UV and X-ray photoelectron spectroscopy and their applications to chemical systems



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Photoelectron spectroscopy employs the precise measurement of the kinetic energy of photoelectrons ejected from a sample upon excitation by monochromatic radiation. The kinetic energy of the photoelectron can be related to its binding energy and is a function of the specific nucleus and chemical environment. This dissertation investigates the theoretical and experimental aspects of photoelectron spectroscopy and their applications to a variety of chemical problems. The theoretical aspects involve semiempirical methods such as EH(extended Huckel), CNDO(complete neglect of differential overlap), and INDO(incomplete neglect of differential overlap) molecular orbital calculations and photoionization cross sections calculated within the orthogonalized plane-wave approximation. The MO calculations can extract information on the energies, character, and ordering of the occupied molecular orbitals in a molecule; they are used as a criterion in assigning photoelectron spectra. The applications presented are the calculations of one-electron orbital energies and molecular wavefunctions of the occupied orbitals of chromyl chloride, hydrazoic acid, the azide ion, N,N-dimethylnitramine, and N,N-dimethylnitrosamino. EHMO theory is also used to develop a model for the adsorption of CO on several W(100) sites. The concept of a 'surface molecule' in which CO is bonded to an array of tungsten atoms has been employed for the adsorption study. The theoretical studies of photoionization cross sections are important since it is possible to deduce ionic state identifications from analysis of the cross sections of the photoelectron bands. Three different studies of photoionization cross sections involve (1) the use of differential photoionization cross sections as a function of excitation energy in assigning photoelectron spectra, (2) vibrational transition probabilities in photoelectron spectra, and (3) photoionization cross section dependence on molecular structure within the fourteen electron series C[lowered 2]H[lowered 2], HCN, and N[lowered 2] and the sixteen electron series B[lowered 2]H[lowered 6], C[lowered 2]H[lowered 4], H[lowered 2]CO and O[lowered 2]. Ultraviolet radiation such as the He I and Hell resonance lines at 584 A and 304 A (21.2 eV and 40.8 eV, respectively) and X-ray radiation such as the K[lowered proportional to] line of Al (1486.6 eV) are used as ionization sources in the photoelectron spectroscopic studies of chromyl chloride, diamine molecules and nitrogen containing compounds. Photoelectrons ejected from the samples upon excitation by photons are sorted according to their kinetic energies in an electron energy analyzer. The resulting photoelectron specturm is an energy distribution curve representing the number of electrons counted at a given kinetic energy. It is essentially a plot of relative intensity versus ionization energy. The experimental works include (1) the Hel photoelectron spectrum of chromyl chloride vapor, (2) the HeII photoelectron spectrum of H[lowered 2], (3) the UV (HeI and HeII) spectra of gaseous HN[lowered 3], N[lowered 2]O, (CH[lowered 3])[lowered 2]NNO[lowered 2], and (CH[lowered 3])[lowered 2]NNO, the X-ray spectra of the core and valence bands of HN[lowered 3], (CH[lowered 3)[lowered 2]NNO[lowered 2] and (CH[lowered 3])[lowered 2]NN0 in the condensed phase and the azide ion in the form of LiN[lowered 3], and (4) the X-ray photoelectron spectra of the core levels of some diamine molecules, namely urea, thiourea, guanidine hydrochloride, nitroguanidine, and cyanoguanidine. The results of this investigation exemplify how the combination of UV and X-ray photoelectron spectroscopy with quantum mechanical methods can provide a complete and firm assignment of the occupied energy levels of a molecular system. The above are outlines of the theoretical and experimental aspects of this dissertation. There will be additional abstract before each individual topic.