Light and Radical Ions

Abstract

If an electron is removed from most neutral molecules, a radical cation is formed. Until recently, the gas phase, or isolated molecule, study of radical cations has largely been the domain of the mass spectroscopist. However, in the last three to five years, there has been great interest in the interaction of radiation and molecular ions. In the last issue (1981) of the Annual Review of Physical Chemistry, two articles dealt directly with the subject, High Resolution Spectroscopy of Molecular Ions (1) and Fast Ion Beam Photofragment Spectroscopy, (2) with another two indirectly addressing it (3,4). It is certainly a measure of the vitality of the area that there exists anything left to review one year later. Limiting the scope of this review, however, rather than finding new material, turned out to be the larger problem. This review is confined to work involving the interaction of light (visible and near UV and IR radiation) with molecular ions. Because the spectroscopy of diatomic ions was treated in detail last year (1), only polyatomic ion work is covered, and its spectroscopic aspects emphasized. Furthermore, this review is restricted to positive ions and does not consider the large body of literature (5) on photodetachment and photoelectron spectroscopy of negative ions. When an isolated molecule, ion or not, absorbs a photon and under­ goes a transition to an excited electronic state, it may decay radiatively or nonradiatively. If the radiative decay pathway is negligible, then absorp­ tion spectroscopy is the usual means for probing the electronic transition. However, absorption spectroscopy of ions has been mostly ineffectual. Gas phase ion densities exceeding 108/cm3 (10lO/cm3 in plasmas) can only be obtained with great difficulty in the laboratory, and absorption spectroscopy is just not sensitive enough to be useful at these low