High-resolution studies of electron scattering by molecular oxygen

Abstract

In a new experimental system synchrotron radiation from the SRS (Daresbury, UK) has been used to photoionise Ar at threshold and to produce an electron beam with an energy resolution of about 3.5 meV (FWHM). The authors have observed the variation of the total scattering cross section of O2 in a supersonic beam over the energy range 0.1-1.3 eV. Rotational effects are strongly apparent and the results suggest that there are important contributions to the scattering from the higher partial waves l=4 and 6 as well as l=2. This apparatus shows the potential to open up the field of electron-molecule scattering at rovibrational resolution to a large variety of molecules.

Resonance and Threshold Phenomena in Low-Energy Electron Collisions with Molecules and Clusters

“…Using optimized conventional electron sources, Ibach's group (1991Ibach's group ( , 1993) have achieved energy widths down to 1 meV in electron scattering from molecules adsorbed at surfaces under ultrahigh vacuum and low current conditions. Photoelectron sources have been used for angle differential gas phase scattering experiments by Gallagher and coworkers (van Brunt and Gallagher 1978;Kennerly et al, 1981), Field et al (1988Field et al ( , 1991a and, more recently, by Gopalan et al (2003). In order to avoid the Doppler effect, differentially-pumped supersonic beam targets were used (see, e.g., Go¨tte et al, 2000).…”

“…The center-of-gravity of the fine-structure split v 0 ¼ 4 anion state is located 0.09 eV above the neutral ground state of O 2 (X, v 00 ¼ 0) (Land and Raith, 1974), and the states with v 0 ! 4 are thus subject to autodetachment with decay widths G v 0 (E), rising strongly (/ E 5/2 ) with electron energy E, as characteristic for a d g -wave shape resonance for the electron-O 2 system; for v 0 ¼ 6, the width is about 1 meV (Field et al, 1988;Allan, 1995;Higgins et al, 1995). Correspondingly, the formation of long-lived oxygen anions in electron collisions at sub-eV energies requires rapid collisional stabilization of the temporary O À 2 anion, as possible in high-pressure media (Bloch and Bradbury, 1935;Christophorou, 1978;Hatano and Shimamori, 1981) or for oxygen bound in aggregates (homogeneous or heterogeneous clusters) (Ma¨rk, 1991;Illenberger, 1992;Hatano, 1997).…”

Attachment of Electrons to Molecules at meV Resolution

“…More recently, quasi-monoenergetic electron sources, based on controlled photoelectron production, were developed for use in electron scattering (Kennerly et al 1981;Field et al 1988Field et al , 1991 and in electron attachment (Ajello and Chutjian 1979;Chutjian et al 1984Chutjian et al , 1985. In a basic paper, Gallagher and York (1974) addressed the relevant aspects of such sources and described a cw laser photoionisation scheme involving metastable Ba* atoms to generate monoenergetic electrons of fixed low energy (17 meV) at high brightness.…”

“…In a basic paper, Gallagher and York (1974) addressed the relevant aspects of such sources and described a cw laser photoionisation scheme involving metastable Ba* atoms to generate monoenergetic electrons of fixed low energy (17 meV) at high brightness. More recently, Field et al (1988Field et al ( , 1991 used monochromatised synchrotron radiation to photoionise Ar at the (l1s', J=l) autoionisation resonance and thereby produced very slow electrons (~4 me V), which were efficiently drawn out with a weak electric field, accelerated and used for elastic and inelastic electron scattering studies from a nozzle target beam at a typical resolution around 4 meV.…”

Electron Scattering with Photoelectrons

“…Even then rotational spacings are of the order of meV's or less and it becomes apparent that what is required is a source of electrons with energy width of a few meV (and no worse) and a kinetic energy in the hundreds of meV range. This has proved rather a tall order but we have been fortunate enough to approach it in recent experiments [1,2]. Another requirement is that the current in the electron beam be at least a good fraction of a picoamp, at any rate for crossed beam experiments.…”