From Ground to Excited State: Revealing Bond Polarity Change via K^–2V Spectroscopy and Brillouin Inverse Theorem (BIT)

We report carbon and fluorine K –2 V double core hole (DCH) spectra of CH 3 F, recorded by single-photon hard X-ray photoelectron spectroscopy. A striking inversion in intensity between the K − 2 6 a 1 ( σ C F * ) and K –2 7 a 1 (3 s ) 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 = ⟨ ψ V ψ 1 s | 1 r 12 | ψ 1 s ψ 1 s ⟩ (V = σ*, 3 s ). The intensity ratio between σ* and 3 s 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 σ * / W 3 s ≫ 1, the bond evolves from (iono)covalent to ionic. Conversely, when W σ * / W 3 s ≪ 1, the bond evolves from (iono)covalent to covalent. This method is extended to CS 2 and CO 2, establishing definitively K –2 V spectroscopy as a direct probe of bond polarity.