Tunable VUV pump and tunable VUV probe experiments on the photodissociation dynamics of atmospheric molecules using slice velocity imaging for detection

University Of California-Davis, Davis CA

Investigators

Abstract

With this award, funded by the Chemical Structure, Dynamics, and Mechanisms - A Program in the Division of Chemistry, Prof. William M. Jackson from the University of California, Davis will characterize the photochemical channels of molecules important in planetary atmospheres, comets, and the interstellar medium in the spectral region between 190.8 and 73.8 nm. The initial studies will be done with the nitrogen, oxygen, nitric oxide, and carbon monoxide molecules. The first three molecules are important for the chemistry of the upper atmosphere, while the last, carbon monoxide is the second most abundant molecule in the interstellar medium and is important in the early development of solar systems. A unique apparatus has been developed with two tunable VUV lasers based on resonance enhanced four wave mixing that has a bandwidth of 0.4 wavenumbers and is tunable between 190.8 and 73.8 nm. One of these tunable lasers can be used to excite the molecule to a particular energy state characterized by the electronic, vibrational, and rotational energy in the molecule and the other can be used to ionize the product atoms that are formed in the dissociation. These lasers are connected to a slice imaging apparatus that can determine the mass as well as the velocity and angular distributions of the product atoms. This information provides critical information about how the specific internal energy of an excited molecule affects the conservation of spin and the fine structure distributions of the product atoms and it provides a critical test for the best theoretical calculations for excited state potential energy curves as well as the dissociation process that occurs on these curves. Simple molecules such as nitrogen, oxygen, nitric oxide, and carbon monoxide only absorb high-energy solar light. This light cannot be seen by the naked eye and it will not travel in air. Yet this light from the sun determines how long molecules such as nitrogen, oxygen and NO can exist in the earth's and other planets atmospheres. It provides the energy for the chemical reactions occurring in these regions as well as those that occurred during the early years of planet formation in this and other solar systems. To truly understand the chemical reactions that do occur in the atmosphere, it is important to understand how this solar light forms the reactive atoms and fragments. These reactive species, in turn, undergo chemical reactions with stable trace species such as carbon dioxide that help determine the climate here on earth. Solar light with different amounts of energy can form nitrogen and oxygen atoms from nitrogen and oxygen molecules that can react differently with both the trace species such as carbon dioxide as well as with the oxygen and nitrogen molecules themselves. The work that will be performed on this grant will provide the information about how the energy of the atoms that is formed changes with the solar energy of the solar light that is absorbed. This information can then be put in the models that are used to predict the future behavior of trace components in our atmosphere as well as the atmospheres of other planets and astronomical bodies.