Photochemistry of chlorine oxides in the ultraviolet and vacuum ultraviolet (VUV)

General context: Chlorofluorocarbons (CFC), which are emitted at earth's surface on a global scale, are capable of ascending to high altitudes before being photolysed by strong UV radiation. This photolysis leads to the formation of atomic chlorine which intervenes in the catalytic cycles of stratospheric oxygene chemistry. It is well known today, that the increase of stratospheric chlorine provokes a net depletion of the ozone layer, especially over the polar regions.

Among the numerous stratospheric reactions cycles [1], the bimolecular reaction between two molecules of ClO (or ClO + BrO) leads to the formation of the OClO radical. The latter can be observed in-situ because of its strong UV/VIS absorption [2]. Cox et al. proposed that at stratospheric temperatures a dimer of ClO can be formed by a termolecular reaction [3]. The important question about these chlorine oxides concerns their interaction with the stratospheric reaction cycles, in particular as to whether they contribute to a net reduction of ozone. In this context, it is substantial to know the branching ratios of primary photoreactions following UV excitation.

Experiments performed and principal results: The aim of my studies was the determination of the branching ratios of the primary photoreactions of OClO and Cl₂O₂. OClO, an explosive gas, was produced in the laboratory in-situ by the reaction NaClO₂ + Cl₂ ® 2 OClO  + NaCl. For the synthesis of Cl₂O₂, its stratospheric formation reaction (2 ClO + M ® Cl₂O₂ + M) was reproduced in a standard flow tube (for details see my JPC1996 paper > publications). I used a two-colour laser experiment l_pump = 308-410 nm, l_probe = 113.56 nm) in order to promote the molecules into their 1^st excited states (pump photon) and detect the primary photoproducts as well as remaining reactant molecules by means of time-of-flight (TOF) mass spectrometry. All species were ionised by one photon ionisation. Concerning the photochemistry of OClO, we showed that the predissociation OClO (A(²A₂)) ® ClO (X²P) + O (³P) is the principal decay route of OClO (A(²A₂)). The photoreaction OClO (A(²A₂)) ® Cl (²P) + O₂ (a¹D_g, X(³S_g^-)), which could result in a contribution to stratospheric ozone depletion because of the formation of atomic chlorine, was shown to be a minor fragmentation pathway with a quantum yield inferior to 7 % upon excitation of the OClO bending modes around 400 nm. Concerning the photochemistry of Cl₂O₂ we determined its ionisation energy, which was not known previously, using synchrotron radiation (BESSY I, Berlin). We found that IE (Cl₂O₂) = 11.05 ± 0.5 eV (112.2 ± 0.6 nm). With the help of quantumchemical calculations, we were able to conclude that the isomer ClOOCl is formed during the termolecular formation of Cl₂O₂ (see article 1). Preliminary results on its primary photoreactions showed that the Cl atom forming reaction channel may have been overestimated in previous experiments.