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

Abstract

The VUV photo-absorption spectrum of hexafluorobenzene, C6F6, recorded using the synchrotron radiation in the 4–10.75eV photon range exhibits broad bands between 4 and 8eV, and fine features above 8eV. 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 7.36eV, confirms earlier observations and is evidence of a further valence excited electronic state. Above 8.3eV 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 0.066eV, 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.