Luminescence of the Tetrachloroferrate(III) Ion

Excitation of a series of compounds containing the ${\mathrm{FeCl}}_4^{-}$ ion with ultraviolet or blue radiation at 85°K gave in each case a broad featureless luminescence band in the near infrared. Analysis of the decay curves obtained by using monochromatic excitation pulses and different observation wavelengths indicates that two emissive processes contribute to the emission. Excitation into a $\mathrm{d\to \; d}$ absorption band produces ${}^4{T}_1{\mathrm{(G)\to }}^6{A}_1$ luminescence, whereas excitation into the low energy tail of the charge transfer absorption gives a second luminescence process, there being little or no tendency for the ion to cross from this luminescent state to the ⁴T₁(G) although the latter is probably the lowest excited state. The ${\mathrm{FeBr}}_4^{-}$ ion under similar conditions gives a broad, near infrared luminescence band which also appears to consist of two transitions.