Isotope effect in the photochemical decomposition of CO{sub 2} (ice) by Lyman-{alpha} radiation

Journal Article · Sun Apr 21 00:00:00 EDT 2013 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.4800929· OSTI ID:22105479

Chunqing, Yuan; Yates, Jr., John T. ^[1]

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1. Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904 (United States)

The photochemical decomposition of CO{sub 2}(ice) at 75 K by Lyman-{alpha} radiation (10.2 eV) has been studied using transmission infrared spectroscopy. An isotope effect in the decomposition of the CO{sub 2} molecule in the ice has been discovered, favoring {sup 12}CO{sub 2} photodecomposition over {sup 13}CO{sub 2} by about 10%. The effect is caused by electronic energy transfer from the excited CO{sub 2} molecule to the ice matrix, which favors quenching of the heavier electronically-excited {sup 13}CO{sub 2} molecule over {sup 12}CO{sub 2}. 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 CO{sub 3} species is observed for the photolysis of the {sup 12}CO{sub 2} molecule compared to the {sup 13}CO{sub 2} molecule in the ice. Only 0.5% of the primary photoexcitation results in O-CO bond dissociation to produce trapped-CO and trapped-CO{sub 3} product molecules and the majority of the electronically-excited CO{sub 2} molecules return to the ground state. Here either vibrational relaxation occurs (majority process) or desorption of CO{sub 2} occurs (minority process) from highly vibrationally-excited CO{sub 2} molecules in the ice. The observation of the {sup 12}C/{sup 13}C isotope effect in the Lyman-{alpha} induced photodecomposition of CO{sub 2} (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.

• OSTI ID: 22105479
• Journal Information: Journal of Chemical Physics, Vol. 138, Issue 15; Other Information: (c) 2013 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ABSORPTION SPECTROSCOPY
ALPHA PARTICLES
CARBON 12
CARBON 13
CARBON DIOXIDE
CARBON MONOXIDE
CARBONATES
DESORPTION
DISSOCIATION
EV RANGE
GROUND STATES
ICE
INFRARED SPECTRA
ISOTOPE EFFECTS
ISOTOPE RATIO
ISOTOPE SEPARATION
MOLECULES
PHOTOLYSIS
TRAPPING
VIBRATIONAL STATES