Ionization yield in a three step photoionization process of an optically thick atomic column: numerical modelization and experiments in thulium

Among the mainly interesting parameters of an atomic vapour laser isotope separation process is the ionization yield. This parameter can be controlled as long as the laser beam spatial intensity distribution and temporal shape are well defined and not subjected to unexpected disturbing effects such as coherent propagation phenomena, leading to spatial and temporal reshaping with hot spots and pulse lengthening and delays. On an other hand, economical considerations require optically thick atomic columns which can favour such effect. In this work is studied the photoionization yield in atomic thulium vapour when propagation effects occur. Experimental results are compared with those of a coherent propagation code describing a four level system and including inhomogeneous broadening as well as degeneracies. A three step photoionization scheme is chosen