Photophoresis of small-size particles in a gas excited by resonance radiation

The motion of small aerosol particles in uniform gas excited by a resonance radiation is caused by the specific mechanism. In the case of a small deviation of the laser radiation frequency from that of the electronic or the vibration-rotation transition, due to the Doppler effect, only the molecules whose velocity projection on the radiation propagation direction lies within a certain velocity range can be excited. For this reason, the relationship between the excited and the nonexcited molecules in the aforesaid velocity range differs from the corresponding relationship in the velocity range, which is symmetrical with regard to the center of the Maxwell distribution. A resultant force, which is applied by the gas to aerosol particles, arises due to the different scatterings of the excited and nonexcited molecules by the surface of aerosol particles.