Threshold Photoelectron Spectrum of the Phenoxy Radical

The high-resolution photoion mass-selected threshold photoelectron spectrum (ms-TPES) of the phenoxy radical (C₆H₅O^•), produced by pyrolysis of anisole, was investigated at the VUV beamline of the Swiss Light Source. Adiabatic ionization energies have been determined to be 8.56, 9.42, 9.76, and 9.94 eV to the X̃⁺¹A₁, ã⁺³A₂, Ã⁺¹A₂, and b̃⁺³B₂ cation states, respectively, supported by DFT, WFT, and composite-method calculations. A ring deformation mode was found to be active upon ionization by Franck-Condon analysis and responsible for the vibrational structure of the TPES in all four ion states. While the X̃⁺¹A₁ and ã⁺³A₂ states' assignment agrees with the literature, we revise the energetic order of the Ã⁺¹A₂ and b̃⁺³B₂ cation states in the ms-TPES, based on a pronounced lifetime broadening of the excited triplet state. This is rationalized by strong coupling between the triplet states as confirmed by EOM-EE-CCSD calculations indicating a conical intersection with a low-lying seam to the ã⁺³A₂ state. Our study provides a well-resolved spectrum, to be used for the isomer-selective assignment of reactive species in combustion and catalysis and also serves as benchmark to evaluate theoretical methods to address closed- and open-shell singlet and triplet cation intermediates.