When an excited atom de-excites spontaneously emitting a photon, it recoils in the direction opposite to that of the emitted photon. Consider now a laser-like situation, in which one has attained population inversion so that a large number of atoms are in an excited energy level E2, with energy level E1 meagerly populated (E1 < E2 ). A photon with energy E2 - E1 passes by, causing stimulated emission of photons with identical phase and energy in the same direction as that of the incident photon. Do all the atoms recoil SIMULTANEOUSLY in the opposite direction?
Hi Gupta
In an atom excitation and deexcitation levels do not occur instantaneouly. You should consider the quantum probability of absorption and emission of particles, which are related to the energy width and lifetime of quantum atomic energy levels. Larger energy widths corresponds to shorter lifetimes of excited excited levels and so on. Thus you cannot assume one photon would be able to excite many atoms in a short period. Moreover should consider that atom recoil depends on 1/A. A: atomic number. So, atomic recoil linear momentum may be very small for large atoms.
Thanks Cesar. A single photon is likely to excite all those atoms which are within a radius of about former's wavelength.
Hi Gupta
The only case I can agree is in case of N atoms in a trap, the so called "superatom", which has N excited states and one Rydberg excitation collectively shared among the other atoms. But only one photon excites the system. There is among N interacting Rydberg atoms a mechanism called Rydberg blockade that prevents the absorption of multiple photons in the trapped system.