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 () 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 .Optimum results were obtained at 1046.9 cm^–1, 69 cm^–1 to the red of the¹²CHClF₂ v ₉ 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.Issued as NRCC 20112     

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.Issued as NRCC 23639Summer Student 1983, Acadia University, N.S., Canada     

A waveguide reactor for IR multiphoton dissociation and its application to13C laser isotope separation

A waveguide reactor for infrared multiphoton dissociation reactions has been presented and applied to laser isotope separation of¹³C. The reactor is a sealed vessel containing a hollow waveguide for a CO₂ laser, and the laser beam is condensed in the waveguide. The waveguide is a pipe of total-reflection glass or metal-coated glass with a funnelled inlet; it is 50 cm long with a 3 mm inner diameter. The isotope separation of¹³C has been made by multiphoton dissociation of CHF₂Cl using a TEA CO₂ laser. The result has shown that the waveguide reactor increases the dissociation yield by four to ten times as much as that without the waveguide at the pulse rate of up to 150 Hz, while the separation factor remains on the same level. It is also found that an optical breakdown which is induced at the wall surface plays a significant role to reduce the isotope selectivity.This work is for a partial fulfilment of B.S. degree of Osaka Electric Communication University     

N.M.R. relaxation time studies in liquid methyl iodide

The spin-lattice relaxation times of the various nuclei in methyl iodide, methyl iodide-d ₃, and carbon-13 methyl iodide (¹³C, ¹H, ²D) were measured between 210 and 350 K. The separation of the proton-proton intermolecular relaxation was accomplished by a dilution study in methyl iodide-d ₃; the resulting intermolecular contribution agreed well with the existing theories for this mechanism. It was found that the spin-rotation interaction contributed significantly to the intramolecular relaxation of both the protons and the carbon-13. For both nuclei the separation of the spin-rotation interaction from the intramolecular dipole-dipole interaction was accomplished without making any assumptions about the temperature dependence of the spin-rotation relaxation time. The resulting spin-rotation relaxation times for both carbon-13 and protons offer evidence that the large spin-rotation effects are due to the methyl group reorientation.     

Scaling-up13C separation by infrared multiphoton dissociation of the CHClF2/Br2 system

¹³C separation at a laboratory scaled-up level by the¹³C-selective InfraRed MultiPhoton Dissociation (IRMPD) of CHClF₂ in the presence of Br₂ has been investigated in a flow reactor. With a complete scaled-up system including a flow reactor, an industrially reliable TEA CO₂ laser with longer pulse duration and a product-separation set-up for¹³C separation, it has been attempted to optimize the parameters suitable for large-scale production of the carbon isotope. The optimization of¹³C separation parameters, such as laser fluence, laser frequency and the partial pressure of CHClF₂ and Br₂ was tested under static conditions. By irradiation with longer pulses, a lower optimum pressure for a high¹³C-production rate was determined. Furthermore, the separation process was scaled in the flow system to examine the¹³C-production rates,¹³C atomic fractions in the CBr₂F₂ products and¹³C depletions in the CHClF₂ reactants at different flow rates and laser repetition frequencies. The data obtained from the flow tests demonstrated a 40 mg/h production rate for CBr₂F₂ at 65%¹³C by using a 40 W (4 J, 10 Hz) laser beam focused with a lens of 120 cm focal length. If the reliable TEA CO₂ laser is operated with 100 W (10 J, 10 Hz) output, the production rate of CBr₂F₂ for¹³C at 60% of 200 mg/h can be attained. The measurements of the spatial profile of the focused laser beam imply a 2 g/h production rate for the 60%¹³C product for an incident power of 200 W (20 J, 10 Hz).     

Parametric study of the IRMPD of CF2HCl molecules with the 9P22 CO2 laser line

The separation of ¹³C by infrared multiple photon decomposition (IRMPD) of CF₂HCl has been parametrically studied in relation with some key parameters such as the laser fluence, the number of laser pulses, and the gas pressure. The process selectivity, the depletion of the ¹³C isotope in the residual gas, the relative amount of ¹³C separated per pulse, and the energy expenditure were determined as a function of the above mentioned parameters, and conclusions were drawn concerning the efficiency of the separation process. An isotopic selectivity of about 40 was obtained in the investigated range of pressure. At 10 Torr of pure CF₂HCl, the ¹³C depletion in the residual gas was 45% when only 300 laser pulses were used. A maximum amount of 4.6×10^–10 kg ¹³C separated per pulse was obtained.     

