The VUV photochemistry of radicals: C3H3 and C2H5.

The VUV photochemistry of the radicals propargyl, C3H3, and ethyl, C2H5, is investigated using synchrotron radiation. The radicals are produced by flash pyrolysis from propargyl bromide and ethyl iodide, respectively. It is shown that dissociative photoionisation of propargyl is associated with loss of a H-atom, leading to formation of c-C3H2+. The process sets in between 12.5 and 13 eV, in agreement with computations. The alternative pathway, leading to C3H(+) + H2 is only tentatively observed at energies above 15.5 eV. The onset for dissociative photoionisation of ethyl according to C2H5 --> C2H3(+) + H2 is observed at 11.6 eV, roughly 1 eV higher than expected from ab initio calculations. In addition we monitor the hitherto unobserved three-body dissociative ionisation of the precursor molecules. The process C2HsI --> C2H3 + H2 + I sets in above 12.7 eV, the dissociative ionisation of C3H3Br --> C3H+ + H2 + Br above 15 eV.