The reaction between electronically excited carbon atoms, C(D), and acetylene was studied at two average collision energies of 45 kJ mol and 109 kJ mol employing the crossed molecular beam technique. The time-of-flight spectra recorded at mass to charge m/e537~C3H ! and m/e 536(C3 ) show identical patterns indicating the existence of a carbon versus atomic hydrogen exchange pathway to form C3H ...