Site-specific fragmentation of acetone adsorbates on Si(100) in the carbon1s absorption edge

We have investigated photon-stimulated ion desorption from acetone adsorbat es on Si(100) at 93 g. The mass spectra and the excitation-energy spectra f or fragment-desorption yields, along with photoabsorption spectra, were mea sured in the soft X-rays with energy of 270-320 eV. Methyl fragments (CHn+, n=1-3) are preferentially produced at the resonance that has been assigned to the transition of methyl carbon core level to a mixture of pi*(CH3) and 3p Rydberg. Both the desorption mass patterns and the excitation spectra f or the fragment-ion yields showed a remarkable change when the samples were changed from thin layers to crystalline thick layers. The desorption mecha nism is considerably changed depending on the layer thickness. (C) 2000 Els evier Science B.V. All rights reserved.