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Imaging of orbital electron densities by electron momentum spectroscopy - a chemical interpretation of the binary (e,2e) reaction

TitleImaging of orbital electron densities by electron momentum spectroscopy - a chemical interpretation of the binary (e,2e) reaction
Publication TypeJournal Article
Year of Publication2001
AuthorsBrion, CE, Cooper, G, Zheng, Y, Litvinyuk, IV, McCarthy, IE
Date PublishedJUL 15

It is shown that electrons in molecules behave in an essentially orbital-like fashion and that imaging of the spherically averaged orbital electron density can be achieved in momentum space by electron momentum spectroscopy (EMS). EMS measurements, using the binary (e,2e) reaction under binary encounter collision conditions, are demonstrated to effectively probe valence electron (frontier orbital) electron transfer out of a molecule by providing imaging of the spherically averaged Dyson orbital electron momentum density distribution corresponding to the ionization process. Experimental EMS cross-sections for the outermost valence (frontier) electrons of HF, H(2)O. NH(3), CH(4), H(2)S and a range of other molecules are found to be in excellent agreement with MRSD-CI calculations of the respective Dyson orbital densities using the plane wave impulse approximation. High level Hartree-Fock and density functional theory (DFT) (B3LYP and B3PW91) calculations, with large, saturated and diffuse basis sets. demonstrate that the Dyson orbital densities and thus the EMS experiments are also extremely well described by the respective initial state (i.e. neutral molecule) canonical molecular orbital(CMO) Hartree-Fock independent particle electron density distributions, with as good or sometimes an even better description being given by the respective (correlated) Kohn-Sham orbital (KSO) densities of DFT. in this sense EMS is shown to provide experimental imaging of orbital electron densities, with the emerging electrons having a delocalised orbital character immediately prior to `knock-out'. The present experimental and theoretical findings also lend support to the earlier predictions of Fukui as to the possibility of observing the `orbital pattern' experimentally {[}Int. J. Quant. Chem. 12 (Suppl. 1) (1977) 277] as well as to the recent views of Stowasser and Hoffmann {[}J. Am. Chem. Sec. 121 (1999) 3414] concerning the `reality' of KSOs. Further supporting evidence for the present findings is provided by a consideration of the results of frontier orbital theory applied to chemical reactions involving electron transfer, such as electrophilic attack {[}J. Chem. Phys. 20 (1952) 722]. The present results and interpretation are also strongly supported by scanning tunneling microscopy (STM) theory {[}Phys. Rev. B 31 (1985) 805] and in particular by recent STM experiments on adsorbed C(60) molecules {[}Chem. Phys. Lett. 321 (2000) 78] which show images which correspond very closely with DFT calculations of the electron density distribution in the HOMO orbital of the neutral molecule. The EMS measurements and associated theoretical calculations, together with the evidence from frontier orbital theory and the STM experiments, strongly suggest that delocalised CMO, or often, even better. the KSO, densities provide an operational definition of orbital electron densities, and thus of orbitals, appropriate for use in sdiscussions of chemical bonding as well as for predicting the outcome of chemical reactions and physical processes involving electron transfer. (C) 2001 Elsevier Science B.V. All rights reserved.