Knowledge Bank

A weak absorption which appears in the infrared spectrum of matrix isolated bromine, in the frequency range $299-311cm-1$, is attributed to the presence of a molecular aggregate likely to be the double molecule $(Br2)2$. Matrix isolation studies were carried out at $22\circ$ and $77\circ K$ using various matrix materials including argon, krypton, xenon, nitrogen, sulfur hexafluoride, carbon dioxide and carbon monoxide. Concentration and temperature dependent studies of matrix samples and spectral investigations of polycrystalline films of solid bromine contribute to the aggregate of data which suggest that the spectral feature results from a molecular species present in the gas phase and not from some esoteric coupling in or local polarization effect induced by the condensed state. Revealing temperature dependent studies were carried out over the temperature range $22-100\circ K$ on samples deposited at $22\circ K$ and $77\circ K$. Spectral effects resulting from slow warming of a matrix isolated sample deposited at $22\circ K$ are characterized by a nonreversible intensity enhancement followed by an annealing out of the absorption. The latter is thought to represent a relaxation of the reagent bromine into the normal halogen crystal structure which shows no infrared absorption in the fundamental region. A similar annealing out was observed for solid bromine. A weakly bonded bromine dimer might be expected to give rise to a single broad band $(\Delta \sim 10cm-1)$ in the infrared near the reported fundamental frequency for diatomic bromine $(323cm-1)$. The existence of such a dimer or double molecule in the gas has been postulated and some experimental evidence in support of this has been reported. Although infrared absorption by bromine has been observed before, the present data are the first which suggest absorption by an aggregate species.