13C isotope has been separated in the form of enriched product C2F4 by selective multi-photon dissociation (MPD) of Freon-22 (CHClF2) using the 9P(26) laser line of a transversely excited atmospheric CO2 laser. The non-linearity factor, γ, that determines the dependence of the yield of 13C isotope on the fluence (J/cm2) has been determined for various laser rotational lines (9P(20)–9P(26)) and the advantage of a lower γ in the case of 9P(26) is highlighted for macroscopic production of 13C isotope. It is also shown that a higher value of the optimum fluence at 9P(26) not only results in a higher enrichment efficiency but in a relatively lower value of γ also. The laser pulse energy is efficiently utilized for selective MPD of Freon-22 by focusing the pulse energy repeatedly with the help of a novel linear multi-pass cavity (LMPC). The novelty of this optical arrangement lies in its ability to maintain the laser fluence around an optimum value for a desired enrichment of 13C in the product. This also ensures a higher quantity of enriched product because of the higher reaction volume. The advantage of the LMPC over the conventionally used Herriott multi-pass cell has also been presented. The gain in reaction volume in the present optical cavity having 20 passes with a constant fluence in each pass is as high as 12. Isotope-selective MPD of Freon in a LMPC with constant fluence in each pass showed a distinct advantage in energy utilization to separate 13C isotope over the gradually reducing fluence case.