Transient absorption spectra of chlorine and bromine

Abstract
New absorption spectra of chlorine (2500 to 2100 Å) and bromine (2900 to 2400 Å are reported. They were obtained by the flash photolysis of a mixture of halogen and inert gas. The spectral lifetimes were never > 50 μs and the intensities were approximately in phase with the light output of the photolysis lamp. Both spectra consist of a long, slowly converging v " = 0 progression of bands which degrade to the red. Whereas the bromine bands obtained on medium and large quartz spectrographs revealed no fine structure, the chlorine spectrum taken on a grating instrument was seen to possess a diffuse, rotational structure. A rotational analysis of the chlorine spectrum was made and band head wavelengths and molecular constants for both halogens are recorded. By means of an isotope effect a v ' numbering of the chlorine bands has been put forward. It is thought th at the spectra result from a two-quantum process and th a t the final transition is from 3 Π o+, u to a higher level, possibly 0 + g . A vibrational perturbation occurs in the chlorine spectrum and predissociation is postulated. The effect of filters, inert gases and oxygen on the spectra is discussed and reasons are suggested why analogous spectra for bromine chloride and iodine were not observed.

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