M. Gauthier

Efficient production of13C2F4 in the infrared laser photolysis of CHClF2

We report the isotopically selective decomposition of chlorodifluoromethane. Chlorodifluoromethane is used industrially in high volume for the production of tetrafluoroethylene and its polymers; thereby it is an attractive working substrate for a medium scale isotope separation process, both in terms of its price and availability.We have studied the infrared multiphoton decomposition of carbon-13 substituted chlorodifluoromethane molecules present at their natural abundance (1.11%). A well defined CO2 laser pulse (80 ns FWHM) was used and both the yield of carbon-13 enriched product and the net absorption of laser radiation were measured. These measurements were made as a function of substrate pressure (10-800 Torr), CO2 laser line (9P 12–9P 32) and fluence (2–8 J cm−2) and were used to determine the energy expenditure per carbon atom produced (ε) at specified product carbon-13 content in the range 30%–96%. The results of these parametric studies were interpreted in terms of the kinetics of multiphoton absorption and dissociation, and allowed an initial optimization of the experimental conditions to minimize ε.Optimum results were obtained at 1046.9 cm−1, 69 cm−1 to the red of the12CHClF2v9 band center. Irradiation of 100 Torr of chlorodifluoromethane at 3.5 J cm−2 gave tetrafluoroethylene containing 50% carbon-13 for an absorption of 140 photons (0.017 keV) per carbon atom produced. This efficiency compares favourably with existing carbon-13 enrichment technologies and would require an absorption pathlength of only 2 m to absorb half the incident photons.

Laboratory scale-up of two-stage laser chemistry separation of13C from CF2HCl

The laboratory scale-up of a two-stage laser enrichment process for carbon isotopes, involving infrared multiphoton dissociation of freon-22, is described. Unmodified commercial equipment and materials were used. An initial study of the effect of fluence, laser frequency, freon-22 pressure and pressure of argon, nitrogen and trifluoromethyl chloride was made in short irradiation cells (constant fluence) in order to define optimum process parameters. The process was then scaled to higher throughput in longer cells (1–5 m) in which compensation for beam-energy depletion by absorption was made by reduction in the beam area by focussing. From the scale-up experiments, measurements of yield and enrichment of the tetrafluoroethylene product gave demonstrated production rates. These, coupled with measurements of the absorption, allowed extrapolation to production rates assuming total utilization of the available output energy.Using a 100 W TEA CO2 laser (10 J, 10 Hz) we have demonstrated production rates of 0.20 g h−1 carbon-12 at 99.99% carbon-12, 11 mg h−1 carbon-13 at 72% carbon-13 and 2 kg per annum carbon-13 at 50%. Energy absorption measurements imply a capability to produce 3 kg per annum carbon-13 at over 95% carbon-13 in a two-stage process.The apparatus was used to produce gram quantities of carbon-13 depleted freon-22 (99.99% carbon-12). A comparison of the infrared multiphoton dissociation of this material with that of natural freon-22 (1.11% carbon-13) showed that under the conditions required to give selective dissociation of13CF2HCl that12CF2HCl was excited as a result of a dominantly radiative interaction and that collisional transfer from13CF2HCl molecules played a minor role.

Isotopically selective multiphoton decomposition of CF3Br and CF3I: separation of selective and non-selective product sources☆

Abstract The decomposition yield and the isotopic selectivity of carbon-13 production in the infrared multiphoton decomposition of CF 3 Br and CF 3 I have been determined from the observed product C 2 F 6 . These two observables have been studied in focussed beam geometry as a function of accumulated number of pulses from the CO 2 laser, pressure of absorber and energy per pulse. The isotopic selectivity is independent of fluence but decreases with pressure, mostly due to equilibrium thermal processes. The amount of C 2 F 6 formed increases as a function of (pressure) 2 and of (pulse energy) 2 . This is explained in terms of a simple diffusional quenching model where energy pooling is the dominant reaction leading to dissociation.

