REVIEW OF SCIENTIFIC INSTRUMENTS 83, 023106 (2012)
A momentum imaging microscope for dissociative electron attachment H. Adaniya, D. S. Slaughter, T. Osipov, T. Weber, and A. Belkacema) Lawrence Berkeley National Laboratory, Chemical Sciences, Berkeley, California 94720, USA
(Received 1 November 2011; accepted 28 January 2012; published online 23 February 2012) We describe an experimental approach to image the three-dimensional (3D) momentum distribution of the negative ions arising from dissociative electron attachment (DEA). The experimental apparatus employs a low energy pulsed electron gun, an effusive gas source and a 4π solid-angle ion momentum imaging spectrometer consisting of a pulsed ion extraction field, an electrostatic lens, and a time- and position-sensitive detector. The time-of-flight and impact position of each negative ion are measured event by event in order to image the full 3D ion momentum sphere. The system performance is tested by measuring the anion momentum distributions from two DEA resonances, namely H− from H2 O− 2 (2 B1 ) and O− from O− 2 ( u ). The results are compared with existing experimental and theoretical data. © 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3685244] I. INTRODUCTION
Dissociative electron attachment (DEA) is one of the most fundamental electron-molecule collision processes, in which a low energy (∼15 eV or less) electron is captured in a molecule to form an unstable molecular anion (resonant state), which subsequently dissociates into a negative ion and neutral fragment(s).1–10 This reaction process is known to involve complicated dynamics of many particles11–20 due to the coupling of nuclear and electronic motion, thereby providing a unique challenge to theory and experiment. DEA is an important process in the field of gaseous electronics21 and in condensed matter, as shown by Sanche and coworkers22–24 and Illenberger and co-workers.25 They found that sub-excitation and sub-ionization electrons, which are created with high abundance when high-energy radiation interacts with soft matter, trigger single- and double-strand breaks of DNA molecules by DEA. More recently, DEA has found emerging applications in nano-fabrication.26, 27 II. APPARATUS A. Collision chamber
We have developed a sophisticated “DEA microscope,” to measure the three momentum components of the negative ion simultaneously, consisting of a pulsed electron beam, an effusive gas target, a pulsed extraction field, an ion optic lens, and a two-dimensional (2D) time- and position-sensitive detector. The DEA microscope enables a kinematically complete description of the two-body breakup process along with the capability to clearly discriminate between two-body and many-body dissociation. The full 4π solid angle detection of this technique eliminates the inversion algorithm required in velocity map imaging,28–30 and all ions are detected regardless of their breakup symmetry. For performance evaluation, the system was tested by the measurement of the O− ion momentum distribution from DEA to molecular oxygen and H− ion distribution from gaseous water via one of the three DEA a) Electronic mail:
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resonances. We have recently reported, elsewhere, detailed investigations of the three DEA resonances in water and the 8 eV Feshbach resonance in carbon dioxide, employing the technique presented here.19, 20, 31, 32 An effusive molecular target was introduced by a stainless steel capillary of 0.3 mm internal diameter and 20 mm length. The capillary was heated and maintained at a suitable temperature to reduce condensation of the target and any impurities on the internal surfaces. For targets with relatively low vapor pressure such as water, the sample reservoir was heated to achieve the desired driving pressure, which was typically a few Torr. A commercially acquired electron gun was employed to produce a pulsed low-energy electron beam. The electron beam was collimated to a spot of 1 to 2 mm diameter in a uniform magnetic field, of ∼25 G, which was generated over the electron beam and collision region by a pair of 0.75 m diameter Helmholtz coils driven by a currentstabilized dc power supply. The magnetic field prevented most of the scattered electrons from entering the spectrometer. Using capacitive pulsing of the electron gun grid electrode, 80 ns pulses with a rise/fall time of
