Electron transfer dynamics in halobenzenes at ice and metal interfaces

Electron transfer reactions manifest themselves in many physical, chemical and biological processes at a fundamental level, for example photosynthesis. Past studies have shown that dissociative electron attachment (DEA) cross sections of halogenated organic molecules are enhanced on polar solids as compared to in the gas phase. Despite its important implications, such as catalysis of ozone depletion, a rigorously proven model is still missing. This work aims to understand the mechanisms for this enhancement. Halobenzenes (C6H5X, X = F, Cl and Br) coadsorbed on amorphous D2O ice on Cu(111) are studied with two-photon photoemission spectroscopy, whereby changing the halogen, the energetics of the system is changed systematically. The C6H5X are shown to be ionised/dissociated by photoexcited excess electrons in the ice conduction band via DEA. The photoinduced reaction rate constants can be measured by surface workfunction changes, which is due to a build up of the negative charge at the surface. The dependence of the rate constants on incident photon flux, photon energy and different electron environment (C6H5X film thickness and solid phase of ice) are studied. By performing comparative analysis, it is shown that the enhancement of DEA cross section is due to pre-existing electron traps and delocalised electrons, which is in agreement with recent theoretical studies. An important implication of this study is that a spectroscopic signal from the transient ionic state is seen in the form of workfunction change. Electron transfer mechanisms at metal-organic interface have also been investigated, as it is interesting from a fundamental view point and is important to molecular electronics. A molecular derived spectroscopic signature of C6H5F/Cu is observed with non-linear photoemission and compared to C6H6/Cu and C6F6/Cu (from past studies). Additionally, electron transfer efficiency is studied by performing coverage dependent workfunction studies of C6H5X/Cu. Together, these experiments show that the LUMO of the substituted benzene molecules shift to lower energies for less electronegative substitution on adsorption on Cu, reflecting the gas phase trend. This provides with a neat systematics to tailor metal-organic interface energies.