Overtone pre-excitation - infrared multiple photon dissociation under collisional conditions : new potential for laser isotope separation

With a view towards developing a method for molecular laser isotope separation (MLIS) of carbon, silicon and nitrogen, this work investigated the overtone pre-excitation - infrared multiple photon dissociation (OP-IRMPD) of CF3H, SiHCl3 and NH3. The study of isotopically selective OP-IRMPD of CF3H resulted in a qualitative understanding of the dynamics of the OP-IRMPD process under collisional conditions. This understanding prompted us to make some crucial improvements which made the OP-IRMPD based approach to laser separation of carbon isotopes economically feasible. The typical value of isotopic selectivity reached in our experiments is 2000-3000 at the highest yield and can be further increased up to at least 7000-8000 with a simultaneous drop in productivity. We have also performed an estimation of the required parameters for an industrial-scale separation and of the energy consumption by a single separation act. The experimental study of OP-IRMPD of the SiHCl3 molecule has shown a limited applicability of this approach on this molecule for highly selective silicon isotope separation due to the low discrimination in the dissociation of pre-excited vs. ground state molecules under the conditions of moderate dissociation yield. Overtone pre-excitation of the ammonia molecule makes its multiphoton dissociation considerably easier and allows a high level of discrimination in dissociation of pre-excited vs. ground state molecules. However the high fluence of the dissociation laser required for effective multiphoton excitation of NH3 makes the OP-IRMPD approach on this molecule impractical for MLIS. We have shown that the probability of the dissociation of a pre-excited ammonia molecule depends on the pre-excitation level and on the wavelength of the dissociation laser.