Knowledge Bank

We present a development of the dipole moment and polarizability operators of X$2$Y$4$ molecules, using a tensorial formalism} {\bf 217}, 239-248, (2003)}$,$}, accepted, (2005).} analogous to the one developed for tetrahedral and octahedral molecules} {\bf 228}, 620-634, (2004).}. These operators are involved in the calculation of the intensities of rovibrational transitions as well as in the calculation of the Stark effect. Expressions for the matrix elements are derived. A model for the study of the Stark effect in isolated bands of such molecules is proposed and has been used to predict the Stark spectra of the $\nu 12$ band of ethylene. Values of the polarizability coefficients have been calculated using {\em ab initio} methods.