The investigators will combine experimental measurement and theoretical calculation to determine molecular nitrogen excitation and emission properties that are critical to the rate processes of nitrogen rich thermospheres. Molecular nitrogen is transparent to solar radiation from the infrared to the far ultraviolet where the forbidden Lyman-Birge-Hopfield and Vegard Kaplan band systems show weak, discrete absorption transitions. Strong absorption of solar radiation occurs in the extreme ultraviolet where the singlet ungerade electronic states show strong discrete blended structure. These states are closely spaced in energy and are strongly coupled. Many of them are strongly predissociative and a major source of chemical radicalization in the atmosphere. The strong coupling among the ungerade states results in very large rotational dependence of transition dipole matrix elements and predissociation yields. The strong rotational dependence in cross sections and predissociation yields requires that laboratory measurements must be translated to projected atmospheric temperature through modeling to avoid large errors. The impact of the strong rotational dependence has not been considered in many atmospheric models. Predissociation yields will be determined from a comparison of emission and excitation cross sections. The measured emission cross sections, together with the already measured, high resolution photoabsorption cross sections, will be utilized to determine the diabetic electronic transition moments. A coupled-channel model will be used to analyze the measurements and calculate spontaneous transition probabilities and predissociation yields. The research in this program connects the community concerned with the understanding of the thermosphere with researchers in laboratory geophysics and theoretical atomic and molecular physics. The program addresses fundamental physical properties of molecular nitrogen, which are critical to understanding the upper atmospheres of Earth, Titan and Triton. As such, the results of the work are applicable beyond the restricted targets identified here, and become a knowledge base for a broad range of disciplines. The program has extensive participation of undergraduate, graduate students and post-doctoral scholars.
Molecular nitrogen is the major atmospheric component of Earth. N2 excitation and emission by solar photoelectrons are important processes in the upper atmosphere. This project has investigated excitation and emission cross sections of N2 singlet ungerade states by experimental measurement and theoretical calculation. Accurate cross sections, oscillator strengths, and predissociation rates have been obtained at rotational level. The strong interaction between Rydberg and valence singlet ungerade states of N2 leads to large irregularity in energy positions, and spectral intensities of the transitions between the ground and singlet ungerade states. Since the temperature of Earth's upper atmosphere is much higher than room temperature, physical parameters measured in the laboratory are often insufficient for analysis of N2 day glow emissions. The theoretical model refined and extended by the results on present and earlier experimental measurements is capable of giving accurate photoabsorption cross sections, oscillator strengths and predissociation rates for many rotational levels. It is now possible to accurately model transitions between ground and singlet ungerade states at any temperature, excited by photon or electron. The funding of this project has supported researches that produce 6 publications in scientific journals and many presentations at scientific conferences. N2 cross section, oscillator strength and predissociation rates obtained in this work are fundamental physical parameters and constitute a knowledge database. They have wide applications in Earth and planetary atmospheres as well as plasma science. They have been used to place an upper limit of N2 abundance in the plumes of Enceladus (a satellite of Saturn). In addition to Earth atmosphere, the excitation and emission cross sections are also important for the investigation of the atmosphere of Titan. This project has also helped to provide training and support for three graduate students, including two female students. The theoretical and experimental parts of the project are a part of a Ph. D and MS thesis. All these students have obtained their Ph. D. or MS degrees.
