Semiconducting zinc diphosphide (ZnP2) crystals have been found to exhibit a very distinct IHS, which consists of eight lines from n = 4 to n = ii [i0]. It has become a widely held view that such spectrally inverted phenomena can be consistently interpreted as the observation of a new type of optical interband transitions in crystals, i.e., transitions between two conduction bands or two valence bands [~]. They can be dubbed second-order transitions if the usual interband transitions between the upper valence band and the lower conduction band, which results in fundamental absorption, are classified as first-order transitions. I It turned out that second-order transitions can also be accompanied by a hydrogen-like line structure. In contrast to excitons, however, this structure converges in the long-wavelength region of the spectrum, forming an inverse hydrogen-like series of absorption and emission lines. This unusual spectrum is du~ to a new quasiparticle, a bielectron or a bihole, which is a bound state of two carriers of like sign. Figure i shows a portion of the band structure of a crystal, illustrating the possibility of observing second-order transitions and IHS provided that E 0 18 eV, exceeding the threshold ionization energy of the cations (Ba2+). Radiative transitions of an electron from the valence band of 2pF- to the cation band, which is accompanied by the emission of photons with an energy of 5-7 eV may be one mechanism of the decay of this state. This new mechanism of fast luminescence (less than 10 -9 sec), and effect of radiation-induced "floating" of holes in the ionic BaY 2 crystal [13, 14], is a brilliant example of second-order phototransitions.