Akshaya K. Nayak

Infrared multiphoton dissociation in focused laser beams: I. Generalization

Various models for IR multiphoton dissociation of polyatomic molecules in focused laser beams are discussed. To appreciate the fluence dependence of the dissociation yield, the essential laser beam characteristics, like its mode structure, propagation and focusing aspects are dealt with in some detail. Specific applications to isotope separation are discussed in a subsequent paper.

Control strategies for laser separation of carbon isotopes

Laser isotope separation (LIS) by infrared laser chemistry of polyatomic molecules has come a long way since its discovery. The last decade has seen considerable efforts in scaling up of the process for light elements like carbon, oxygen and silicon. These efforts aim at ways to improve both the enrichment factor and the throughput. The achievement is quite significant especially for carbon isotope separation wherein macroscopic operating scales have been realized. We report our studies on the IR laser chemistry of two promising systems, viz. neat CF2HCl and CF3Br/Cl2. We have investigated conditions for optimizing the dissociation yield and selectivity using natural samples containing l.l%C-13.Wealso highlight our current efforts for scaling up the process. These include the design aspects of a photochemical reactor with multipass refocusing Herriott optics for efficient photon utilization, development of a cryogenic distillation set up and a preparative gas chromatograph for large scale separation/collection of the isotopically enriched photoproduct in the post-irradiation stage.

Infrared multiphoton dissociation in focused laser beams: II. Application to T/D/H isotope separation

Abstract In the preceding paper it was shown that multiphoton dissociation (MPD) yield vs fluence can be expressed by a “power-law” model. Here the model is exploited for the purpose of presenting and analysing our experimental data on T/D/H separation. This approach permits a unified description of the experimental system regardless of the potential complexities.

Separation of tritium from deuterium by pulsed NH3 laser-selective multiple photon dissociation of CTCl3

The multiple photon dissociation of ppm level CTCl3 in CDCl3 and the selectivity of T/D separation were investigated using a pulsed ammonia laser. The effect of laser frequency, fluence and buffer gas pressure on the dissociation rate and isotopic selectivity were studied. The depletion of CDCl3 was not observed within experimental errors. A lower limit of single step selectivity factor was found to be >5000 at 133 Pa substrate pressure.

TEA CO2 laser-induced telomerization chemistry in conventional and waveguide photoreactors

Abstract Experimental approaches to the chemical synthesis of gas phase radicals by IR multiple photon excitation of polyatomic molecules are considered. The transverselyxcited atmospheric (TEA) CO2 laser-induced telomerization reaction of C2F4 and CF3I was studied in conventional and waveguide photolysis reactors. The difference in the product distribution in the two cases can be explained in terms of the various competing rate constants and the concentration of the perfluoroalkyl radicals generated in the irradiated volume.

Efficient optical filter for TEA CO 2 laser pumped mid IR molecular lasers

Trifluoromethyl iodide, CF(3)I, has been shown to be an efficient optical filter for TEA CO(2) laser pumped several mid IR molecular lasers. Its absorption characteristics and effectiveness of filtering capacity were examined using a pump TEA CO(2) laser and a 12.08-microm NH(3) laser.

Improved performance of an NH3 laser by temporal shaping of the CO2 laser pulse

Abstract Redistribution of the energy between the gain switched peak and the tail of the CO 2 laser pulse has resulted in 30% improvement in the conversion efficiency of an NH 3 laser. This technique has been shown to minimise the saturation effect normally operative in such systems as a result of strong pumping.

Exploitation of self-focusing in the operation of optically pumped molecular lasers

We have demonstrated that in optically pumped molecular lasers, where self-focusing plays a dominant role, use of multitransverse mode pumping has a decided advantage over TEM(oo) mode pumping.

TEA CO2 laser induced breakdown versus selective photochemistry in C2F5H and C2F5H/C2F5T mixtures

Abstract In the context of laser separation of tritium isotope using C2F5T in mixtures with C2F5H, we have studied the laser induced dielectric breakdown (LIDB) as a function of the system pressure using 10.6 μm radiation of a TEA CO2 laser. By taking also into account our isotope selective results, we have found that LIDB does not imply difficulties by affecting the laser selective process. Analysis of the experimental results shows that LIDB works by cascade ionization and the recombination constitutes the major loss process. The LIDB threshold fluence Φth for the tritiated mixture was found to be lower than the corresponding value in pure C2F5H indicating preionization in the medium due to β emission of tritium.

Product distributions in UV and IR laser chemistry of CnF2n+1I (n = 1, 2) in the presence of H2

The UV and IR photolysis of trifluoroiodomethane and pentafluoroiodoethane in the presence of hydrogen were studied. The products were quantitatively analysed by IR spectrophotometry and temperature-programmed gas chromatography. The photolysis product distribution using different experimental parameters was obtained. The difference between the product distributions in IR multiphoton dissociation (IRMPD) and conventional UV photolysis has been explained in terms of the various competing rate constants and the concentration of the perfluoroalkyl radicals generated in the irradiated volume.