Electronic spectroscopy of bromopentafluorobenzene by high-resolution vacuum ultraviolet photoabsorption, electron impact, and photoelectron spectroscopies
Introduction
Bromopentafluorobenzene, C6F5Br, is a brominated compound which is used in the production of pesticides and the preparation of catalysts [1]. It has also been proposed as a potential source of active CFx radicals and ions for silicon etching in industrial plasma reactors [2].
As industrial use increases the probability of C6F5Br release into the terrestrial atmosphere, it is important to explore the behaviour of this molecule when subjected to solar radiation. C–Br bonds are much weaker than CF-bonds in perfluorinated molecules and can be photolysed by solar radiation relatively easily [3], releasing highly reactive bromine atoms which can increase stratospheric ozone depletion. Accordingly it is necessary to characterise the electronic states of C6F5Br since its photolysis rate will be determined by the excitation energy and symmetry of the lowest excited states. The formation of CFx radical species needed in semiconductor plasma reactors for silicon etching will also be dependent upon the dissociation dynamics of these states. However, to date there is little data, either experimental or theoretical, on the excited states of neutral and ionic bromopentafluorobenzene. Indeed, to the authors’ knowledge, the only spectroscopic data on C6F5Br are three He(I) photoelectron spectra [4], [5], [6] recorded at much lower resolution (200 meV) than the present work. There are no previous photoabsorption studies or electron impact investigations.
The present work aims to elucidate the neutral and ionic electronic excited states of C6F5Br on the basis of new high-resolution photoabsorption, electron impact and photoelectron spectroscopic data. Calculated atmospheric photolysis rates are also reported for the first time for this significant industrial molecule.
Section snippets
Experimental
The electronic states of C6F5Br have been studied using different experimental techniques including: VUV photoabsorption using a synchrotron source, which provides data on optically allowed transitions and allows absolute photoabsorption cross-sections to be measured; high-resolution electron energy loss spectroscopy (HREELS) performed under non strictly electric dipolar interaction conditions which allows the character (allowed vs. forbidden) of the transitions to be explored and He(I)
Properties of bromopentafluorobenzene
The structural constants for C6F5Br, determined experimentally for the gas-phase [11], [12], are summarized as follows:
Experimental:
C–C = 1.394 Å
C–F = 1.327 Å
C–Br = 1.879 Å
Angle CCC = 120°
Angle CCF = 120°
The molecule belongs to the C2V symmetry class in its gas-phase electronic ground state. This is in contrast with hexafluorobenzene (C6F6) that has been found to belong to the D6h symmetry group. Despite the different symmetry classes, comparison with C6F6 helped in the assignment of the features observed
Valence and Rydberg states
The high-resolution absolute VUV photoabsorption spectrum of C6F5Br recorded between 4.0 eV and 10.8 eV is shown in Fig. 1. It can be split into two regions; the first between 4 eV and 7 eV is composed of broad bands and the second from 7 eV to 10.8 eV with several broad bands with superimposed fine structure. The shape of the absorption spectrum is confirmed by the high-resolution electron energy loss spectrum recorded between 4 eV and 13.0 eV and shown in Fig. 2. The HREELS result also shows the
Absolute cross-sections and UV photolysis rates
The absolute cross-section values of C6F5Br have been used to model its atmospheric destruction by UV photolysis as a function of altitude. The photolysis rates of C6F5Br were calculated at a given wavelength as the product of the Solar Actinic Flux [18] and the molecular photoabsorption cross-section by 1 km altitude steps from the surface up to the stratopause (50 km). At each altitude, the total photolysis rate may be calculated by summing over the individual photolysis rates for that
Conclusions
The present work provides the first experimental data on the electronic spectroscopy of C6F5Br in the 4.0–13.0 eV energy region. Most of the valence excited states are slightly shifted to lower energy compared to hexafluorobenzene, an effect ascribed to the presence of fluorine atoms in the molecule. The high-resolution of the measurements has enabled the present analysis to be extended to the fine structure associated with vibrational excitation in the Rydberg state region. In addition, precise
Acknowledgements
The Patrimoine de l’Université de Liège, the Fonds de la Recherche Scientifique (FRS-FNRS) and the Fonds de la Recherche Fondamentale Collective of Belgium have supported this research. M-J H-F wishes to acknowledge the Fonds de la Recherche Scientifique (FRS-FNRS) for her position. SE and NJM acknowledge the support from the British Council and Engineering and Physical Sciences Research Council (EPSRC). The authors wish to acknowledge the beam time granted at the ISA synchrotron facility,
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Directeur de recherches du FRS-FNRS of Belgium.