Photodissociated iodine laser and population inversion
You are making a chemical laser. A laser depend on keeping a positive difference in population between the two levels involved (upper &gt;lower), and of course feedback for a laser oscillator. It does not matter how many I* you make as long as the lasing transition does not go the the lowest ground state level. You must make the transition end at a higher level in the ground state I atoms that has no (i.e. insignificant) thermal population, thus a make a 3 level laser meaning that the laser radiation cannot pump  this lower level to the higher level from which lasing occurs.  You must of course keep producing I* atoms.