Electronic structure of hexafluorobenzene by high-resolution vacuum ultraviolet photo-absorption and He(I) photoelectron spectroscopy

The VUV photo-absorption spectrum of hexafluorobenzene, C6F6, recorded using the synchrotron radiation in the 4-10.75 eV photon range exhibits broad bands between 4 and 8 eV, and fine features above 8 eV. The broad bands are likely to be due to valence states dissociating into neutral fragments. They have been assigned to a π-π∗ transition, two π-σ∗ transitions and one σ-π∗ transition. A small shoulder observed at 7.36 eV, confirms earlier observations and is evidence of a further valence excited electronic state. Above 8.3 eV numerous fine structures are observed and the features are assigned by comparison with the high-resolution HeI photoelectron spectrum to a nd Rydberg series converging to the ground ionic state and a ns series associated with the first electronic excited state limit. The first two nd terms show vibrational structure. The origin of the vibrational structure is discussed in terms of Jahn-Teller distortion. For the 3s term, excitation of the vibration normal mode ν2′ is reported for the first time with a value of 0.066 eV, a value consistent with that of the ion. The HeI photoelectron spectrum of C6F6 has also been revisited and a new interpretation of the vibrational structure for the first and the fourth electronic bands is proposed. Finally, using a previously derived solar flux model, the atmospheric lifetime relative to photolysis is estimated and the consequences for atmospheric chemistry briefly discussed.