Deuterium separation at high pressure by nanosecond CO2 laser multiple-photon dissociation

Abstract

Photochemical deuterium separation is evaluated at pressures up to 1 atm using 2 ns duration CO2 laser pulses to achieve multiple-photon dissociation (MPD) as the isotopic separation step. Photochemical performance is compared for Freon 123 (2,2-dichloro-1,1,1-trifluoroethane), difluoromethane, and trifluoromethane based on deuterium optical selectivity in absorption, photoproduct yield, and single-step deuterium enrichment factor. The absorption coefficient versus energy fluence is measured from 0.01 to 3 J/cm2 fluence for CF3CDCl2, CDF3, and CHDF2; added buffer gas results in an order-of-magnitude increase in the CDF3 absorption coefficient. The deuterium optical selectivity in absorption at 0.5 J/cm2 fluence with added buffer is 80 for CF3CDCl2 at 10.65 μ, 800 for CHDF2 at 10.48 μ, and 2500 for CDF3 at 10.21 μ. The absorption coefficients and hence optical isotopic selectivities are dependent on fluence, and the optical selectivity attains a maximum value of 8000 for CDF3 below 0.01 J/cm2 fluence. The deuterium-bearing MPD photoproducts at high pressure are trifluoroethylene for Freon 123, hydrogen fluoride for trifluoromethane, and both hydrogen fluoride and monofluoroacetylene for difluoromethane. Yield data determined by gas chromatography are analyzed using a model describing MPD due to a focused Gaussian beam in an absorbing medium to remove compositional and geometrical effects; this analysis results in a saturation fluence (at which the dissociation probability approaches 100%) of 12±2 J/cm2 for CF3CDCl2, 20±2 J/cm2 for CDF3 buffered by 1 atm argon, 30±2 J/cm2 for CDF3 buffered by 60–400 Torr CHF3, and 22±3 J/cm2 for CHDF2 buffered by 100–400 Torr CH2F2. Near unity dissociation probabilities are obtained for samples unbuffered by argon with operating pressures up to 40 Torr for Freon 123, and up to 400 Torr for both difluoromethane and trifluoromethane; the methane derivatives are much more resistant to high pressure collisional quenching than Freon 123. The single-step deuterium enrichment factor is 1200±300 for natural isotopic abundance Freon 123 at 30–100 Torr (10.65 μ) determined by mass spectrometry of the main trifluoroethylene photoproduct. The single-step deuterium enrichment factor determined indirectly from gas chromatographic analysis is 2500±500 for natural difluoromethane at 200 Torr (10.54 μ), and 11, 000+4000−2000 for natural trifluoromethane at 100 Torr (10.21 μ). It is concluded that both difluoromethane and trifluoromethane are photochemically satisfactory for viable large-scale laser production of heavy water.