Spectroscopy of Molecular Ions in Noble Gas Matrices

Abstract

Molecular ions are of considerable chemical and physical interest for comparing the spectroscopic and bonding properties of neutral mole­ cules with their positive and negative molecular ions. The study of molecular ions in the gas phase by photoelectron, photoionization mass, and ion cyclotron resonance spectroscopies can be complemented by infrared and optical absorption spectra of the molecular ion trapped in a solid inert gas host. Further ion studies with tunable infrared lasers will be greatly aided by the vibrational data obtained for molecular ions in noble gas solids. The ion-matrix interaction is of fundamental and practical interest as matrix. spectra of ions are related to the gas phase. Charged species in matrices form two general classes described in the literature as and chemically with respect to the counterion. The first ionic species characterized in matrices, Li + 0;, is of the latter type where the lithium cation and the superoxide anion are Coulombically bound together (1, 2), and charge transfer occurs be­ cause this electrostatic attraction more than makes up for the difference between the ionization energy of lithium and the electron affinity of oxygen. The next molecular ions identified in matrices, B2H; and �-, are of the isolated type where the cation is separated by an unde­ termined number of matrix atoms from the anion (3, 4). These ionic systems have been characterized as Coulomb ion pairs, which exist because of essentially zero overlap between the wavefunctions for the electron on the recipient molecule and the cation that provided the electron (5). Clearly, the formation of ions of the isolated type requires