Two-Photon Molecular Electronic Spectroscopy

Abstract

The excitation of rotational, vibrational, and electronic states of mole­ cules by the simultaneous absorption of or more photons has developed into a significant branch of science in the last decade. Numerous compounds have now been studied by workers from several different disciplines: atomic spectroscopy, optical physics, solid state physics, and electrical engineering, as well as chemical physics and molecular spectroscopy. This rich background has given rise to an ingenuity of technique so astonishing that it seems at times almost to obscure the spectroscopic questions that originally motivated this field. It is our purpose in this article to return to the basic chemical questions. We shall review the advances in our knowledge of molecules that have been made using nonlinear spectroscopy, leaving aside all discussion of theory and technique. To begin this work, we had to collect a comprehensive bibliography of compounds that have been investigated by some form of two-photon spectroscopy. We made a computer assisted literature search of the IN SPEC data base (1) from 1973 to 1979 via the DIALOG terminal system. We specified a simple full-text search in the data base of the adjacent words two and photon, appearing with the root molec anywhere in the INSPEC field (abstract, title, or special data-base descriptors). The specific DIALOG command was TWO(W)PHO­ TON(F)MOLEC?. The question mark allowed for various possibilities such as molecules, molecular, etc. The search was run on October 2, 1979, netting 372 references of which some 220 were ultimately