Electron Momentum Spectroscopy Study Of Some Small Molecules Of High Symmetry

Electron Momentum Spectroscopy (EMS) is a powerful tool for investigating theelectronic structures and dynamics of electron scattering. It can measure the bindingenergy and the electron momentum distribution of each orbital simultaneously. With thehigh-performance3rdgeneration electron momentum spectrometer, we can explore thephenomena that can not be discovered in the past, and uncover the profound physicalmechanism behind those phenomena in cooperation with the theoretical study.A new Gaussian fitting program was compiled in the thesis for stripping the twodimensional energy-momentum spectra. The new program is robust and easy to use. Itgreatly reduces the complexity of Gaussian peak fitting.The highly symmetric small molecules usually have relatively simpleelectronic structures. As a result, it is easier to interpret the physical mechanismbehind the experimental phenomena. The main work of this thesis is toinvestigate a few small molecules using our high-performance EMSspectrometer.We successfully measured the I2molecule under different experimentalconditions. For the outer valence orbitals, the spin-orbit relativistic methodpresented the best descriptions for the experimental momentum distributions.The momentum spectra of related orbitals as well as the variation ofbranch-ratios manifest that the relativistic effect in I2can not be ignored. For theinner valence orbitals, the calculation using SAC-CI General-R method with thebasis set cooperating with the relativistic pseudo-potential can simulate thecomplicated satellites structures and their momentum distributions.The Jahn-Teller effect in the momentum distributions of NH3has beenobserved through comparing the distributions of the different slices. For thedegenerate1e orbital, the slice with a higher binding energy has a higherintensity in the low momentum region. This phenomenon has not been observedon the non-degenerate3a1orbital. It was found that the displacements of atomsunder different vibration modes can influence the momentum distributionsdifferently, which can qualitatively explain this phenomenon. We also systematically investigated the distorted wave effect (DWE) inCF4, CCl4, and CBr4molecules. The HOMOs of them are all highly symmetricwith many nodal planes, and the features of DWE have been observed in theexperimental momentum distribution: there is a turn-up in the low momentumregion, and experimental intensitiy decreases as the incident energy increases.The DWEs were also found in other two orbitals. Besides, in the5thorbitals ofthese molecules, a unique phenomenon was observed: the theoretical calculationdoes not well fit the experimental momentum distributions, and displays somecommon characteristics. The mechanism behind the phenomenon is still notclear.