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Isotope effect in the photochemical decomposition of CO2
(ice) by Lyman-α radiation
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The photochemical decomposition of CO2 (ice) at 75 K by Lyman-α radiation (10.2 eV) has been studied using transmission infrared spectroscopy. An isotope effect in the decomposition of the CO2 molecule in the ice has been discovered, favoring 12CO2 photodecomposition over 13CO2 by about 10%. The effect is caused by electronic energy transfer from the excited CO2 molecule to the ice matrix, which favors quenching of the heavier electronically-excited 13CO2 molecule over 12CO2. The effect is similar to the Menzel-Gomer-Redhead isotope effect in desorption from adsorbed molecules on surfaces when electronically excited. An enhancement of the rate of formation of lattice-trapped CO and CO3 species is observed for the photolysis of the 12CO2 molecule compared to the 13CO2 molecule in the ice. Only 0.5% of the primary photoexcitation results in O–CO bond dissociation to produce trapped-CO and trapped-CO3 product molecules and the majority of the electronically-excited CO2 molecules return to the ground state. Here either vibrational relaxation occurs (majority process) or desorption of CO2 occurs (minority process) from highly vibrationally-excited CO2 molecules in the ice. The observation of the 12C/13C isotope effect in the Lyman-α induced photodecomposition of CO2 (ice) suggests that over astronomical time scales the isotope enrichment effect may distort historical information derived from isotope ratios in space wherever photochemistry can occur.
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