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J. Phys. Chem. A 2010, 114, 4870–4874
Fragmentation Channels in Dissociative Electron Recombination with Hydronium and Other Astrophysically Important Species† Oldrˇich Novotny´,*,‡,§ Henrik Buhr,‡,| Julia Stu¨tzel,‡ Mario B. Mendes,‡ Max H. Berg,‡ Dennis Bing,‡ Michael Froese,‡ Manfred Grieser,‡ Oded Heber,| Brandon Jordon-Thaden,‡ Claude Krantz,‡ Michael Lange,‡ Michael Lestinsky,‡,§ Steffen Novotny,‡,⊥ Sebastian Menk,‡ Dmitry A. Orlov,‡ Annemieke Petrignani,‡ Michael L. Rappaport,| Andrey Shornikov,‡ Dirk Schwalm,|,‡ Daniel Zajfman,| and Andreas Wolf‡ Max-Planck-Institut fu¨r Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany, Columbia Astrophysics Laboratory, MC5247, 550 West 120th Street, New York, New York 10027, and Faculty of Physics, Weizmann Institute of Science, RehoVot 76100, Israel ReceiVed: October 31, 2009; ReVised Manuscript ReceiVed: December 17, 2009
We report on our recent studies of dissociative recombination (DR) employing two different fragment imaging detection techniques at the TSR storage ring in Heidelberg, Germany. Principles of an upgraded 3D optical system and the new energy-sensitive multistrip detector (EMU) are explained together with possible applications in reaction dynamics studies. With the EMU imaging detector we succeeded to observe the branching ratios after DR of deuterated hydronium ions D3O+ at energies of 0-0.5 and 4-21 eV. The branching ratios are almost constant at low energies while above 6 eV both oxygen-producing channels O + D + D + D and O + D2 + D strongly increase and dominate by about 85% at 11 eV. To demonstrate further capabilities of our fragment imaging detectors, we also summarize some of our additional recent studies on DR of molecular ions important for astrophysics as well as for fundamental unimolecular dynamics. Introduction Dissociative recombination (DR) of molecular ions with electrons is an essential process controlling the charge density and the chemical composition of the cold interstellar medium (ISM). Experimental data on DR and reliable predictions based on a good knowledge of the underlying quantum mechanisms are required in order to understand the chemical network in the ISM and related processes such as star formation from molecular clouds. Especially in cases of more complex molecular ions, not only the total reaction cross sections are needed but also the chemical composition and internal excitation states of the neutral products.1,2 The basic mechanism of DR3,4 of molecular ions is the resonant capture of an incident electron into the potential surface of a doubly excited state of the neutral molecule within the Franck-Condon region of the ion. Such states are usually dissociative, and their asymptotic energy lies below that of the ion, reflecting the energy gain in binding the free electron. The excess energy is stabilized by dissociation into neutral, often excited fragments. Measurement of the remaining relative fragment kinetic energy provides information on internal states of both the ion and the DR products. Merged beams in combination with the storage ring technique proved to be an ideal tool for investigating the DR of molecular ions at low energies. A large variety of ions were investigated over the past decade.3 For more and more complex ions, †
Part of the special section “30th Free Radical Symposium”. * To whom correspondence should be addressed, oldrich.novotny@ mpi-hd.mpg.de. ‡ Max-Planck-Institut fu¨r Kernphysik. § Columbia Astrophysics Laboratory. | Weizmann Institute of Science. ⊥ Current address: Wihuri Physical Laboratory, University of Turku, 20014 Turku, Finland.
fragment imaging detection techniques are needed in order to disentangle the complexity of the DR process. Position-sensitive fragment measurements offer a range of methods for distinguishing between reaction channels with respect to the number of fragments, the internal product states, and the chemical composition. Reaction geometries and their dependence on the collision angle can be also determined. This opens a wide range of possibilities for sensitively exploring the quantum dynamics in the excited molecular states involved in these reactions. In this paper we give an overview on the partly new, partly upgraded fragment imaging techniques installed at the TSR storage ring in Heidelberg, Germany, together with a brief summary of ongoing experiments on the DR of four astrophysically important molecular ions, CF+, HF+, HCNH+, and H3O+, where in the last cases deuterated isotopologues, DCND+ and D3O+, are used. Results of the fragmentation branching ratios for the DR of D3O+ ions, in particular at high collision energies, will be discussed in detail. Experimental Arrangement In the experiments reported here, the ion beam is produced in one of several ion sources, accelerated in an electrostatic accelerator, injected into the TSR5 heavy-ion storage ring, and stored in an orbit of 55.4 m circumference in a vacuum of 3 × 10-11 mbar. The ions can be further accelerated inside the ring up to the energy limited by its maximum magnetic rigidity, amounting to 3-5 MeV for ions being discussed in this contribution. In two straight sections the ion beam can be overlapped with an electron beam for about 1.5 m each and phase-space compression of the stored ions is applied by electron cooling for several seconds before the measurement, thus defining the ion beam properties. Electron cooling is a wellestablished technique used for cooling ion beams of low mass-
10.1021/jp9104097 2010 American Chemical Society Published on Web 01/14/2010
Dissociative Electron Recombination
Figure 1. Scheme of DR fragment imaging for two-body breakup. The velocity vector of the center of mass of the DR fragments remains unchanged after the dissociation. The relative distance of fragments after the flight time (TOF) reflects the kinetic energy released in the particular dissociation event.
to-charge ratio.6 At the TSR a photocathode-produced ultracold electron beam7,8 is employed, reaching transverse and longitudinal temperatures T⊥ near 1.0 meV and T|| ) 0.02 meV, respectively, in the co-moving reference frame. With this new device we succeded to perform phase-space compression also for beams of heavy singly charged molecular ions (∼30 amu) within only a few seconds,7,9 reaching a beam diameter of
