In this project supported by the Chemical Structure, Dynamics and Mechanisms Program of the Division of Chemistry, Professor William Jackson and his research group at the University of California at Davis will explore how light dissociates the diatomic molecule C2. This molecule absorbs light in the near infrared and visible region, which is unlike the other homogenous diatomic molecules, such as H2, N2, and O2, that are also made up of cosmically abundant atoms. As a result the C2 molecule is unique in that it will readily undergo optical pumping in the collision less environments that are characteristic of certain regions of planetary atmospheres, comets, and the interstellar medium. This leads to higher populations in excited rotational and vibrational levels for this molecule compared to others in this group. This can change the thresholds for photodissociation, and photoionization, for branching between electronic states of the atomic products, and for the coupling between the singlet and triplet manifolds of the excited electronic states of C2.
A focused ArF laser is used to prepare C2 in excited vibrational states of the ground singlet sigma state and the first excited triplet pi state in a seeded pulsed supersonic molecular beam. Then an unfocused ultraviolet (UV) and/or vacuum ultraviolet (VUV) laser sources will be used to prepare C2 in a particular excited electronic state from which the C2 radical can be either be photodissociated or photoionized. If it is photoionized the C2+ product can be detected via the time-of-flight mass spectrometer (TOF-MS). Photodissociation will produce carbon atoms in the triplet P, singlet D and singlet S electronic states. These atoms can be ionized with the VUV and UV lasers in the interaction region of the slice imaging apparatus. This strategy will allow us to investigate how electronic, vibrational, and rotational energy affect the threshold energy for photodissociation into carbon atoms in the triplet P, singlet D and singlet S electronic states, and determine precise thresholds for photoionization of C2.
This research will reveal the properties of the C2 molecule, which remains unstudied relative to other cosmically abundant diatomic molecules such as hydrogen (H2) and oxygen (O2). The research has implications for our understanding of the evolution of the universe and the origins of life because the ratio of carbon to oxygen in the interstellar medium determines whether the region will be oxygen rich or oxygen poor. The oxygen poor regions will evolve and produce carbon rich grains that will have the components that can form the basis of life. The studies in the Jackson laboratory will help to determine the ultimate stability of this molecule since it will define just how fast it will decay when exposed to UV radiation in the interstellar and interplanetary media. Participants in this research program include postdoctoral associates, graduate and undergraduate students.
Project Outcomes on NSF Grant CHE-0957872 A unique apparatus, Fig. 1, consisting of two tunable VUV lasers coupled to a slice imaging apparatus for characterization of the atomic products that can be used for photochemical studies of N2, NO, CO2, and CO in the wavelength region from 80 to 100 nm. Figures 2 and 3 show how images collected with this apparatus can be analyzed to produce Total Kinetic Energy Release, TKER, spectra that show the probability for producing atomic or molecular fragments with a particular kinetic energy. This information can then be used to determine how much of a particular reaction channel is produced at a particular photolysis wavelength region. The wavelength region between 80-100 nm is important in the thermosphere of the Earth's atmosphere because any molecules present there will absorb this high-energy solar radiation. We have used the apparatus developed under this grant to study for the first time the metastable atoms produced in the photodissociation of the molecules. We discovered that many of the reaction products are forbidden by the spin rule that states the total spin of the products are equal to the total spin of the reactants. In the case of N2 the branching ratio between spin allowed and the spin forbidden channels were measured between 12.5-14.1 eV. Not only are the spin-forbidden channels observed but they continued to be formed even when the spin allowed channels were energetically available. This shows how important it is to experimentally determine these ratios by directly exciting the nitrogen molecule with VUV radiation, which we have done. The spin forbidden reactions produce metastable excited N(2D) atoms. These atoms have enough energy to undergo endothermic chemical reactions and they live long enough to collide with N2 and O2 in the rarefied atmosphere between 150 and 200 km of the thermosphere. These excited metastable atoms will affect the chemistry and deposition of solar radiation in this atmospheric region. From a practical point of view it is important to characterize all of the reactions in the earths atmosphere so that we can accurately predict how solar radiation will affect the ultimate fate of the molecules that reach the thermosphere. It is also important from a fundamental point of view to understand what happens to the chemistry of excited molecules when you deposit large amounts of energy into them. Similar studies have been preformed on NO, CO2, and the CO molecules. Again, new unexpected chemical reaction channels have been observed and in some cases there branching ratios have been measured. In the case of NO we have observed reaction channels into neutral atoms above the ionization potential of the NO molecule. This shows that these channels can still compete with molecular ionization in this high-energy region of the solar spectrum. The CO molecule is a photodissociation product of CO2, it is isoelectronic with N2, and it is the second known most abundant molecule in the universe. We have therefore used our apparatus to study the photochemistry of this molecule and again we find that unexpected reactions occurred in the photodissociation. We observed C and O atoms in excited state that had not been predicted by theory. These excited atoms are also metastable and they will react with different rate constants in the regions of the universe where the densities are high enough so that they collide with H2 before they radiate the energy. The reactions can produce products that will lead to more complex molecules that are ultimately observed in the interstellar medium and protoplanetary disks that are the precursors for planetary formation. The amount of CO2 is known to be increasing in the atmosphere and since all of these molecules are not destroyed in the lower atmosphere this increase ultimately will be limited by photochemical decomposition. The oscillator strength of the absorption bands of CO2 in the low energy region between 8.53-10.6 eV are 340 times weaker when compared to the higher energy between 10.6 eV-13.8 eV, therefore, it is important to determine the chemistry in this higher energy region. It is in this region where the CO2 photodissociates with a high probability and this will ultimately limit its rise in our atmosphere. Preliminary results show that the spin selection rules can also be broken at these high energies and that the molecule can undergo internal conversion to the ground state surface. This molecule is one of the simplest examples of the complex behavior of the photochemistry of polyatomic molecules. The results of the studies on this molecule will be a benchmark for the theoretical understanding of much more complex molecules by allowing theorist to test ideas about the conversion of electronic to vibrational energy in an isolated molecule as well as how facile it is to flip spins in the outer electrons.
