Doppler Cooling of Lithium
Different laser cooling mechanisms have been rising important questions from thermodynamics and statistical physics point of view ever since the beginning of this research field (over 40 years ago). Sisyphus cooling is especially well known in this respect providing experimentally accessible regime to study deviations from thermal equilibrium. Here we discuss the Doppler cooling mechanism (the most basic laser cooling mechanism which works even for a simple two-level atom) and show that it also supports deviations from Gaussian statistics for a certain parameters range. We study experimentally the Doppler cooling in Lithium and point out an interesting deviation from the simple, two-level theory, namely cooling at resonance. We develop a realistic theory which accounts for all energy levels of lithium atoms and all laser fields and show its successes and failures.