Ground-state populations of cationic and neutral Y, Ba, Cu, and neutral YO, BaO, and CuO are monitored by laser-induced fluorescence during laser ablation of YBa2Cu3O7-. 193-nm photons completely dissociate CuO allowing the observation of a subsequent reaction of the Cu atom with O2 to form CuO. As a function of O2 pressure, the CuO signal increases to a maximum at approximately 120 mTorr of O2, presumably minimizing , and then falls to negligible levels by 400 mTorr. Ablation at 248 and 351 nm dissociates a smaller fraction of the CuO, i.e., nascent CuO is observed in the absence of O2. © 1991 The American Physical Society.