Ionization Potentials of the Tetraphosphorus Molecule

The He(I) and He(II) photoelectron spectra of the P₄ molecule have been recorded and compared with the results of an ab initio calculation of the electronic structure using s- and p-type Gaussian orbital basis functions. The agreement for the first three ionization potentials is excellent and is taken to show that in a sufficiently good s,p basis, there is no need for 3d AO's, contrary to the claims of others. The “σ” bonds are unavoidably bent in a description which does not use (or need) 3d AO's. The first, second, and fourth ionizations show appreciable Jahn-Teller splitting, but vibrational structure is observed only in the third ionization (²A₁, v₁’ excited). The optical spectrum of P₄ can be explained using configuration interaction among the valence-shell ¹T₂ configurations, and the low oscillator strengths of certain bands are shown to result from strong mixing among allowed transitions, rather than from symmetry restrictions. Electron density plots of the valence-shell MO's are also presented.