Selective chemical bond breaking characteristically induced by resonant core excitation of ester compounds on a surface

Inner-shell excitation possesses an attractive potential to induce selective chemical bond breaking due to its special localization and atomic selectivity. This paper reviews our recent works about site-selective (or site-specific) photon stimulated desorption (PSD) of core-excited ester compounds, poly(methyl methacrylate) (PMMA) thin films, condensed methyl isobutyrate (MIB) multilayers, methyl ester terminated self-assembled monolayers (SAMs), partially deuterided SAMs and ethyl ester terminated SAMs. Exact selective bond breaking was observed for all molecular systems except for MIB multilayers: COO-R (R = CH3, C2H5) breaking in the σ*(COO-R)-C1s, O1s(OR) excitations and CO-OR in the σ*(CO-R)-C1s, O1s(OR) excitation. Such resonant excitations take place within the functional group locally and the initial memory of the excited site and its localization character is effectively conserved during the PSD process even after subsequent Auger decays and ionic dissociation/desorption. Variation of selectivity observed here indicates the importance of the molecular environment and the contribution of the indirect dissociation process due to statistical energy relaxation and x-ray-induced electron stimulated desorption. The obvious polarization dependence measured only for selectively desorbing ions also supports the above characteristics. Neutral fragments desorbing from core-excited PMMA follow the profile of the total electron yield spectrum but not the partial ion yield of corresponding ions. This result indicates that ionic dissociation locally promoted by Coulomb repulsion plays an important role to reveal site-selectivity in chemical reactions induced by core-level excitation