Optical Spectroscopy of Condensed Phase Interfaces

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Abstract
Why interfaces? A natural question with which t o begin our discussion is why should spectroscopes be concerned about interfaces, particularly those between condensed phases. Spectroscopists have traditionally worried about how the interaction of electromagnetic radiation with chemical and physical systems could be used to gain fundamental understanding into questions of structure and dynamics, but now are being asked to extend those measurements from simple bulk phase model systems and compounds to a new class of problems. These problems are characterized by features which are strange indeed to spectroscopists familiar with elucidating molecular parameters from measurements on randomly oriented (or completely oriented in the case of single crystals) ensembles of molecules. These new systems are of molecular dimensions in extent; they are typically partially ordered by the bulk phases which they terminate; they are physically inaccessible in many cases; and they reside in the presence of a huge excess of a bulk phase which is often of similar chemical and physical makeup. Clearly in the face of these daunting challenges there must be a compelling answer to our question, and it is simply this. For many structures, in nature and in technology, it is the interfacial regions which dominate the behavior. From the molecular biologist interested in recognition of self, to the electrical engineer trying to design high reliability VLSI circuitry, to the chemist interest in electrocatalysis, a common theme emerges. The properties of the interface must be understood, and spectroscopy can provide much of this understanding.