Threshold electron attachment and electron impact ionization involving oxygen dimers

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

“…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). The built-in stabilization capability of clusters was, e.g., demonstrated in a recent RET study involving oxygen monomers and dimers (Kreil et al, 1998): while O À 2 formation from O 2 molecules is very inefficient (and only possible through efficient postattachment interactions with the Rydberg ion core) (Walter et al, 1986;Harth et al, 1989) the rate coefficient for O À 2 formation from oxygen dimers was found to exceed that involving monomers by four orders of magnitude (Kreil et al, 1998).…”

Dissociative attachment and vibrational excitation in low-energy electron collisions with chlorine molecules

“…To our knowledge, no measurements of fragmentation of chlorine clusters upon electron impact ionization have been reported to date. In the lack of better information we use the known fragmentation behaviour of O 2 dimers [44] to estimate the fragmentation of Cl 2 dimers; note that both (O 2 ) 2 and (Cl 2 ) 2 are weakly bound van der Waals clusters. For 70 eV electron impact ionization of O 2 dimers, about 4% of the ionized products appear as O + 4 ions in the mass spectrum [44].…”

“…In the lack of better information we use the known fragmentation behaviour of O 2 dimers [44] to estimate the fragmentation of Cl 2 dimers; note that both (O 2 ) 2 and (Cl 2 ) 2 are weakly bound van der Waals clusters. For 70 eV electron impact ionization of O 2 dimers, about 4% of the ionized products appear as O + 4 ions in the mass spectrum [44]. Thus the measured Cl + 4 /Cl + 2 intensity ratio in figure 3(c) suggests a neutral chlorine dimer to monomer ratio of about 1%.…”

“…For clusters, the p-wave selectivity which holds for the Cl 2 molecule at low electron energies is no longer strict (see e.g. [44,55,56]). In such a case the RET signal close to threshold would have to be mirrored-in the spirit of the quasi-free electron model for s-wave attachmentby a non-zero free electron attachment signal close to threshold with a tendency to increase towards zero energy.…”

Clear Experimental Evidence forp-Wave Attachment-Threshold Behavior in Electron Attachment to Chlorine Molecules

“…2) for several nozzle temperatures and stagnation pressures. At p 0 1 bar and T 0 500 K (used for the attachment data shown below), the Cl 4 =Cl 2 intensity ratio amounted to 0.0004 from which we can infer (using the fragmentation pattern of O 2 dimers [38]) that the dimer density is only about 1% of the Cl 2 monomer density. In the absence of both lasers, the exothermic bimolecular reaction K 2 Cl 2 !…”