Laser isotope separation of carbon by multiple IR photon and subsequent UV excitation of CF3I molecules

A new approach to laser isotope separation is considered. It is based on collisionless multiple photon ir laser excitation and subsequent uv laser dissociation of vibrationally excited molecules. TEA CO₂ and excimer XeF, XeCl lasers are used for ir excitation and uv dissociation, respectively. The products of photolysis (C₂F₆) are enriched with¹²C.     

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

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 Φ_th 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.     

Definitive Carbon-13 Chemical Shift Assignments in 2-Substituted Adamantane Derivatives by Deuterium Isotope Effects

Deuterium isotope effects on carbon-13 chemical shifts in 5-[²H]-isotopomeres of ten 2-substituted adamantane derivatives were determined and used for complete assignment of their carbon-13 NMR spectra.

The carbon-13 NMR spectra of ten 2-substituted adamantane derivatives have been assignated by consideration of deuterium isotope effects. Some four- and five-bond downfield deuterium effects on certain chemical shifts have been measured and attributed to remote hyperconjugative interactions.     

Rotational-coherence molecular laser isotope separation

We have proposed a laser isotope separation method, utilizing rotational coherence of a simple molecule. In the scheme, photoexcited molecules are isotopically separated by difference of rotational period between them. To illustrate this method, two-pulse photodissociation of mixed ⁷⁹Br₂/⁸¹Br₂ isotopes has been investigated theoretically. The photodissociation probabilities of ⁷⁹Br₂ and ⁸¹Br₂ have been calculated as functions of time delay between the photoexcitation and dissociation laser pulses. We have demonstrated that isotope enrichment factor of ⁷⁹Br relative to ⁸¹Br can be changed from 0.34 to 1.8, by simply changing the time delay only by 0.2 ns. Additionally, we have shown that this method is effective for heavy isotopes, based on mass dependence of the isotope enrichment factor.     

Simultaneous measurements of absorption and vibrational relaxation time in CF2HCl molecules

Summary We describe in this paper a simple interferometric technique which allows a direct and simultaneous determination of energy absorption and of the vibrational-translational relaxation time of polyatomic molecules strongly excited by reasonant, infrared (10.6 μm) laser radiation. In particular, we have applied this method to the study of freon-22 (CF₂HCl), a medium-size molecule that, apart from its potential application in¹³C isotope separation, shows interesting absorption features in the 10 μm region. The results are also compared with our previous findings in SF₆. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Macroscopic Isotope Separation of 13C by a CO2 Laser

By photochemical dissociation of the rare carbon isotope component of CHClF₂ by means of a CO₂ laser with an average power of 150 W, Q-switched at 10 kHz, we have demonstrated the separation of more than 1 mol of ¹³C, enriched to 50% (2 mol of total carbon). It is contained in about 1 mol (101 g) of the product C₂F₄. The total throughput of the starting material was 29 kg. The experiment was run day and night for 2 weeks, almost only controlled by a computer. We obtained production rates of 5 mmol/h, corresponding to about 0.5 kg ¹³C per year.     

Isotopic ratio measurement of methane in ambient air using mid-infrared cavity leak-out spectroscopy

We report on infrared laser spectroscopic measurements of the isotopic composition of methane (¹²CH₄, ¹³CH₄) in natural air samples with a cavity ring-down technique. A CO overtone sideband laser is utilized to excite a high-finesse cavity which provides an effective optical absorption path length of 3.6 km. We achieved a detection limit of 105 ppt methane in ambient air using an integration time of 20 s. This corresponds to a minimum detectable absorption of 1.9×10^-9 /cm. Rapid determination of the ¹³C/¹²Cisotopic ratio of methane in ambient air without sample preconcentration or gas processing is realized. The present system requires only few minutes for an isotopic ratio measurement with a precision of 11%o . Received: 14 July 2000 / Revised version: 25 October 2000 / Published online: 6 December 2000     

Basic features of boron isotope separation by SILARC method in the two-step iterative static model

