Abstract
Angle-resolved photoelectron spectra were obtained on a series of nitrogen-containing aromatic systems using synchrotron radiation as the ionizing source. The angular distribution parameter, ..beta.., was measured in the photon energy range 12-27eV for the first three orbitals of 2-fluoropyridine, pentafluoropyridine, and pyridine, and for the first four orbitals of pyrimidine and pyrazine. The angular distribution parameter as a function of photon energy is shown to be of value in the assignment of bands in photoelectron spectra and has been applied to the photoelectron spectra of pyridine. The fluoro-substituted pyridines provided information on the trend of the ..beta.. values for nonbonding orbitals with predominant nitrogen lone-pair character. The diazines demonstrated that ..beta.. values as a function of photon energy could be used for orbital assignments even in the case of partial band overlap. For pyridine the overlap of the first two ionic levels, consisting of a ..pi.. and a nonbonding orbital, is so severe that unambiguous assignment has been difficult but is now possible. Experimental evidence is presented for the sequence of these levels to be the nonbonding (n) orbital at lower ionization energy followed by the ..pi.. orbital. The relevance of systematic investigations of the angle-resolved photoelectron spectroscopy of amore » homologous series of molecules as a function of photon energy is discussed.« less
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