Hexalithiobenzene: A molecule with exceptionally high second hyperpolarizability

The electronic structure of the fully lithiated benzene molecule has been c alculated at different levels of approximation with particular attention pa id to its second hyperpolarizability. It has been estimated that lithiation of the benzene molecule may lead to the enormous increase of the second hy perpolarizability. For hexalithiobenzene the second hyperpolarizability ter m responsible for the third harmonic generation has been estimated to be of the order of 10(7) e(4) a(o)(4) E-h(-3). The reasons for the very large in crease of the dipole polarizability, and in particular, of the second hyper polarizability upon lithiation have been analysed in terms of the electroni c structure, orbital energy levels, and the excitation spectrum of the hexa lithiobenzene molecule. The main factor responsible for the particularly la rge values of the studied properties is the electronic valence shell struct ure of the lithium atom. Although the hexalithiobenzene molecule is used as a model for the study of the lithiation effect, several stable organometal lic systems involving alkali metals are known. The present study suggests t hat among these systems one may find materials of extremly high nonlinear o ptical efficiency.