From Ground to Excited State: Revealing Bond Polarity Change via K^-2V Spectroscopy and Brillouin Inverse Theorem...

We report carbon and fluorine K^-2V double core hole (DCH) spectra of CH₃F, recorded by single-photon hard X-ray photoelectron spectroscopy. A striking inversion in intensity between the $K^{-2}6a_1\left({\sigma }_{CF}^{*}\right)$ and K^-27a₁(3s) resonances reflects a polarity switch in bonding character depending on the K-shell vacancy site. Within the Brillouin Inverse Theorem (BIT) framework, the transition strengths are shown to depend on the bielectronic integral $W_V=⟨{\psi }_V{\psi }_{1s}\left|\frac{1}{r_{12}}\right|{\psi }_{1s}{\psi }_{1s}⟩$ (V = σ*, 3s). The intensity ratio between σ* and 3s Rydberg states encodes the bond natures in the initial and final states, revealing both the character of the DCH bond and the recombination pathways to reconstruct the ground-state density. When $W_{{\sigma }^{*}}/W_{3s}\gg 1$, the bond evolves from (iono)covalent to ionic. Conversely, when $W_{{\sigma }^{*}}/W_{3s}\ll 1$, the bond evolves from (iono)covalent to covalent. This method is extended to CS₂ and CO₂, establishing definitively K^-2V spectroscopy as a direct probe of bond polarity.