A Comparative Study of Core-Induced Photon Stimulated Desorption and Electron-NEXAFS of Condensed H2O, NH3, and CH4

Abstract

Photon stimulated desorption of neutrals and ions from condensed films of H2O, NH3 and CH4 has been investigated in the C 1s, N 1s and O 1s core excitation regimes, respectively, and compared to absorption data obtained via electron-NEXAFS. Desorption of neutral molecules and of H+ as well as fragment and cluster ions has been observed. The PSD yields of neutrals, molecular ions and ionic clusters, as well as the corresponding total-yield and/or Auger-NEXAFS spectra show identical structures. This suggests that X-ray induced ESD, probably via double and multiple valence ionization, is the dominant mechanism for desorption of these species. The PSD yield of H+ ions, however, exhibits significant discrepancies from these XESD derived NEXAFS spectra. On H2O and NH3, where it is by far the most abundant ion, it is dominated by a narrow peak right at the onset of desorption. The intensity of this threshold peak shows a remarkable polarization dependence. On CH4, the H+ signal is small, and no enhancement is observed at any photon energy; there is an overall shift of 0.2 eV of the H+-NEXAFS structure to lower energy as compared to absorption. The mechanisms of these effects are discussed.