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

It is shown that electrons in molecules behave in an essentially orbital-li ke fashion and that imaging of the spherically averaged orbital electron de nsity can be achieved in momentum space by electron momentum spectroscopy ( EMS). EMS measurements, using the binary (e,2e) reaction under binary encou nter collision conditions, are demonstrated to effectively probe valence el ectron (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 cro ss-sections for the outermost valence (frontier) electrons of HF, H2O. NH3, CH4, H2S and a range of other molecules are found to be in excellent agree ment with MRSD-CI calculations of the respective Dyson orbital densities us ing the plane wave impulse approximation. High level Hartree-Fock and densi ty functional theory (DFT) (B3LYP and B3PW91) calculations, with large, sat urated and diffuse basis sets. demonstrate that the Dyson orbital densities and thus the EMS experiments are also extremely well described by the resp ective initial state (i.e. neutral molecule) canonical molecular orbital(CM O) Hartree-Fock independent particle electron density distributions, with a s good or sometimes an even better description being given by the respectiv e (correlated) Kohn-Sham orbital (KSO) densities of DFT. in this sense EMS is shown to provide experimental imaging of orbital electron densities, wit h the emerging electrons having a delocalised orbital character immediately prior to 'knock-out'. The present experimental and theoretical findings al so lend support to the earlier predictions of Fukui as to the possibility o f observing the 'orbital pattern' experimentally [Int. J. Quant. Chem. 12 ( Suppl. 1) (1977) 277] as well as to the recent views of Stowasser and Hoffm ann [J. Am. Chem. Sec. 121 (1999) 3414] concerning the 'reality' of KSOs. F urther supporting evidence for the present findings is provided by a consid eration of the results of frontier orbital theory applied to chemical react ions involving electron transfer, such as electrophilic attack [J. Chem. Ph ys. 20 (1952) 722]. The present results and interpretation are also strongl y supported by scanning tunneling microscopy (STM) theory [Phys. Rev. B 31 (1985) 805] and in particular by recent STM experiments on adsorbed C-60 mo lecules [Chem. Phys. Lett. 321 (2000) 78] which show images which correspon d very closely with DFT calculations of the electron density distribution i n the HOMO orbital of the neutral molecule. The EMS measurements and associ ated theoretical calculations, together with the evidence from frontier orb ital theory and the STM experiments, strongly suggest that delocalised CMO, or often, even better. the KSO, densities provide an operational definitio n of orbital electron densities, and thus of orbitals, appropriate for use in sdiscussions of chemical bonding as well as for predicting the outcome o f chemical reactions and physical processes involving electron transfer. (C ) 2001 Elsevier Science B.V. All rights reserved.