Submitted to: CHEMICAL PHYSICS
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2008
Publication Date: 6/12/2008
Citation: Han, J., Heaven, M.C., Schnupf, U. 2008. Spectroscopy, Dissociation Dynamics and Potential Energy Surfaces for CN(A)-Ar. Chemical Physics. 128(22):224309. Interpretive Summary: Radicals in gas phase and aqueous solutions play a significant role in radiation chemistry and biology, heterogeneous atmospheric processes and are often implicated as enzymatic intermediates. However, the understanding of the structure and spectroscopy of even simple radicals is very limited. Currently, the world scientific community is interested in investigating the specific effects that govern the interactions of free radicals with their substrates as they undergo chemical reactions. Understanding how to activate chemicals, e.g. via laser excitations, so they will undergo selective chemical reactions is of outmost importance to the chemical industry but also for atmospheric/environmental remediation. An excellent case in point is the cyano radical, CN, because of its importance in combustion and atmospheric chemistry. This study of the interaction of the CN radical with Argon (Ar) is a continuation of experimental and theoretical studies of a series of simple radicals. In the current paper a structural and spectroscopic characterization of the CN-Ar interaction complex in its highly reactive A-state is presented. Employing only the newest and best experimental and computational tools, we can relate previous information from structural/spectroscopic observations obtained by other researchers to details on the basic structure of CN and Ar-CN. This work allows a better understanding of the energetics involved to precondition the CN radical via laser excitation to probe the interaction of Ar with CN. These studies will lead to more efficient designs for chemical reactions involving simple radicals used in numerous commercial applications.
Technical Abstract: The A2Pi-X2Sigma+ band system of CN-Ar has been examined using fluorescence depletion and action spectroscopy techniques. Eight vibronic bands of the complex were observed in association with the monomer 3-0 transition. Pump-probe measurements were used to characterize CN(A2Pi3/2, v=3) fragments from direct photodissociation of CN(A2Pi, v=3)-Ar, and CN(X2Sigma+, v=7) fragments from CN(A2Pi, v=3)-Ar predissociation. The latter showed a marked preference for population of positive parity diatomic rotational levels. Bound state calculations were used to assign the A-X bands, and to obtain fitted potential energy surfaces for the A state. The average potential obtained from fitting had a well depth of De=137.8 cm-1. High-level ab initio calculations were used to obtain equilibrium Jacobi coordinates of Theta e=94 degrees and Re=7.25 bohr. The near symmetric character of the fitted potential energy surface was consistent with the symmetry preference observed in the predissociation dynamics.