Band Spectra of PbSe, SnSe and PbTe in Absorption

A vibrational analysis has been made of band systems in the visible region due to PbSe, to SnSe and to PbTe. These bands were observed in absorption. In each case the band heads degrade toward the red. The vibrational coefficients ${\omega }_e$ and ${\chi }_e{\omega }_e$ for the ground state and for the first excited state are compared with those of the band systems in PbO, SnO, PbS and SnS. The expected trend in these vibrational constants is observed in each case, namely a decrease in ${\nu }_h$, ${\omega }_e$, and ${\chi }_e{\omega }_e$ with increasing number of electrons in the molecule.