Ionization yield of two-step photoionization process in an optically thick atomic medium of barium

The kinetics of a two-step photoionization process in optically thick atomic medium of barium (Ba) is studied using the rate equation approach. In the first step, Ba atoms get resonantly excited by laser radiation from their ground state to an intermediate excited state and subsequently are ionized in the second step by another laser radiation. The absorption of exciting radiation is taken into account along its propagation direction (optically thick). However, the medium is assumed to be optically thin for the ionizing radiation. A numerical simulation is done to estimate the ionization yield for time-varying Gaussian shaped laser pulses. The required energy density of the laser pulse to saturate the excitation transition throughout the thick medium is calculated. The effect of optical delay between the laser beams on the ionization yield is simulated. The calculated ionization yield from the simulation is compared with the measured values.