Bromine predissociation

Spontaneous predissociation of the B state as well as direct photodissociation has recently been observed by Lum and McAfee, using excitation at S14.3 nm [in a single axial mode of an argon-ion laser Oinewidth 10 nm)] of a molecular beam of bromine. Predissociation is the predominant decay mechanism of BrifB) excited at S14.S nm. Similar results for 558 nm excitation have been reported by Moore et al ... 

The emission from molecular halogens in the 3n0u+ state resulting from shock heating could, in principle, arise from both inverse predissociation and direct recombination on account of the large thermal populations of the excited atoms at these high temperatures. Emission from this state has been observed hitherto at high temperatures from iodine, bromine, and chlorine... 

Usually, the processes are stopped by addition of a quench gas to the main filling gas. Vapours of polyatomic molecules such as ethanol, ether, ethyl formate, methane, bromine or chlorine may be applied. Because of the lower ionization energy of these molecules, the positive charge of the ions is transferred to the molecules and these dissipate their energy by dissociation or predissociation. Chlorine and bromine exhibit strong absorption of the photons emitted they dissociate, recombine and return to the ground state via a series of low-energy excited states. 

Elements.—The use of a tunable narrow-frequency laser to produce an isotopically selected chemical reaction has been described by Leone and Moore 1 this approach should permit efficient isotope separations to be performed. In the example reported, natural Br2 (79Br/81Br = 1) is photo-predissociated by selective excitation into the 3n0+ state. Bromine atoms, enriched in one isotope, react with HI before scrambling occurs, to produce 80—85% enriched H81Br. 

The absorption coefficient of Bt2 in the visible region is intermediate between those of Oi and l2- Hence for Bt2, fluorescence studies are more difficult, and bromine has only recently attracted attention. As for I2, large variations of lifetime and cross-sections as a function of v have been reported, and the short lifetime of the excited state (less than anticipated from the absorption coefficient) indicates that for Bt2 the radiative lifetime contributes little to the measured lifetime, and that non-radiative processes are very important. Capelle, Sakurai, and Broida suggest that the predissociation rate is proportional to the inverse square of the molecular mass. However, the relatively large bandwidth used by these workers (0.1—0.8 nm) makes such a conclusion doubtful, and the variation of lifetime with vibrational-rotational state would seem to be more complex, as recently found by McAfee and Hozack. ... 

It is very likely that an excited halogen molecule suffers induced predissociation which is the first (initial) step of the given elementary reaction. Por this reason it may be considered that the halogen atoms are the primary active centers of halogen-sensitized photochemical reactions in virtually every case. A peculiar case of photosensitization with bromine has been observed for carbon monoxide oxidation [278]. The process Brg + hv ->2Br has been found to be followed by Br + O2 + CO -> BrO + CO2, etc. 

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