What is the relationship between reabsorption and heating in laser cooling?

[bznm]

Homework Statement

I have to study laser cooling. I'm actually trying to understand the Doppler limit on temperature.
On the net I have found "at a certain point the cooling mechanism is foiled by heating due to the random absorption and reemission of photons. (http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/lascool.html)" (A similar sentence is in Phillips' Paper "Laser cooling and trapping of neutral atoms")

I can't understand in which way emission can heat the atom... Can you explain me? A lot of thanks

 

[bznm]

In this paper http://users.phys.psu.edu/~dsweiss/PRA%20RC%20laser%20cooling%20in%20optical%20lattices.pdf it says: "Laser cooling requires spontaneous emission,
and spontaneously emitted photons can be reabsorbed by other atoms. Reabsorption causes heating, which makes equilibrium temperatures increase with the density and number of atoms".

I don't understand why reabsorption causes heating...