Spectra and structure of nitrous oxide dimers and clusters

Supersonic expansions of nitrous oxide seeded in argon were studied by single pass FTIR absorption at right angles with the jet axis. In very dilute mixtures, 0.2% N2O, dimer bands at 1282.4 and 2228.0 cm−1 were observed corresponding to the ν1 and ν3 stretches of the monomer. Cluster bands (due to clusters of undetermined size) were readily observed in 20%–100% N2O mixtures at 1165.1, 1295.2, 2245.6, 2467.4, 2578.3, 2811.7, and 3505.8 cm−1. All bands are shifted from the corresponding monomer bands more than in the crystal and in the same direction. Changing the dilution from 10% to 20% shifted all cluster bands 1–2 cm−1 towards low frequencies but still far away from the crystal frequencies. This is interpreted as being due to cluster melting. No increase in bandwidth is associated with the shifts, indicating that, if present at all, the coexistence region described by Berry and co‐workers is rather limited in N2O clusters.

Multiphoton decomposition of hexafluoroacetone: Effects of pressure, fluence, wavelength, and temperature on the decomposition yield and C‐13 and O‐18 isotopic selectivity

The yield and isotopic selectivity of the infrared multiphoton induced decomposition of hexafluoroacetone have been studied in both parallel and focused beam geometries as a function of temperature, pressure, fluence, and photolysing wavelength. At low conversions, the chemical stoichiometry of the decomposition process is simple, CF3COCF3    nhν    CO + 2CF3. Several potential interferences are discussed but it is concluded that the yield of CO directly reflects the yield of the primary dissociation event. Pressure effects are complex and not amenable to quantitative description, however, varying parameters such as fluence and photolysing wavelength lead to predictions which give a clear qualitative picture. In particular, it is possible to distinguish parametric variations due to spontaneous dissociation and dissociation dependent upon energy pooling. Isotopic selectivity provides a powerful insight in this area. Preliminary data on the effect of temperature have been obtained at constant incident fluen...

The selective decomposition of CF3 13COCF3 molecules at natural abundance: Multiphoton dissociation at threshold

The first example of a system is reported in which isotopic selectivity increases dramatically with increasing substrate pressure. Hexafluoroacetone is studied using infrared laser induced multiple photon decomposition. The parametric variations of pressure, laser frequency, and fluence to the reaction yield are reported. (AIP)

Second-stage enrichment in the laser separation of carbon isotopes

There is a general agreement that efficient infrafed laser induced separation of carbon isotopes requires a two-stage process. An efficient first stage 1%→50%13C enrichment was shown by Gauthier et al. [1] to be feasible and competitive with conventional technology. In this work, second-stage CO2 laser enrichment of equimolar mixtures of12CHClF2 and13CHClF2 has been demonstrated yielding tetrafluoroethylene containing 95% or 99%13C. Forward enrichment by selective decomposition of the13CHClF2 fraction was very efficient, absorbing only 6 and 16 eV, respectively, per carbon atom produced at 95% and 99%13C content.

Separation of deuterium by IR multiphoton decomposition of chlorodifluoromethane. IR multiphoton absorption by and decomposition of a CF2DCl/CF2HCl mixture

Abstract IR multiphoton absorption by various pressures of CF 2 HCl or of 1??? CF 2 DCl in CF 2 HCl was studied at several 9P and 10R lines of the CO 2 laser using temporally unmodified pulses. Values of 〈 n 〉 for the individual molecules, and thus the optical selectivities, obtained from these data indicate that the molecule is not a promising candidate for deuterium separation in these frequency ranges. A measurement of D-isotope depletion during the multiphoton decomposition of the same mixture at 9P42 (1025.2 cm −1 ) gave a minimum estimate of the isotopic selectivity of the dissociation process of 35.

Viable Commercial Ventures Involving Laser Chemistry Production: Two Medium-Scale Processes

Consideration of viable commercial ventures involving laser chemistry must take into account not only the cost structure of the laser production step but must also consider all other steps in the venture through to provisions of goods or services to clients. Recent work on the two-stage infrared laser isotope separation of carbon-13 and the selective wavelength synthesis of vitamin-D is reviewed. The processes, which may be viewed as surrogates for direct carbon dioxide and excimer laser driven synthetic schemes, are discussed in terms of their technical description and the business opportunity they present.

Isotopic labelling at natural abundance. Photolysis of pentafluoroethyl in the infrared multiphoton decomposition of pentafluoroethyl iodide

Abstract The combination of isotopically selective infrared multiphoton dissociation with gas chromatography-mass spectrometry provides a powerful assessment of reaction mechanisms. In the case of IR MPD of pentafluoroethyl iodide strong evidence for the homogeneous gas-phase photolysis of the transient C 2 F 5 radical is obtained.