Fragment-Ion Desorption From Sulfur-Containing Amino Acids By Localized Core-Level Excitation

The desorption of fragment ions from sulfur-containing amino acids following the sulfurK-edge photoexcitation has been investigated using monochromatized synchrotron radiation. The samples investigated are pellets of neutral L-cystine, L-cysteine and L-methionine. It was found that only atomic S+ions are desorbed following the S1s→ σ*resonant excitations, while various kinds of molecular fragment ions containing carbon, nitrogen and oxygen atoms are desorbed by the low-energy electron irradiation. The photon-energy dependencies of theS+yields approximately resemble those of the total electron yields (TEY's). On the other hand, the edge-jump ratios, defined asIon/Ioff, whereIonis the intensity at the core-to-valence resonance energy andIoffis that at off-resonance energy, for theS+-yield curves are by more than ten times higher than those of the corresponding TEY curves. The edge-jump ratios and the partial electron yield spectra revealed that the low-energy electrons produced following the Auger decays scarcely contribute to the ion desorption. Also, it was elucidated that theS*desorption is induced by the direct core-to-valence excitation which is followed by the SKLLAuger decay. It was concluded that the core-to-valence resonant photoexcitation and succeeding specific S–S and S–C bond scissions play a major role in the ionic fragmentation in the present amino acids.