Q:

What causes this IR emission in UV 365nm LED?

I've just got access to spectrometer and measuring spectrum of everything. But when I took a look at 365nm UV led from chinese UV flashlight I got something really strange - it had some IR at exactly half the energy. Any suggestions on what it might be? 
I double checked that it is actually IR, and not a grating glitch - IR-cut filter does remove it. 
UV peak: 370.5nm
IR peak: 741.4nm

A:

As boyfarrell pointed out, the IR peak that you observe is due to second order reflection from the diffraction grating of the spectrometer. Strong spectral lines are often seen in the second order (with the advantage of better resolution). You can easily verify it by inserting a long-pass filter that blocks 370 nm (some orange glass or even sunglasses) between the LED and the spectrometer. You will see that both peaks disappear simultaneously. 

