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Iodine molecules ($\ce{I2}$) can absorb in the visible region and dissociate into $\ce{2 I^.}$ radicals. One of the I atoms is in ground electronic state $\mathrm{^2P_{3/2}}$ and the other I atom is in the excited $\mathrm{^2P_{1/2}}$. So, photodissociation of $\ce{I2}$ produces 50% of the species in the excited state and 50% in ground state.

The $\mathrm{^2P_{1/2}\rightarrow^2P_{3/2}}$ of iodine has been extensively used in laser production, in chemical iodine laser or methyl iodide photodissociation laser for example. But in those cases, most of the $\ce{I}$ atoms are produced in the excited state. The criteria for laser production is population inversion, where >50% of the species are in the excited state.

Here I am not sure if the photodissociation of iodine molecule satisfies the criteria of population inversion, as exactly 50% of the species is in excited state. So my question is—can we consider 50% excited species as population inversion and if so can photodissociated $\ce{I_2}$ generate laser?

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  • $\begingroup$ You are making a chemical laser. A laser depend on keeping a difference in population between the two levels involved, 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 should make the transition end at a higher level in the ground state I atoms that has no thermal population, (i.e. a make a 3 level laser) so that the laser radiation cannot pump this level to the higher level from which lasing occurs. You must of course keep producing I* atoms. $\endgroup$
    – porphyrin
    Jan 9 at 19:46
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You are making a chemical laser. A laser depend on keeping a positive difference in population between the two levels involved (upper >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.

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  • $\begingroup$ So, if there are two levels (as in the question), there would be no laser, as the emission from excited states would be reabsorbed by the ground state atoms. Three level laser would solve the problem. Is that correct? $\endgroup$
    – S R Maiti
    Jan 9 at 22:10
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    $\begingroup$ Not quite, for two levels the stimulated emission and absorption are at the same wavelength so the population of the two levels equalise. Look up 3 and 4 level lasers. $\endgroup$
    – porphyrin
    Jan 10 at 9:44

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