Is the infrared sunlight collimated at ground level? Is the infrared light from the sun (above 700nm) collimated like the visible spectrum or does it become diffuse due to the atmosphere? I mean during direct sunlight, so a clear sky.
Basically, I want to know if it is possible to use infrared light only to power a heat engine or not.
 A: This picture

says yes, as the image of the sun is seen in infrared. Go to the link to see the effect of changing infrared wavelengths.
A: You need to specify the exact wavelength. In general the atmosphere is transparent to near-infrared radiation from about 700-2400 nm, with some notable absorption bands due to water and carbon dioxide. Beyond that, the mid- and far infrared can be heavily absorbed, particularly by water vapour. There are transmission windows at around 3.5, 5 and from about 8-14 microns. Beyond that, the atmosphere is nearly opaque until mm wavelengths.
A transmission curve for the atmosphere (at zenith) is shown below (from https://en.m.wikipedia.org/wiki/Infrared_window ). I think this is for a "dry" site with low amounts of precipitable water vapour.

This tells you how much direct radiation from the Sun, reaches the ground and arrives as a parallel beam (if the Sun is at zenith). This is what most people understand to be meant by collimated.
Note that the amount of infrared radiation scattered in the atmosphere and hence the diffuse illumination from the sky will be small.
