On the Threshold Photoelectron Spectrum of Fulvenone, a Reactive Ketene Derivative in Lignin Valorization.

Unveiling reaction mechanisms by isomer-selective detection of reactive intermediates requires advanced spectroscopic knowledge. We study the photoionization of fulvenone (c-C 5 H 4 =C=O), a reactive ketene species relevant in catalytic pyrolysis of lignin, which was generated by pyrolysis of 2-methoxy acetophenone. The highresolution threshold photoelectron spectrum (TPES) with vacuum ultraviolet synchrotron radiation revealed well-resolved vibrational transitions, assigned to ring deformation modes of the cyclopentadienyl moiety. The adiabatic ionization energy was determined to be 8.25 ± 0.01 eV and is assigned to the X + 2 A 2 ← X + 1 A 1 transition. A broad and featureless band arising at 9 eV is associated with the A + 2 B 1 ← X + 1 A 1 excitation. A conical intersection is responsible for the ultrafast relaxation of the fulvenone cation from the A + into the X +  state resulting in a featureless and lifetime broadened band. These insights will increase the detection capabilities for fulvenone and thereby help to elucidate reaction mechanisms in lignin catalytic pyrolysis.