Knowledge Bank

We have observed orange-red emission in $K2Cr2O7$ single crystals at low temperature. At 12 K the emission maximum is 685 nm and the lifeline is 0.5 msec. The appearance of the spectrum is that of weak structure superimposed on a broad, stronger continuum emission (bandwidth of $3500cm-1$ FHHM). The intensity is strongly temperature dependent as it decreases by a factor of ten between 12 and 17 K. In contrast, single crystals of $Na2Cr2O7$ and $(NH4)2Cr2O7$ are not luminescent at any temperature. A similar emission is found for dilute, frozen aqueous solutions of $K2Cr2O7$ at 77 K. Frozen solutions of sodium dichromate and anmonium dichromate also emit, but much more weakly and at a slightly longer wavelength. Addition of KCl to solutions of $Na2Cr2O7$ generates the $K2Cr2O7$ emission; similarly, addition of CsCl or RbCl to solutions of $Na2Cr2O7$ also gives rise to a luminescent sample with the maximum slightly to the blue of the potassium dichromate emission. In this talk we will discuss these cation dependent spectra and attempt to identify the emitting species.