[en] Third harmonic generation (THG) in a phase-matched mixture for the first time has been produced by utilizing a new type of the sealed-off sapphire cell. Maximum energy conversion with the efficiency of 4% for the pulse of train of mode-locked YAG: Nd³₊ laser has been achieved. The nonlinear medium response (THG) induced by a train of ps pulses allows one to determine and discriminate between limiting influence of simultaneously occurring processes such as multiphoton ionization, Kerr-effect induced phase-mismatching, self-phase modulation. 17 refs