Ion-momentum imaging of resonant dissociative-electron-attachment dynamics in methanol

Ion Momentum Imaging of Dissociative Electron Attachment Dynamics in Methanol D.S. Slaughter, 1, ∗ D.J. Haxton, 1 H. Adaniya, 1 T. Weber, 1 T.N. Rescigno, 1 C.W. McCurdy, 1 and A. Belkacem 1 Lawrence Berkeley National Laboratory, Chemical Sciences, Berkeley, California 94720,USA A combined experimental and theoretical investigation of the dissociative electron attachment (DEA) dynamics in methanol are presented for the Feshbach resonance at 6.5 eV incident electron energy. Highly-differential laboratory-frame momentum distributions have been measured for each fragmentation channel using a DEA reaction microscope. These measurements are combined with calculations of the molecular frame electron attachment probability in order to investigate the dy- namics of the dissociating methanol transient negative anion. In contrast to previous comparisons between water and methanol [1, 2] we find subtle differences in the dissociation dynamics of the two fragment channels that are direct evidence of planar symmetry-breaking of warm methanol in its electronic ground state. We also find that the DEA fragmentation does not strictly follow the axial recoil approximation and we describe the dynamics that enable an accurate prediction of the fragment angular distributions. PACS numbers: 34.80.Ht I. INTRODUCTION Low-energy free electrons are widely considered to play an important role in the radiation-induced chemistry of bio-molecules [3] and organic chemistry in the interstellar medium [4]. Dissociative electron attachment (DEA) is one of the primary fundamental interactions that drives free electron chemistry, and attracts considerable inter- est, not only for the need to understand electron-induced molecular breakup and negative ion production in nature, but also in systems of technological interest [5]. Both gaseous methanol and its ice are ubiquitous in interstellar clouds and comets, but the mechanisms of methanol synthesis in these systems are not well under- stood, although low-energy electrons are likely to play an important role [6, 7]. Free-electron driven chemistry also provides many reaction pathways leading to more complex molecules of which methanol may be an inter- mediate [4]. On Earth, methanol has widely been pro- posed as a potential large-scale alternative to fossil fuels, as it can be produced both agriculturally and syntheti- cally. Today, industrial production of methanol is typi- cally achieved by reaction of synthesis gas (mixtures of H 2 , CO 2 and CO) in the presence of a Cu/Zn/Al 2 O 3 cat- alyst at high temperatures and pressures [8]. A similar scheme has been proposed for storage of CO 2 and elec- trical energy [9] and it is possible that DEA or the time- reverse process, associative detachment, play important roles in these industrial applications. Several experimental studies have been conducted on DEA to methanol in the past, including measurements of the translational kinetic energy release (KER) [1, 10] and velocity slice imaging [11] of the dissociation prod- ucts. K¨ uhn et al. [10] found evidence of hydrogen scram- bling in the production of OH − in the 10.5 eV resonance in their measurements of DEA relative cross sections and DSSlaughter@lbl.gov KER spectra in methanol over electron energies spanning 0 eV to 17 eV. Curtis and Walker [1] extended the study to the H − ion channel, making comparisons between the three Feshbach resonances in methanol and the Fesh- bach resonances in the relatively well-understood water molecule,[1, 11–20] in addition to comparing the DEA spectra to the corresponding parent Rydberg states in both VUV photo-absorption spectra and near-threshold electron energy loss spectra. They identified each of the first two Feshbach resonances to be due to promotion of an electron from either of the highest lying occupied or- bitals, (7a ) 2 or (2a ) 2 , of methanol in its ground state, having planar (C s ) symmetry, and simultaneous elec- tron attachment to the 3s Rydberg orbital; the 6.5 eV and 8 eV resonances being 2 A [(2a ) 1 (3s) 2 ] and 2 A [(7a ) 1 (3s) 2 ], respectively. More recently, Ib˘anescu et al. [21] provided further insight on these resonances by comparing their high- precision measurements of DEA ion yields, photoelectron spectra and vibrational excitation cross-sections from several alcohols, characterizing a previously unknown shape resonance at around 3 eV. Ib˘anescu and Allan [22] followed this with a time-dependent density functional theory description of the dynamics of the first two Ry- dberg states in methanol, predicting that the dynamics of dissociation of the 1 2 A Feshbach resonance in the methanol anion would follow the dynamics of the parent Rydberg excited state [23] of the molecule. Prediction and measurement of the dynamics of the methanol anion dissociation processes are formidable challenges from both theoretical and experimental per- spectives. In general, detailed measurement and analysis of the dynamics of DEA to polyatomic molecules have, until recently, been limited to small molecules containing only a few atoms. Our understanding of multidimen- sional dynamics resulting from DEA are hindered by the few elements of symmetry and many degrees of freedom that exist in larger polyatomics. Methanol is one of the simplest systems that possesses relatively weak planar symmetry and many vibrational modes, presenting an ac