Ionization band profile analysis in valence photoelectron spectroscopy

Analytical descriptions of partially resolved and unresolved vibrational progressions in valence photoelectron ionization bands are discussed. The ²T_2g valence ionization of Cr(CO)₆ is examined as an example of a partially resolved vibrational progression, and the Jahn-Teller split ²E′ and ²E″ valence ionizations of Fe(CO)₅ are examined as examples of overlapping ionizations with unresolved vibrational progressions. Asymmetric Gaussian band shapes are obtained when vibrational broadening is responsible for the overall contour of the molecular ionization. Attempts to model the valence ionizations with symmetric Gaussian peak models can lead to serious misrepresentations of the ionization bands. This is found to be particularly important for overlapping ionizations such as the Jahn-Teller split bands of Fe(CO)₅, where attempts to model the contours with symmetric Gaussian peak shapes lead to physically unreasonable band positions, relative amplitudes, halfwidths and band areas. The treatment of ionizations with resolved vibrational fine structure is also addressed with the ²A_1gionization of osmocene. A series of related Gaussian components are used to model the resolved vibrational progressions. The goodness of fit between the model and the data is discussed in each case. Advantages and limitations in the analytical representation of valence photoelectron data are addressed.