Applications and Impact of THz Spectroscopy
High resolution THz spectroscopy has had a major impact on many important fields of science and technology. The earliest studies in this region were of species such as H2O, O2, NO, CH3F, and OCS and served to both establish spectroscopic methodologies and to provide basic information about molecular structure and interactions [1].
Because the strength of the interactions between electromagnetic radiation and molecular rotation peaks sharply in the THz, this spectral region has also been well suited for the study of reactive species such as free radicals [15].
A variety of spectroscopically based remote sensing applications has grown out of this more basic work. Of these, two have become of major importance. The first is the study of the chemical processes in the upper atmosphere which are important in ozone formation and destruction [26]. We will discuss each of these in more detail below.
Because of these and other applications (e. g. the modeling of atmospheric propagation) the spectroscopic properties of virtually all of the important atmospheric and astronomical species have been collected into data bases. These data bases have become the standard for many applications and play an important role in the development of the spectral region. The Submillimeter, Millimeter, and Microwave Spectral Line Catalog has been maintained by the Jet Propulsion Laboratory for many years [6]. Likewise, the HITRAN Molecular Spectroscopic Database has been maintained by the Air Force [7]. While the latter began primarily as an infrared database, the growth in both infrared and submillimeter experimental technologies has been such that for many molecular species the best spectral data base results from a weighted fit of infrared and microwave data to a theoretical model.
Although most of the spectroscopic work in this spectral region historically has been referred to as millimeter and submillimeter spectroscopy, in this chapter we will for the most part use the term THz. An interesting study of the relationships among the communities that work in this spectral region can be done by using an Internet search engine to explore 'THz' and 'submillimeter' Boolean combined with 'spectroscopy'.
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