Transferability of the valence ionization intensities of chemical functional groups between molecules

The transferability of the valence ionization intensities of chemical functional groups is investigated with three common laboratory ionization sources (Ne Iα, 16.7 eV; He Iα, 21.2 eV; and He IIα, 40.8 eV) for a series of molecules. Each molecule contains two different functional groups that are well separated electronically and spatially by an alkane chain linker. The optimum length of the alkane chain between the functional groups is investigated. In this initial study, the functional groups are RCl, RBr, RSH, and RFc, where R is an alkane chain and Fc is ferrocene with an alkane chain bonded to one cyclopentadienyl ring. An additional feature of this study is that each functional group contains a molecular orbital with nearly pure atomic character, allowing comparison to theoretical atomic ionization cross-section ratios. In general, the observed photoelectron intensity changes are more gradual than those predicted by either theoretical atomic photoionization cross sections or by a complete Gelius analysis based on molecular orbital compositions obtained from electronic structure calculations. Experimentally, there is a high degree of transferability of the relative ionization intensities of the chemical functional groups between the molecules of this set. These results indicate that an empirical library of the relative ionization cross sections of chemical functional groups as a function of photon energy will be a useful aid to the study of more complex molecules and will provide an experimental foundation for further theoretical studies of molecular photoelectron cross sections.