The shape of spectral lines: The importance of the far wings

Spectroscopy, the study of the interaction of radiation and matter, provides most of the information we have gleaned about the composition, structure, and evolution of the universe. As is well known, by measuring the frequencies of spectral lines in absorption or emission, one can uniquely infer the presence of atoms or molecules as well as their physical state and environment (e.g., solid or gaseous, neutral or ionized, moving or stationary, etc.). Furthermore, by studying the intensities of these lines, one can determine the abundance (i.e., number of a particular species per unit volume). Although less well known, the shape of the spectral lines, in particular, the structure of the far wings, plays a very important role in many important atmospheric phenomena such as the greenhouse effect or the absorption of harmful ultraviolet radiation. Although first measured more than 50 years ago, the anomalous absorption of radiation by water vapor in the earth's atmosphere was postulated to be due to far wings of allowed lines. However, only within the past few years has a quantitative verification of this hypothesis been possible through the development of an accurate theoretical description of the shape of self-broadened water lines. During the summer, work has been done on improving this theory and in comparing the results to other theories valid near the center of the lines. The relevance of this work to measurements of greenhouse gases, of earth-based measurements of the 3 K cosmic background radiation, of satellite-based measurements of the atmospheres of the earth and other planets, and other similar problems will be discussed briefly.