Mechanism of state-specific enhancement in photon-stimulated desorption as studied using a polarization-dependent technique

In the present study, we investigated the mechanisms of state-selective fragment-ion. desorption from condensed chlorobenzene following C1s, C12s, and C12p core-level excitations by measuring the polarization dependences of NEXAFS (near-edge X-ray absorption fine structure) spectra recorded with desorbed ion yields and total electron yields (TEY). TEY-based NEXAFS shows no significant polarization dependence at any edges. In contrast, ion-based NEXAFS demonstrates a variety of marked polarization dependences. C12p -> 3d pi* excitation induces a significant enhancement of Cl+ desorption yield at normal incidence (polarization parallel to surface), while C12s -> sigma*(C-Cl) excitation stimulates a substantial enhancement of Cl+ desorption at grazing incidence (polarization perpendicular to surface). C 1s -> sigma*(C-Cl) enhances D+ desorption at grazing incidence, while C 1s ->pi*(C=C) suppresses D+ desorption at all angles. All polarization dependences consistently reveal that excited molecules with upward-pointing bonds at top-most surfaces lead to efficient fragment desorption. This orientation effect is understood in terms of competing processes between the ultra-fast dissociation and charge-neutralization relaxation taking place during core-excited states. We demonstrate that the polarization-dependent ion-NEXAFS technique can distinguish direct state-specific fragmentation from indirect desorption caused by the collision of secondary electrons. (c) 2005 Elsevier B.V. All rights reserved.