Orbital imaging by multichannel electron momentum (ie. (e,2e)) spectroscopy. The ionization, excitation and fragmentation of molecules using techniques of electron energy loss spectroscopy and coincidence counting, and also synchrotron radiation studies of photoionization. Current studies are in the following areas:
(a) Electron momentum spectroscopy (EMS) is used to obtain electron densities (psi(p))2 f or individual atomic and molecular orbitals. This provides orbital imaging and a direct experimental evaluation of molecular wave functions. The method provides a sensitive probe of molecular structure, chemical bonding and reactivity in the chemically important outer spatial regions of the electron distribution. Recent applications are to molecules of biochemical and biomedical interest such as aminoacids and heterocyclic systems. Comparisons are made with SCF, CI and DFT calculations.
New applications are in progress to study larger biochemicals, transition metal complexes, chemically reactive species (excited atoms, metastables, ions, free radicals, etc.) as well as condensed matter (solids, specially prepared surfaces, adsorbed gases and aligned molecules) using reflection geometry (e, 2e) spectroscopy under UHV conditions. Results are expected to find application in wave function evaluation and design, testing and developmentof new quantum mechanical theoretical methods such as DFT, computer aided mo lecular design and drug screening, molecular recognition, momentum space chemistry, catalysis, the design of new materials, nanostructures, time-correlated position sensitive imaging and the increase of fundamental knowledge relating chemical, biochemical and physical properties to details of electron motion.
(b) High resolution electron energy loss spectra of valence and inner shell electrons in molecules. This covers energy transfer in the range 10-1000 eV (equivalent wavelengths 1240-12 Å), spanning the UV and soft X-ray regions.
(c) Photoionization, photoabsorption, and ionic photofragmentation of molecules in the 10-400 eV (1240-31Å) region. These studies are made using the virtual photon field of a fast electron and yield quantitative results (i.e. total and partial absolute oscillator strengths (cross-sections)) entirely equivalent to those which could be obtained with synchrotron radiatio n. Coincidence techniques are used with electron energy loss spectroscopy to give simulations of photoionization mass spectrometry (e,e+ion) and photoabsorption (e,e) at continuously tuneable energies. The dipole-induced breakdown pattern of molecules is elucidated. Synchrotron radiation is also used.