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 Title: Valence-hole Electron Configurations: A New Global Electronic Structure Paradigm For C2 And Beyond Author(s): Jiang, Jun Contributor(s): Field, Robert W; Nauta, Klaas; Schmidt, Timothy Subject(s): Electronic structure, potential energy surfaces Abstract: The global electronic structures of C$_2$\,$^1\Pi_g$ and $^3\Pi_g$ states, up to the energy of the first symmetry-allowed C(2s2p$^3$)+C(2s$^2$2p$^2$) dissociation channel, are modeled using a diabatic state interaction picture. The experimental observations ($v$=0-8 of $C^1\Pi_g$; $v$=0-12 of $d^3\Pi_g$ and $e^3\Pi_g$, and five $v$ levels of the coupled $3,4^3\Pi_g$ states) are reproduced with an average residual of 0.3\,cm$^{-1}$. The key concept behind the diabatization is the valence-hole electron configurations, for which an electron in the anti-bonding $2\sigma_u$ orbital is excited to a higher energy orbital. For C$_2$, when this electron is excited to a bonding orbital (i.e.\,$3\sigma_g/2\pi_u$$\leftarrow$$2\sigma_u$), the valence-hole configuration is the lowest energy configuration within a given symmetry manifold. These valence-hole configurations have a nominal bond-order of three, and correlate to the excited C(2s2p$^3$)+C(2s$^2$2p$^2$) channel with 2p$\leftarrow$2s electron promotion in one of the carbon fragments. A $diabatic$ valence-hole state with bond-order of three is expected to have a high dissociation energy, and in our model, it dissociates into the lowest symmetry-allowed C(2s2p$^3$)+C(2s$^2$2p$^2$) channel. As $R$ increases, this deeply-bound valence-hole state crosses several other low-lying electronic states (derived from the $2\sigma_u^22\sigma_g^2$ configuration), all of which dissociate into the low-energy C(2s$^2$2p$^2$)+C(2s$^2$2p$^2$) channels without 2p$\leftarrow$2s promotion. The curve-crossings between the valence-hole and the $2\sigma_u^22\sigma_g^2$-type states in C$_2$ are analogous to the ionic (A$^+$B$^-$)/covalent (AB) curve crossings in more ionic species. In both cases, the electronic structure landscape of the low-lying valence states is systematically disrupted due to their curve crossings with a single diabatic state (valence-hole or A$^+$B$^-$). We believe that the valence-hole state is an important but hitherto neglected feature in the electronic structure model. The C$_2$ molecule offers a perfect platform to study the role of valence-hole states in causing fundamental changes of the global electronic structure landscape. The valence-hole-induced curve crossings in C$_2$ occur at much lower excitation energy than the analogous ones in N$_2$, CO, NO, and O$_2$. The effects from these curve crossings are gloriously and uniquely sampled in the molecular constants of low-lying states of C$_2$. Issue Date: 2021-06-24 Publisher: International Symposium on Molecular Spectroscopy Genre: Conference Paper / Presentation Type: Text Language: English URI: http://hdl.handle.net/2142/111530 Date Available in IDEALS: 2021-09-24
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