). By the CMI method all the fragment ions originating from the dissociation of a single parent ion can be detected in coincidence, so that the decomposition pathways can be identified uniquely. Two types of two-body fragmentation pathways from the doubly charged parent ion were unambiguously identified: (i) two path- ways in which one of the two side C–C bonds is broken, C 4 H 2þ 6 ! CH þ m þ C 3 H þ 6m ðm ¼ 2; 3Þ; ð1Þ and (ii) two pathways in which the central C–C bond is broken, C 4 H 2þ 6 ! C 2 H þ n þ C 2 H þ 6n ðn ¼ 2; 3Þ: ð2Þ The existence of the m = 3 and n = 2 pathways proves that a hydro- gen atom (or a proton) migrates prior to the C–C bond breaking. From the relative yields of the four fragmentation pathways and the distributions of the momentum vectors of the singly charged fragment ions, we discuss the fragmentation processes of doubly charged 1,3-butadiene molecules induced by an intense laser field. 2. Experiment The experiments were carried out both at The University of Tokyo and Vienna University of Technology. The experimental apparatus used in Tokyo has been described elsewhere in detail [12,14]. Briefly, a Ti:Sapphire chirped pulse amplification (CPA) laser system (Pulsar 5000, Amplitude Technologies) was used to generate laser pulses with a duration of 40 fs, a repetition rate of 5 kHz, and a center wavelength of 795 nm. The laser pulses 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.11.008 * Corresponding author. Fax: +81 3 5689 7347. E-mail address: kaoru@chem.s.u-tokyo.ac.jp (K. Yamanouchi). Chemical Physics Letters xxx (2009) xxx–xxx Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett ARTICLE IN PRESS Please cite this article in press as: H. Xu et al., Chem. Phys. Lett. (2009), doi:10.1016/j.cplett.2009.11.008