Low-energy electron-induced chemistry of condensed-phase hexamethyldisiloxane: Initiating dissociative process and subsequent reactions

Thin films of condensed hexamethyldisiloxane (HMDSO) have been exposed to electron irradiation at incident energies between 5 and 15 eV and analysed afterwards by thermal desorption spectrometry (TDS). Formation of products is observed at energies at and above 11 eV and quantified at 15 eV by comparison with reference samples of known composition. Gas-phase measurements aiming at detection of dissociative election attachment (DEA) were, in addition. performed to obtain more insight into the dominant electron-induced dissociation channel expected to initiate further reactions in the condensed phase. Apart from CH4 which is the most obvious product present in exposed films of HMDSO, tetramethylsilane (TMS) and smaller amounts of C2H6 have been detected. The quantity of the products is by one to two orders Of magnitude smaller than the amount of decomposed HMDSO. In addition, signals ascribed to unquantified amounts of larger siloxanes have been observed. The present results together with previous gas-phase results from literature suggest that dissociative ionisation leading to Si-C bond rupture and release of a methyl radical is the most important electron-driven initial reaction step. Possible mechanisms of the subsequent reactions induced by the fragments of the initial dissociation reaction are reviewed and discussed in relation to the observed product quantities. (C) 2009 Elsevier B.V. All rights reserved.