[en] The three-colour three-step three-photon ionization scheme of Yb such as 6¹S₀(0 cm^-1) → 6³P₁(17992.008 cm^-1) → (7/2,3/2)₂4f¹³6_s²6p(35196.98 cm^-1) → A.I.(52353 cm^-1) si chosen to examine the population dynamics of the isotopes consisting of Yb medium. The Bloch equation is numerically solved, where the interacting lasers are supposed to be on-resonant with the transition lines of ¹⁶⁸Yb. The effects of quasi-monochromatic single-mode laser bandwidth and Doppler broadening are investigated. It is revealed that the ionization scheme chosen can be used for a photoionization of ¹⁶⁸ Yb resulting in the selectivity of 100%, but also that the laser beam propagation in a thick medium for ionizing the element can be affected by at least two isotopes of ¹⁷¹Yb and ¹⁷³Yb.