Relative efficiencies of O(1S) production from photodissociation of Co2 between 1080 and 1160 Å

Temperature dependence of the photodissociation of CO2 from high vibrational levels: 205-230 nm imaging studies of CO(X1Σ+) and O(3P, 1D) products

The 205-230 nm photodissociation of vibrationally excited CO₂ at temperatures up to 1800 K was studied using Resonance Enhanced Multiphoton Ionization (REMPI) and time-sliced Velocity Map Imaging (VMI). CO₂ molecules seeded in He were heated in an SiC tube attached to a pulsed valve and supersonically expanded to create a molecular beam of rotationally cooled but vibrationally hot CO₂. Photodissociation was observed from vibrationally excited CO₂ with internal energies up to about 20 000 cm^-1, and CO(X¹Σ⁺), O(³P), and O(¹D) products were detected by REMPI. The large enhancement in the absorption cross section with increasing CO₂ vibrational excitation made this investigation feasible. The internal energies of heated CO₂ molecules that absorbed 230 nm radiation were estimated from the kinetic energy release (KER) distributions of CO(X¹Σ⁺) products in v″ = 0. At 230 nm, CO₂ needs to have at least 4000 cm^-1 of rovibrational energy to absorb the UV radiation and produce CO(X¹Σ⁺) + O(³P). CO₂ internal energies in excess of 16 000 cm^-1 were confirmed by observing O(¹D) products. It is likely that initial absorption from levels with high bending excitation accesses both the A¹B₂ and B¹A₂ states, explaining the nearly isotropic angular distributions of the products. CO(X¹Σ⁺) product internal energies were estimated from REMPI spectroscopy, and the KER distributions of the CO(X¹Σ⁺), O(³P), and O(¹D) products were obtained by VMI. The CO product internal energy distributions change with increasing CO₂ temperature, suggesting that more than one dynamical pathway is involved when the internal energy of CO₂ (and the corresponding available energy) increases. The KER distributions of O(¹D) and O(³P) show broad internal energy distributions in the CO(X¹Σ⁺) cofragment, extending up to the maximum allowed by energy but peaking at low KER values. Although not all the observations can be explained at this time, with the aid of available theoretical studies of CO₂ VUV photodissociation and O + CO recombination, it is proposed that following UV absorption, the two lowest lying triplet states, a³B₂ and b³A₂, and the ground electronic state are involved in the dynamical pathways that lead to product formation.

Low-energy electron scattering cross section for the production of CO within solid films of carbon dioxide

We report absolute electron scattering cross sections sigma(p) for the production of CO within thin solid film of carbon dioxide (CO(2)) condensed on a solid Ar substrate. The CO fragments, which remain trapped within the bulk of the carbon dioxide film, are detected in situ by recording energy losses to their lowest triplet electronic state a (3)Pi using high-resolution electron-energy-loss spectroscopy. The production of CO is studied as a function of the electron exposure, film thickness, and incident electron energy between 2 and 30 eV, a range within which most of the secondary electrons are created in systems irradiated by high-energy particles. The energy dependence is characterized by a feature around 4 eV with sigma(p)=(7.0+/-4.0)x10(-18) cm(2), a minimum around 7 eV, a strong rise up to a large and broad maximum around 15 eV with sigma(p)=(5.4+/-2.5)x10(-17) cm(2), a decrease to a minimum around 18.5 eV, and finally a monotonous increase up to 30 eV. The CO production is discussed in terms of the formation of electron resonances or transient anion states, which may lead directly to the fragmentation of the molecule via dissociative electron attachment or indirectly by decaying into an entirely repulsive part of the corresponding excited neutral and positive ion states.