No abstract

Using two different crossed-beams high-resolution electron attachment instruments (employing either a trochoidal electron monochromator or a hemispherical electron monochromator) we have determined the cross section curve for H − production from H 2 via the 4 eV resonance at two different temperatures. These relative partial cross sections have been calibrated by comparing present values for the 14 eV resonance with absolute total cross sections determined previously. Taking into account the experimental energy distribution and the rotational excitation and its influence on the cross section shape we obtain very good agreement with theoretical predictions in terms of both the shape and magnitude of this resonance peak.

Electron monochromators which are operated within an axial magnetic (guiding) field are especially suitable for the production of monochromatic electrons at low energies. Although in principle the technology of such devices has an appreciable historic background, we have discovered experimentally important new features, which cannot be understood using the previously published theories of operation. An in-depth study of the electron trajectories in a crossed electric and magnetic field using Simion1 showed a number of possible pitfalls, which have to be avoided in construction and operation. From our simulations we derived a novel design and operational method, which is currently under evaluation. We have already demonstrated that using this novel design an electron energy resolution of about 50 meV is realistic.

We have studied negative ion formation following low-energy electron impact (0-10 eV) to OClO including a time-of-flight analysis of the ions. The dominant feature is formation of a transient negative ion (OClO − * ) by resonant electron capture in the vicinity of 0.7 eV which decomposes into Cl − , O − , ClO − and O − 2 . The total dissociative attachment (DA) cross section at the resonance maximum (0.7 eV) is near 10 −15 cm 2 with ClO − +O the dominant channel. Although the DA reaction O 2 + Cl − is exothermic by nearly 4 eV the Cl − ion appears with a mean kinetic energy of only 0.4 ± 0.1 eV indicating effective vibrational excitation of neutral O 2 . Also observed at higher target gas pressures is the parent anion ClO − 2 which arises from an effective charge transfer process involving a metastable complex formed in the collision between the dominant DA product ClO − and the sample molecule OClO. Possible implications of these results for the heterogeneous chemistry of OClO on polar stratospheric cloud particles are considered.

No abstract