Hyperthermal molecular beam scattering: K–O2 ion/neutral product angular, energy, and branching ratio analysis

Publisher Website
Google Scholar
Abstract
Angular distributions of K+ ions and K atoms from collisions of a beam of hyperthermal K atoms with a cross beam of thermal O2 molecules were determined in the range from 18 to 38 eV (lab). A pronounced rainbow was observed in the ion‐pair channel at a reduced angle of 240 eV deg. A smaller rainbow was also found in the neutral channel at 220 eV deg. Energy loss distributions for both product species were also determined at 28 eV (lab) by time‐of‐flight measurements. Both K+ ions and K atoms from K+O2 were detected concurrently. A pseudorandom pulsing method was used to increase the duty cycle. The TOF spectra indicated overlapping energy‐loss distributions corresponding to ground and excited state formation for both the neutral and ion products. Excited state to ground state branching ratios for both the neutral and ion products were determined as a function of reduced angle. The neutral branching ratio shows a pronounced peak at ∼180 eV deg., while the ion ratio increases rapidly above 200 eV deg. From these ratios the differential cross sections were calculated for neutral K in which either K or O2 is electronically excited. These cross sections are similar to the differential cross sections of K+ ions. The neutral to ion branching ratios, also determined, exhibit a broad minimum in the vicinity of the rainbow angle. Differential cross sections for neutral and ion scattering were integrated to give the total neutral/ion branching ratio of ∼4. A preliminary analysis of the experimental differential cross sections has been performed via an atom–atom model. Although the model has substantial and expected deficiencies, the analysis strongly indicates that the quartet surfaces emanating from the neutral ground electronic asymptote must be substantially more repulsive than the corresponding doublet surface. An analysis of the low velocity features in the TOF measurements of K indicates production of the lowest electronically excited state of O2 (i.e., 1Δg) as well as electronically excited states of K. A similar analysis of the TOF spectra of K+ suggests the occurrence of autoionization from the neutral ground state surfaces to the K+(1S0)+O2(1Δg) surface.