Is it possible for a dark colored powder (dark brown) to emit in the visible region and if so would it be possible to record its emission spectrum?
It is definitely possible, although many nanopowders do not emit in solid state (i.e. without being dispersed in a medium) because of aggregation induced quenching. A good example of this is carbon nanodots, very emissive in solution, yet quenched in solid-state. Most dye molecules have the same behaviour. If your sample emits in powder form, you can record its emission in "front-face" geometry within a spectrofluorometer. In this geometry, the illuminated portion of the sample (hosted within a suitable sample holder, such as an aluminum cilinder) is facing (45 degree angle) the detector. Some emission will be collected from the detector, i.e. you can get the spectrum you're looking for.
Regards
Fabrizio
Dark brown appearance infers that your material is somewhat less absorbing in the orange-red range. If your emission line or lines fall into this range and sensitivity of your spectrometer is sufficiently high, you can get a measurable signal. The efficiency of emission, however, won't be high so I would doubt that your material would be useful from a practical point of view, if you intend it to be a phosphor in a powder state.
Many years ago, I had synthesized dark brown GaN powders with Zn-doping. The powders emitted blue when excited by 325 nm He-Cd laser beam. Anyway, the color of the powder depends on the syntesis temperatrue which influences the desoprtion of the carbon from the precusors. At low synthesis temperature, the products look black; at high synthesis temperatrue, the products look grey, while it looks dark brown at medium temperature.
As long as the powder isn't black it can emit, even in the visible spectral range.
However, quantum efficiency won't be good as the material will absorb its own emission partially.
High quality luminescent materials should have close to zero absorption in the range where they emit. But still, even if absorption is high, they can show emission, albeit weak.
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