In this paper we develop a new static model for boron isotope separation by the laser assisted retardation of condensation method (SILARC) on the basis of model proposed by Jeff Eerkens. Our model is thought to be adequate to so-called two-step iterative scheme for isotope separation. This rather simple model helps to understand combined action on boron separation by SILARC method of all important parameters and relations between them. These parameters include carrier gas, molar fraction of BCl₃ molecules in carrier gas, laser pulse intensity, gas pulse duration, gas pressure and temperature in reservoir and irradiation cells, optimal irradiation cell and skimmer chamber volumes, and optimal nozzle throughput. A method for finding optimal values of these parameters based on some objective function global minimum search was suggested. It turns out that minimum of this objective function is directly related to the minimum of total energy consumed, and total setup volume. Relations between nozzle throat area, IC volume, laser intensity, number of nozzles, number of vacuum pumps, and required isotope production rate were derived. Two types of industrial scale irradiation cells are compared. The first one has one large throughput slit nozzle, while the second one has numerous small nozzles arranged in parallel arrays for better overlap with laser beam. It is shown that the last one outperforms the former one significantly. It is argued that NO₂ is the best carrier gas for boron isotope separation from the point of view of energy efficiency and Ar from the point of view of setup compactness.     

Application of the overtone CO laser to atmosphere gas analysis

In this paper, we consider the applicability of the overtone CO laser to atmosphere gas analysis. The atmosphere transmittance was calculated for lasing lines at the overtone frequencies of the CO laser. The results of the search for informative wavelengths for sensing the minor gas components of the atmosphere N₂O, NO₂, H₂CO, CH₄, C₂H₂, HCl, HBr, and HCN are presented.     

Study of uranium isotope separation using CO2 laser and CO laser

First, the kinetic investigation of UF₆ + HCl reaction and the isotopic selectivity under CO laser irradiation is performed. On this investigation, the kinetics of UF₆ + HCl reaction by using an intracativity CO laser and CO₂ laser irradiation system, and the isotopic selectivity for this process are studied theoretically. It is found that under the resonant CO laser and CO₂ laser irradiations, the laser-catalyzed reaction rate can increase, and a good selectivity can be achieved. The uranium isotope separation factors β calculated are about 2.44 ∼ 4.05 at laser intensity 50 ∼ 100 W cm⁻² and temperature 235 K.     

A New Laser Photochemical Process for Isotope Enrichment

Selective laser excitation of a specific isotopic species to its lowest electronic singlet (S₁) and triplet (T₁) states respectively, then followed by a chemical reaction for isotope separation is examined theoretically. It is found that the direct excitation to T₁ state from ground state (S₀) is an unique excitation channel for obtaining a controllable chemical reaction and for achieving a high isotope separation factor. This is due to the following important facts: 1) Resonant energy transfer from a selectively excited triplet isotopic species to an unexcited species of the other isotope present in the mixture is missing (or is very small); 2) The lifetime of the T₁ state is normally longer than that of the S₁ state; 3) There is no intersystem crossing channel that exists for the deactivation of excited isotopic species from T₁ state.

Numerical calculation is performed for the chlorine isotope separation in a mixture of thiophosgene (0.5 torr) and diethoxyethylene (1.5 torr). The result indicates that the isotopic enrichment factor is at least 2.5 × 10³ times larger for selective T₁ excitation than that for S₁ excitation.     

High-pressure continuously tunable CO2 lasers and molecular laser isotope separation

The acronym MLIS (molecular laser isotope separation) defines the laser process whereby the isotopes of uranium can be separated by mid-infrared laser/s when the molecule employed is UF₆. The theoretical and spectroscopical data to configure and enable experiments and demonstrations in the laboratory is adequate. However, the engineering and commercial aspects require innovative technology solutions that are not presently available in the literature on these topics. This paper is an overview of the most salient features of MLIS and its potential utility at an industrial level.     

Transient molecular absorptions induced by the absorption of CO2 laser radiation

Laser-induced molecular absorptions are reported for the species SF₆, CF₃I, and CF₃Br. Induced absorptions are observed at frequencies ～ 40 cm^-1 lower than that of a CO₂ TEA laser which is used to vibrationally excite the molecules in a low pressure cell. The dependence of the effect on pump laser intensity indicates that the species that exhibit the induced absorption have absorbed more than one laser photon. Applications of this phenomenon with respect to laser induced isotope separation are discussed.     

Self- and foreign-broadening and shift coefficients for C2H2 lines at 1.54 μm

In this article we present experimental data for line parameters measurements of several transition lines of overtone bands of C₂H₂ at 1540 nm. The measurements were done with a spectrometer based on a semiconductor diode laser and a direct absorption spectroscopy scheme. Broadening and shift coefficients have been measured for the pure gas or in mixtures with N₂, O₂ and CO₂ as perturbing gas. Received 6 November 2000 and Received in final form 13 March 2001