  For the first time, we report that a single crystal of (K-crypt)2Bi2
containing [Bi2]2+ displays ultra-broad near-infrared photoluminescence (PL)
peaking at around 1190 nm and having a full width at the half maximum of 212
nm, stemming from the inherent electronic transitions of [Bi2]2+.The results
not only add to the number of charged Bi species with luminescence, but also
deepen the understanding of Bi-related near-infrared emission behavior and lead
to the reconsideration of the fundamentally important issue of Bi-related PL
mechanisms in some material systems such as bulk glasses, fibers, and
conventional optical crystals.
