The peak intensity of radiation from Mars is about 14,000 nm. In what spectral band is this?
You say peak intensity, but a more appropriate term is peak wavelength. Intensity is in watts per square meter; power on a surface. The unit nm is a length, specifically the length of an electromagnetic wave.
A wavelength of 14,000 nm is in the far-infrared range of the electromagnetic spectrum.
Being that Mars is cold, its peak radiation wavelength is going to be fairly long. Compared to stars with much more energetic photons, colder planets tend to release light of longer wavelengths.
This is important because when scientists use telescopes to view Mars, they can get brighter images in the infrared spectrum, which can reveal different things than a visible light, or bolometric spectrum would.
The best way to classify light on the electromagnetic spectrum is to find a nice chart with wavelengths and frequencies next to their respective bands.
This website has a chart of the spectrum you can use.