IR multiphoton absorption and isotopically selective dissociation of CHCIF2 in a herriott multipass cell

Abstract

The small absorption is a major problem in isotopically selective IR multiphoton dissociation. Usually the radiation diverges before it is fully absorbed. To solve this problem, we have demonstrated the application of a refocusing (Herriott) multipass cell. It can generally help in laser isotope separation to use the photons more efficiently. Employing such a cell and a Q-switched CO2 laser at high repetition rate, the dissociation yield of CHCIF2 was 23 times higher than in a single pass. The number of passes used (up to 60) was more than is conventionally possible in such small cells. The increased number was permitted by making use of spherical aberration. With 18 passes, we also measured the multiphoton absorption for various wavelengths and pressures, in part separately for12CHCIF2 and13CHCIF2, and also for two-wavelength irradiation. Appropriate change of pressure or wavelength increased the absorption. But the corresponding increase of the dissociation was larger in every case. To explain this and other observations, we invoke the molecular distribution over the energy levels.