Light from the sun drives the chemistry of the atmosphere. The chemistry is initiated by the absorption of light by various gases. Furthermore, each gas absorbs the various colors (frequencies) of the light (the electromagnetic spectrum) with varying efficiency. This efficiency is an important factor for how deeply into the atmosphere a given color can penetrate and consequently at what altitudes a particular chemical pathway will be important. In the upper atmosphere, very energetic photons (colors of high frequencies) representing the vacuum ultraviolet (VUV) region of the electromagnetic spectrum are weakly absorbed by molecular oxygen, the second most abundant gas in the atmosphere. Molecular oxygen is therefore important for the removal of these photons. The absorption efficiency (or cross section) of oxygen in this spectral region is particularly difficult to measure because of a complicated system of narrow absorption lines known as the Schumann-Runge bands. Accurate measurements require careful work with high resolution of the electromagnetic spectrum. Other important processes in the upper atmosphere involve the absorption of VUV by nitrogen oxide (NO) which similarly has a complicated spectrum whose absorption cross sections are inadequately known. This project is a continuation of research supported by NSF under Grant ATM-8318960. VUV cross section measurements of oxygen and nitrogen oxide will be performed with a recently constructed VUV Fourier transform spectrometer that can achieve a high spectral resolution necessary for determination of absolute cross sections of molecular bands possessing distinct rotational structure. Cross sections are to be obtained at temperatures of 78K and 300K. These data will allow more accurate modeling of the transmission of energetic light through the upper atmosphere than is currently possible and will benefit various research efforts in atmospheric chemistry and aeronomy.
