A Theoretical Model for Indirect Dissociative Electron Attachment Iwona Anusiewicz, ²,‡,§ Monika Sobczyk, ²,‡ Joanna Berdys-Kochanska, ²,‡ Piotr Skurski, ²,‡ and Jack Simons* ,² Chemistry Department and Henry Eyring Center for Theoretical Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112, and Department of Chemistry, UniVersity of Gdansk, 80-952 Gdansk, Poland ReceiVed: July 12, 2004; In Final Form: NoVember 4, 2004 In this paper, we describe a computational model that allows us to avoid having to perform a very large number of tedious calculations on electronically metastable anions when studying indirect DEA processes. By indirect, we mean that the electron attaches to an orbital in one region of the molecule but a bond is subsequently broken in another region. For such events, one must describe the coupling between two diabatic anion states, corresponding to the occupation of orbitals in the two regions of the molecule, to achieve a correct description. We introduce a simple 2 × 2 matrix model as well as physically reasonable and computationally efficient approximations to the diabatic states in regions where they are metastable. We show this model to be highly effective when applied to several indirect DEA processes that we studied earlier with brute-force methods. The main advantage of using this model is that one can avoid having to carry out a large number of calculations on metastable anion states; only one or two such calculations are required. I. Introduction A. Direct Dissociative Electron Attachment. In a direct dissociative electron attachment (DEA) process, a free electron having kinetic energy E strikes a molecule A-B in which the fragments A and B are chemically bonded and enters directly into an antibonding orbital (e.g., a S-S σ* orbital in MeS- SMe), after which the nascent A-B - anion can undergo either electron autodetachment or A-B bond cleavage to form A + B - . Such a process is illustrated in Figure 1, where the bound A-B and repulsive (A-B) - potential energy surfaces are also shown. Let us consider an example of such a direct DEA event. It has been determined 1 that electrons having kinetic energies near 1 eV can enter into the S-S σ* orbital of dimethyl disulfide (MeS-SMe) and cause S-S bond rupture to form the MeS radical and the MeS - anion. In such processes, the dissociation yields depend on the competition between the rate of motion on the repulsive (A-B) - anion surface (the black curve in Figure 1) and the rate of electron autodetachment (represented by the green arrow in Figure 1). The shorter the autodetachment lifetime, the smaller the DEA yield; the steeper the repulsive curve, the prompter the dissociation, and thus, the higher the DEA yield. B. Indirect Dissociative Electron Attachment. Indirect DEA processes are different because they involve attaching an electron to a vacant orbital in one region of the molecule while breaking a bond in another region. An example of such a process is shown in Figure 2, in which the olefin π* orbital is where the electron attaches but the C-Cl σ bond is where the dissociation occurs. 2 In such indirect DEA processes, there are two anion states that must be considered: one with the excess electron in the olefin π* orbital and the other with the electron in the C-Cl σ* orbital. The appropriate potential energy curves for these two diabatic anion states are shown qualitatively in Figure 3 as functions of the C-Cl bond length (because this is the bond that ultimately is cleaved). Note that the π* anion curve is drawn as nearly parallel to the neutral molecule’s energy curve along the R C-Cl coordinate * Corresponding author. E-mail: simons@chemistry.utah.edu. ² University of Utah. ‡ University of Gdansk. § Holder of a Foundation for Polish Science (FNP) Award. Figure 1. Depiction of electron capture (red arrows), autodetachment (green arrow), and dissociation (black arrows) arising when an A-B molecule is struck by an electron and forms an A radical and a B - anion via direct DEA. Figure 2. Illustration of indirect DEA in which an electron enters a π* orbital and subsequently fragments the C-Cl σ bond to form the Cl - anion and a hydrocarbon radical. 484 J. Phys. Chem. A 2005, 109, 484-492 10.1021/jp046914f CCC: $30.25 © 2005 American Chemical Society Published on Web 12/30/2004