Electronically excited halogen atoms atomic transitions

Electronically excited halogen atoms in the np5 2P% state are optically metastable as the transition... 

Since the spectra show little change in position or intensity as the series is ascended, jt-it transitions are excluded. Figure 6 shows that the spectra are comparatively slightly affected by solution of the chlorides in sulfuric acid absorption cannot, therefore, be due to an n-ir transition from the nitrogen atom. It may be a result of excitation of the unshared electrons on the halogen atoms, consistent with the effect of bromine substitution on the position and intensity of the absorption band. The ultraviolet spectra therefore give no direct information on the structure of the ring. 

Exciplex lasers (also called excimer lasers) use reactive halogen atoms to form excited pseudo-molecules with noble-gas atoms. Molecules such as XeF are stable on ly in excited electronic states and quickly dissociate after transition to the ground state. This makes possible a large population inversion and produces laser action in the ultraviolet region. A simple prototype for such behavior is the Hcj excimer, which is an entry in Table 11.1. 

The gas-phase photochemistry of haiogenated ethenes has been studied in the UV and VUV [60, 61], as well as in the infrared, using multiple-photon-absorption excitation with powerful CO2 laser sources [62-66]. Also, sensitized decompositions, for example using electronically excited Hg( P) atoms, have also been reported [67-69]. The net gas-phase photochemistry of these systems usually involves hydrogen halide elimination via three-and/or four-center transition states, with some evidence for simple bond fission producing halogen atoms in the case of Hgf Pj) photosensitization [70]. 

The electronically excited rare gas atom is sometimes called a super-alkali because its ionization potential is so low. There are also super-halogens and not only super-alkalis (Herschbach, 1966 Bersohn, 1976). The super-halogen has a particularly high electronic affinity NO2, with an electron affinity of about 2.4 eV, is an example so is (CN)2. Excited states of organic molecules are used as effective electron donors. Complexes of transition metals in unusually high oxidation states are keen acceptors and vice versa for complexes where the metal is nominally neutral. 

The electronic spectra of the halogens arise from absorption bands corresponding to the electronic transitions, in which an electron is excited from the antibonding Ttg to the antibonding orbital (equation 6). The separation between these two orbitals decreases as the atomic number of the halogen increases. 

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