Concepts such as heat, temperature, friction, etc are macroscopic, and, as
such, lack most of their meaning at the particle level. Atoms and molecules
absorb and emit energy in quanta called photons. Atomic nuclei also absorb
and emit photons of very high energy, called gamma rays.
"Heat radiation" usually refers to the emission of photons by macroscopic
objects in the wavelength range called infrared, longer that visible red
light but shorter than microwaves. However, cold objects emit radiation in
the microwave region. A particular distribution of wavelengths of these
photons is characteristic of what is called the temperature of the object.
Macroscopic objects also gain or lose heat energy via conduction from or to
surrounding materials. Atoms and molecules emit and absorb radiation over
a broad range of the electromagnetic spectrum. Generally speaking,
rotational energy transitions are in the microwave region of wavelengths,
vibrational in the infrared, and electronic in the visible or ultraviolet.
These individual transitions, however, are associated with sharply defined
wavelengths, as opposed to the broad distributions of wavelengths from
I would suggest reading a chapter entitled "Temperature" in any introductory
college physics text, usually around chapter 15 or so at the beginning of
the Thermodynamics section. It should be understandable even without going
through the earlier chapters on mechanics. Later in the text would be more
challenging chapters on the electromagnetic spectrum and quantum mechanics,
usually in the section entitled "Modern Physics", which refers to physics
since the late 1800s or so. This would include topics such as relativity,
quantum mechanics, particle theory, etc.