Slice Imaging Studies of Vibration, Rotational and Electronic Mediated Photodissociation and Photoionization of C2

University Of California-Davis, Davis CA

Investigators

Abstract

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.