Electron-induced dissociation of hydrocarbon multilayers

We have recently observed large cross sections (σ = 1.8 × 10^-15 cm²) for electron induced dissociation (EID) of physisorbed cyclohexane in multilayers on Pt(111) surfaces. This is a general phenomenon for hydrocarbon multilayers and for other physisorbed hydrocarbon species at monolayer or submonolayer coverages. While it is certainly not new to identify electron induced effects in adsorbed layers, the large EID cross sections reported here and their implications for physisorbed hydrocarbons is not generally appreciated. Two consequences of this chemistry are discussed. First, these EID cross sections are so large that serious artifacts in temperature programmed desorption (TPD) can be caused even in the short time it takes to perform a TPD experiment using a conventional quadrupole mass spectrometer (QMS) which can bombard the sample with electron fluxes in the μA regime. This problem in TPD can be overcome by utilizing a biased grid between the sample and the QMS ionizer region to stop low energy electrons. However, the secondary electron emission inherent in many electron spectroscopies (XPS, UPS, AES, etc.) may cause similar effects that are difficult to eliminate. Secondly, the large difference in EID cross sections between physisorbed and chemisorbed species has the exciting consequence that well-defined, complex hydrocarbon surface intermediates, e.g., cyclohexyl, can be prepared cleanly on reactive metal surfaces for chemistry studies.