Determination of atomic level populations by generation of third-harmonic ultrashort optical pulses

A simple method of measuring atomic level populations was implemented by comparing the energies of the third harmonic generated by two identical ultrashort pulses. When the first ultrashort pulse induces a process that removes some of the atoms from the ground (initial) level for third harmonic generation, the energy of the second third-harmonic pulse W_th2 becomes lower than W_th1 as a result of a reduction in the number of atoms. In the experiments К = W_th1 / W_th2 increased as the ultrashort pulse intensity increased. Experimental investigations and theoretical estimates showed that the most probable process causing a reduction in the population of the 6²S_1/2 level of cesium atoms was multiphoton ionization. It was found that the degree of multiphoton ionization could be estimated from K.