Hole-burning spectroscopy of coordination compounds

Abstract

Electronic transitions in condensed phases are subject to inhomogeneous broadening, caused by the variation of local fields. Even in extremely well defined crystals, this broadening is on the order of magnitude of 1 cm −1 (30 GHz), obscuring valuable information about the electronic structure. There are several laser-based techniques that can overcome the inhomogeneous broadening and the homogeneous (or natural) line width may be approached at low temperatures. This latter line width may be as narrow as 30 Hz (1 × 10 −9 cm −1) at liquid helium temperatures. In this article an overview of spectral hole-burning mechanisms and their application to coordination compounds is given, and recent progress in chromium(III) systems is emphasized. In the past, the main subject of investigations by spectral hole-burning has been the temperature dependence of the homogeneous line width and ultimate spectral resolution was sought. However, even at moderate resolutions of ca. 30 MHz (0.001 cm −1), spectral hole-burning is a very powerful technique that is capable of unraveling g-factors in low magnetic fields, dynamics of water molecules of crystallization, spin-lattice relaxation rates and hyperfine (and super-hyperfine) interactions in the excited state and the ground state. Besides being a highly successful spectroscopic technique, spectral hole-burning also has many potential applications in areas such as ultra-high density optical data storage (>100,000 GB/cm 3) and processing, laser frequency stabilization, portable frequency standards, etc. Coordination compounds may be tailored to fulfill the stringent requirements of such applications.