1
THE PHOTOABSORPTION AND PHOTOIONIZATION OF HALOGENATED ETHYLENES: BROMINATED DERIVATIVES.
R. Locht1, B. Leyh1, H.W. Jochims2, H. Baumgärtel2.
1 Molecular Dynamics Laboratory, Department of Chemistry, Institute of Chemistry,
University of Liège, Sart-Tilman par B-4000 Liège 1, Belgium.
2Institut für Physikalische und Theoretische Chemie, Freie Universität Berlin,
Takustrasse 3, D-14195 Berlin, Germany.
In our program on the dynamics of highly excited states of the halogenated derivatives of ethylene, particular attention has been paid to the brominated derivatives [1]. The main motivation is not only their importance in chemical and technological applications but also the scarcity, or even the lack of spectroscopic data related to these compounds.
In 2000, a thorough investigation of C2H3Br has been presented. The vacuum UV
photoabsorption spectrum [2] as well as the threshold and He(I)-photoelectron spectrum [3] have been published. The CIS-spectra of the first six electronic states were measured [3]. The unimolecular photodissociation dynamics of C2H3Br+ in the C2H3++Br channel has
been studied by the maximum entropy (MEM) method [4]. Finally, the ion-pair formation and dissociative photoionization of this molecule have been investigated by mass spectrometry and TPEPICO techniques [5]: the C2H3++Br¯,
C2H3++Br, C2H2++[H,Br] and Br++[C2,
H2] channels were considered.
This research program has been continued by recording the photoabsorption spectrum and the dissociative photoionization mass spectrometry of brominated Figure 1.
2 atives of ethylene, e.g. C2HBrF2 and C2BrF3 in the 5-20 eV photon energy range. The
present work has been performed on the 3m-NIM-2 beamline equipped with a 600 ℓ/mm Al grating and entrance and exit slits of 30 µm and 50 µm widths respectively. In addition, the photoelectron spectrum and the kinetic energy distributions of the fragment ions observed at the energy of the HeI (21.2 eV), NeI (16.67-16.85 eV) and ArII (13.47 eV) resonance lines have been recorded in the laboratory.
Fig. 1 shows an example of photoabsorp-tion spectra obtained respectively for (1) C2HBrF2, (2) C2BrF3 and
(3) C2ClF3 in the 6-12 eV
photon energy range. The spectra related to the latter two compounds clearly differ from the former by the @→@* transition. This transition, located at about 6.8 eV for C2HBrF2, is
blue-shifted by about 0.8 eV when the H atom is substituted by F. By contrast, the substitution of the Br atom by Cl has no significant influence on either the shape or the energy position of the spectral features. Further-more, the H-substitution leads to a severe quen-ching of the Br (or Cl) lone pair excitation (9-11 eV photon energy range). A deeper analysis of these spectra, combined with all our previous results on about more than ten halogenated ethylene derivatives, is in progress.
The dissociative photoionization of C2HBrF2 and C2BrF3 has been recorded by
mass spectrometry at variable wavelength. Almost all fragment ions present in the 20 eV photon energy mass spectrum have been considered.
Fig. 2 shows a typical example of results obtained by HeI photoelectron spectroscopy and mass spectrometric photoionization. The photoionization efficiency curve (PIC) of the molecular ion C2HBrF2+ and its first derivative (dI+/dE) as observed
3 in the threshold region are displayed in fig. 2 (bottom). This figure clearly shows the complex pattern of autoionization and direct ionization. The best way to disentangle these data is to record both the HeI photoelectron spectrum (PES) and the photoabsorption spectrum (PAS) shown in fig. 1. The latter clearly shows that the ionization threshold region is dominated by Br-lone pair Rydberg transitions. Fig. 2 (upper part) shows the structure for the first HeI PES-band. Several features present in this spectrum correlate with those observed in the PIC by resonant photoionization and are ascribed to direct ionizing transitions. A systematic study is in progress.
Acknowledgments.
The authors gratefully acknowledge the Freie Universität Berlin. R.L. and B.L. are indebted to the Belgian "Fonds National de la Recherche Scientifique" (FNRS) for financial support. The authors wish to thank the BESSY staff, and particularly Dr. G. Reichardt and Dr. W. Braun, for their essential collaboration.
References.
[1]. R. Locht, B. Leyh, H.W. Jochims, H. Baumgärtel, Jahresbericht (2003), 48. [2]. A. Hoxha, R. Locht, B. Leyh, D. Dehareng, K. Hottmann, H.W. Jochims, H. Baumgärtel, Chem.Phys. 260 (2000) 237.
[3]. A. Hoxha, R. Locht, B. Leyh, D. Dehareng, K. Hottmann, H. Baumgärtel, Chem.Phys. 256 (2000) 239.
[4]. A. Hoxha, R.Locht, A.J. Lorquet, J.C. Lorquet, B. Leyh, J.Chem.Phys. 111 (1999) 9256.
[5]. A. Hoxha, B. Leyh, R. Locht, H.W. Jochims, M. Malow, K.-M. Weitzel, H. Baumgärtel, to be published.
