THE DECOMPOSITION OF CARBON DIOXIDE BY Hg 6(3P1) AND Hg 6(3P0) ATOMS

Carbon dioxide has been shown to decompose into carbon monoxide and oxygen atoms, when exposed to radiation at 2537 Å, in the presence of mercury vapor. The rate rises steeply with decreasing substrate pressure, and varies directly with the 1.8 ± 0.1 power of the light intensity. The proposed mechanism attributes reaction to the collision of electronically excited CO2 molecules with Hg 6(3P1) atoms. The suppression of reaction at higher substrate pressures is readily explained in terms of collisional deactivation of the excited CO2 species. Nitrogen was found to increase the rate of CO formation; the maximum effect was obtained for a mixture of 7.4 mm nitrogen and 3.74 mm carbon dioxide, in which case the rate was 1.58 times that for pure substrate. It is shown that nitrogen serves to generate metastable Hg 6(3P0) atoms, which can sensitize the decomposition. The reaction might serve as a chemical method for monitoring Hg 6(3P0) atoms. For CO2–N2 mixtures, the rate was found to rise when the reacting system was exposed to radiation at 4047 Å. This is taken as direct evidence of sensitization by higher states of mercury, generated by stepwise excitation, since radiation at 4047 Å converts Hg 6(3P0) to Hg 7(3S1).