The decomposition mechanism of hard a-C films under irradiation using a microbeam laser (514.5 nm) was studied. The films were magnetron sputtered at different pressures to achieve different degrees of sp(3)/sp(2) bonding ratio. Under laser irradiation, they showed graphitization and decomposition. The graphitization saturated at early exposure of similar to4 min, while the decomposition progressed exponentially with the exposure time, following a first-order reaction mechanism. The decomposition of a-C films under laser irradiation can be modeled using a two-step process with the establishment of a quasi steady-state intermediate. The relative energy barrier and rate constant for film decomposition were calculated. With increasing sp(3)/sp(2) bonding ratio, the films showed increasing resistance against the irradiation-induced changes. This is shown by the decreasing rate constant and increasing energy barrier during the decomposition. (C) 2004 American Vacuum Society.