We have developed a new electron-ion coincidence apparatus combined with synchrotron radiation in order to examine the various ion desorption mechanisms related to the Auger process induced by core excitation. Photon stimulated ion desorption (PSID) of a poly-methylmethacrylate (PMMA) thin film has been investigated by this apparatus. The PSID of PMMA induced by carbon core excitation has been examined using Auger electron yield, total ion yield, resonant Auger electron, and Auger electron-photoion coincidence (AEPICO) spectra. The spectrum of the total ion yield divided by the Auger electron yield shows that the desorption efficiency is largely increased at the resonant excitation of carbon 1s electron in the O-CH3 side chain to sigma*(O-CH3) orbital. In AEPICO measurement, H+ and CHn+ (n = 1-3) ions are observed at various resonant excitations. The AEPICO signal intensity depends on the Auger electron energy. Particularly, the CH3+ ion desorption in coincidence with Auger electron at 270 eV shows strong enhancement with sigma*(O-CH3) resonant excitation. The results of the resonant Auger spectra and AEPICO yield spectra demonstrate the relation of the ion desorption mechanism to the bonding/antibonding character and localized character of the excited sigma*(O-CH3) orbital and the Auger final state. (C) 2001 American Institute of Physics.