Dissociative electron attachment to sulfur dioxide : A theoretical approach

In this article, density functional theory (DFT) and natural bond orbital (NBO) calculations are performed to understand experimental observations of dissociative electron attachment (DEA) to SO$_2$. The molecular structure, fundamental vibrational frequencies with their corresponding intensities and molecular electrostatic potential (MEP) map of SO$_2$ and SO$_2^-$ are interpreted from respective ground state optimized electronic structures calculated using DFT. The quantified MEPs and the second order perturbation energies for different oxygen lone pair (n) to $\sigma^*$ and $\pi^*$ interactions of S-O bond orbitals have been calculated by carrying out NBO analysis. The change in the electronic structure of the molecule after the attachment of a low-energy ($\leq$ 15 eV) electron, thus forming a transient negative ion, can be interpreted from the $n\rightarrow\sigma^*$ and $n\rightarrow\pi^*$ interactions. The results of the calculations are used to interpret the dissociative electron attachment process. The dissociation of the anion SO$_2^-$ into negative and neutral fragments has been explained by interpreting the infrared spectrum and different vibration modes. It could be observed that the dissociation of SO_{2}^{-} into S^{-} occurs as a result of simultaneous symmetric stretching and bending modes of the molecular anion. While the formation of O$^-$ and SO$^-$ occurs as a result of anti-symmetric stretching of the molecular anion. The calculated symmetries of the TNI state contributing to the first resonant peak at around 5.2 eV and second resonant peak at around 7.5 eV was observed from time-dependent density functional theory calculations to be an A$_1$ and a combination of A$_1$+B$_2$ states for the two resonant peaks, respectively. These findings strongly support our recent experimental observations for DEA to SO$_2$ [Jana and Nandi, Phys. Rev. A, 97, 042706 (2018)].