The band spectra of diatomic molecules P2, AS2, Sb2 and Bi2 were investigated in the spectral range 2100–10 000 Å with a medium-size spectrograph, using a discharge tube developed for studies of forbidden lines. A study of the excitation conditions of various bands shows, among other things, that the bands previously ascribed to As2+ actually are emitted from a low-lying electronic state of the As2 molecule. Several new band systems were found, for some of which a vibrational analysis wvas carried out. This permits the identification of three new electronic states for the P2 molecule and two new states for the As2 molecule. Four of these states are located below the previously known excited states of these molecules and belong to the triplet term system. A comparison with the N2 spectrum shows that for this group of diatomic molecules the 3IIg state, located above the 3Σu+ state for the N2 molecule, becomes gradually depressed below the 3Σu+ state as the atomic number of the element increases.
S. MROZOWSKI and C. SANTARAM, "Electronic Spectra of P2, As2, and Sb2 Molecules," J. Opt. Soc. Am. 57, 522-530 (1967)