Photochemically Induced Electronic-To-Electronic Energy Transfer in Geminate CO···O van der Waals Pair Generated through Vacuum Ultraviolet Photolysis of CO₂ in Ar Matrices

Abstract

An intracage geminate CO···O van der Waals pair is generated through vacuum ultraviolet (VUV) photolysis of 0.1% CO2 in Ar matrices at 18 K. Excitation of CO between 157 and 130 nm (AΠ ← X1Σ+ vibronic transitions) results in a strong emission due to the 1S → 1D transition of O at 560 nm and weak emission bands due to CO and O2. The band at 560 nm is absent in the emission spectra measured from CO in Ar (1:1000) or O2 and CO in Ar (1:1:1000) matrices. The observed phenomenon is due to photochemically induced electronic-to-electronic energy transfer. Excited CO and ground-state O(3P) undergo chemical recombination to result in excited CO2, which dissociates to ground-state CO (X1Σ+) and excited O(1S). Other possible pathways are also discussed but shown to be inappropriate.

Wavelength resolved UV photodesorption and photochemistry of CO2ice

“…46,47 The formation of excited O( 1 S) atoms resulting from reactive processes following UV excitation of CO (CO* + O / CO * 2 / CO + O*) have been identied in Ar cryogenic matrices experiments. 48,49 In the solid phase, however, the photodecomposition efficiency is expected to be strongly reduced with respect to the gas phase. This is due either to possible reactive collisions between O and CO fragments on the surface or within the ice matrix, forming CO 2 , or to the electronic quenching of the electronic excited states producing vibrationally hot CO 2 molecules.…”

Molecular Photodynamics in Rare Gas Solids

“…(c) Excitation of guest−host intermolecular transitions, which in the gas phase would correspond to excitation of collision complexes, is a natural consideration in condensed media. Perhaps the best characterized in this family are harpoon reactions initiated by excitation to guest−host charge-transfer potentials. ,, Optical access of intermolecular potentials can also occur among geminate guests, a dissected example being that of CO and O …”

Electron stimulated desorption from solid CO2 and a hydrogen containing CO2 matrix: A comparative study

“…Under nonresonant conditions the electronic energy transfer ͑EET͒ could go via two different routes, via photophysical EET, where excess energy is dissipated to the surrounding ''bath'' of vibrations in a condensed medium, or it can occur through the formation of a chemically bound state and this dissipation process involves electronic, vibrational, and rotational EET. 33 Unfortunately our work does not provide a clear answer to such questions and, hence, further studies are required to clarify these issues.…”

“…This would mean that CO and O form a van der Waals pair, as already recently has been suggested in photolysis experiments. 33 The other possibility would be that the CO ϩ ion suffers postdissociative collisions and dissipates part of its energy to the phonon excitations of the lattice of the CO 2 matrix. Both mechanisms imply that the ion yield will depend on the temperature of the solid because the cross section is expected to vary also as a function of the bend angle in the van der Waals complex ͓the temperature dependence of the branching ratio and the rate constant has been studied so far, for the reaction of O ϩ ( 4 S) with CO 2 see work by Viggiano 34 …”

“…What is the real nature of the transient states of CO 2 in the matrix, i.e., are the neutral fragments CO* and O able to diffuse freely within the matrix and thus lose their kinetic energy on vibrational-rotational excitations of neighboring CO 2 molecules, in order to live for such a long time ͑i.e., the order of second͒ in the matrix, or is a van der Waals complex CO*¯O formed, as it has been recently suggested in Ar matrix experiments? 33 The need for answers to these questions highlights the necessity for further detailed experiments on this and related systems.…”