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
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