Product Ratios in the Photolysis of cis- and trans-Glyoxal at Low Pressures: The Mechanisms of the Thermal and the Photochemical Decompositions

Article abstract of DOI:10.1021/j100276a023

Values of the product ratio H2/CO have been measured for the photolysis of cis-glyoxal excited at 488.0 nm and of trans-glyoxal excited at 454.5 nm for pressures of glyoxal ranging from 0.01 to 0.45 torr.In the low-pressure limit, ratios from the two isomers converge and rise toward a value of 0.5.It is concluded that the collision-free decomposition proceeds largely through crossing from the excited singlet state to high vibrational levels of the ground state, which then decomposes to H2+2CO.With increasing pressure the H2/CO ratio falls sharply, which is attributed to collisional deactivation of the vibrationally excited ground state and its consequent decomposition by a lower energy path to H2CO+CO.These results and conclusions are compared with those found recently in the thermal decomposition and in a molecular-beam study of the dynamics of the photolysis.Product ratios were also measured in the infrared multiphoton decomposition of glyoxal, with a pulsed CO2 laser.The results were similar to those found at 488.0 and 454.5 nm and support the conclusion that the vibrationally excited ground state decomposes by two channels.