Recently, Ramanujam and co-workers described for the first time that surface relief gratings can be inscribed into azobenzene copolymers with a single pair of nanosecond laser pulses. We performed a detailed investigation of the involved processes. Two contributions to the diffraction efficiency are observed, one arising from a surface relief and one from a transient cis-trans grating which decays on millisecond time scales. The stable surface relief, in contrast, is caused by a thermal effect which sets in at a well-defined threshold value of the pulse energy.