Isotopically selective CVD of silicon by IRMPD of Si2F6

The IRMPD of Si₂F₆ by a CO₂ TEA laser was applied to isotopically selective CVD of silicon. A white film, probably consisting of polymers of SiF₂, was deposited on a metal foil during the irradiation of natural Si₂F₆ with the laser radiation at 951.19 cm^–1 and about 1.5 J cm^–2. Upon heating, the film became dark brown, evolving SiF₄. The³⁰Si content was found to be as high as about 20%.     

Enrichment of 13C by IRMPD of CBr2F2

The CO₂-laser-induced infrared multiple photon decomposition of natural CBr₂F₂ in the presence of oxygen has been examined as a function of pulse number (30–1500), reactant pressures (CBr₂F₂, 10–150 Torr and O₂, 5–90 Torr), laser line [9P(8)–9P(32)], and laser fluence (1–3 J cm^–2) to optimize irradiation conditions for ¹³C-enrichment. CF₂O was the main carbon containing product and afterwards was converted into CO₂ via hydrolysis. A small amount of C₂Br₂F₄ was detected only under extreme conditions, for example, at high laser fluences or wavenumbers close to an absorption band. The ¹³C-atom fraction of the final product CO₂ was found to be 20–80%, depending on experimental conditions. The two-stage IRMPD process proposed previously has been examined in further detail in the present study. First, CBr₂F₂ containing about 30% of ¹³C was prepared in the ¹³C-selective IRMPD of natural CHClF₂ in the presence of Br₂. The second-stage IRMPD of the CBr₂F₂ in the presence of oxygen under selected conditions resulted in the high enrichment of ¹³C beyond 90%.     

First-stage enrichment in CO2-laser-induced13C separation by a two-stage IRMPD process: IRMPD of CHClF2/Br2 mixtures

We have been studying the practical CO₂-laser-induced¹³C separation by a two-stage IRMPD process. The IRMPD of natural CHClF₂ in the presence of Br₂ mainly produced CBr₂F₂, which was found to be highly enriched with¹³C. The yield and¹³C-atom fraction of CBr₂F₂ were examined as functions of pulse number, laser line, laser fluence, total pressure, and Br₂ pressure using a CO₂ TEA laser with an output less than 1 J pulse^–1 in order to optimize experimental conditions for¹³C separation. For example, we obtained CBr₂F₂ at a¹³C concentration of 55% in the irradiation of the mixture of 100-Torr CHClF₂ and 10-Torr Br₂ with the laser radiation at a wavenumber of 1045.02 cm^–1 and at a fluence of 3.4 J cm^–2. The mechanism for the IRMPD is discussed on the basis of observed results. Using 8-J pulses, we were able to obtain 1.9×10^–4 g of¹³C-enriched CBr₂F₂ (¹³C-atom fraction, 47%) per pulse under selected conditions. It is possible to produce 90% or higher¹³C by the second-stage IRMPD of the CBr₂F₂ in the presence of oxygen.     

Carbon-13 separation by IRMPD of trifluoromethane-d0 and -d1

Carbon-13 isotope selectivities and specific decomposition yields in the infrared multiple-photon decomposition of CHF₃ and CDF₃ were examined as a function of wave number using a CO₂ TEA laser. The observed maximum selectivities were 30 for CHF₃ and 55 for CDF₃. The specific decomposition yields in CDF₃ were considerably larger than those in CHF₃.     

13C-selective IRMPD of CBr2F2/Cl2 and CCl2F2/Br2 systems

The CO₂ TEA laser irradiation of CBr₂F₂ in the presence of Cl₂ yielded ¹³C-enriched CBrClF₂ and ¹³C-enriched CCl₂F₂ under selected experimental conditions. As the photolysis proceeded, the ¹³C concentration of CBrClF₂ decreased gradually and that of CCl₂F₂ increased up to 90% or higher. These results can be explained by the mechanism involving the secondary ¹³C-selective IRMPD of the primary product CBrClF₂. On the other hand, the carbon-containing product for a CCl₂F₂/Br₂ system was only CBrClF₂; the further IRMPD of which probably regenerated CBrClF₂ in the presence of Br₂. The decomposition probabilities of ¹²C- and ¹³C-containing molecules in both systems were measured as functions of laser line, laser fluence, and reactant pressures.     

Chemical reactions following the IRMPD of C2F3Cl

C₂F₃Cl is photolyzed with a TEA-CO₂ laser at 1050.44 cm^–1 with focussed fluences up to 280 J/cm². The stable products in the IRMPD of C₂F₃Cl are determined for up to 10 Torr of C₂F₃Cl being photolyzed both neat and with added O₂. C₂F₄ and trans-C₂F₂Cl₂ are found to occur in the greatest yield though C₃F₅Cl, C₃F₄Cl₂, C₄F₇Cl, and C₂F₃Cl₃ also appear to be primary products. When O₂ is present F₂CO, FClCO, and CF₂ClCOF are the exclusive products. The formation of these products are for the most part consistent with a carbene formation dissociation mechanism for C₂F₃Cl IRMPD. C₂F₃Cl₃ may best be explained by another mechanism competitive with carbene formation. Many products attributed to secondary photolysis mechanisms are observed for long photolysis times.This work was performed at Department of Chemistry and chemical Engineering, Michigan Technological University, Houghton, MI 49931, USA     

Production of highly concentrated 13C by continuous two-stage IRMPD. CBr2F2/HI,CCl2F2/HI,and CBrClF2/HI mixtures

Difluoromethane CH₂F₂ containing 90–98% ¹³C was obtained in the selective IRMPD of mixtures of CBr₂F₂/HI, CCl₂F₂/HI, and CBrClF₂/HI. In CBr₂F₂/HI mixtures, the intermediate product CHBrF₂ resulting from the reaction between the initial decomposition fragment CBrF₂ and HI underwent secondary selective IRMPD to form highly ¹³C-enriched CH₂F₂ in continuous laser irradiation. The intermediate product in the mixtures of CCl₂F₂/HI and CBrClF₂/HI was found to be CHClF₂, but no significant secondary photodecomposition in CBrClF₂/HI mixtures occurred owing to the low absorption cross section of CHClF₂ at the adopted laser frequencies and fluences. The observed decomposition probabilities and selectivities under different conditions with respect to laser frequency, fluence, and partial pressures of halogenated difluoromethanes and HI suggest that CBr₂F₂ is one of the better candidates for practical ¹³C separation by IRMPD.     

Improved separation of the rare sulfur isotopes by infrared multiphoton dissociation of SF6

The dissociation probabilities of³²SF₆ and some of³⁴SF₆ have been measured at a large number of CO₂ laser lines both at room temperature and at 140 K. The longwavelength wing of this dissociation spectrum is exponential in the wavenumber. Its logarithmic slope is proportional to the inverse temperature. Selectivities are high enough at 140 K, that the photons are consumed only for the rare isotope in the case of³⁴SF₆ and nearly so for³⁶SF₆. For³³SF₆ further improvement of the selectivity would be desirable.     

13C-selective two-stage IRMPD of mixtures of CHClF2 and HI

The ¹³C-selective infrared multiple-photon decomposition (IRMPD) of mixtures of CHClF₂ and HI was examined in collimated and focused beam geometries using a CO₂TEA laser. The carbon-containing products were CH₂F₂ and CHF₂I. The former product showed remarkably high ¹³C atom concentrations beyond 95% under selected experimental conditions, while the latter contained 25% or less. The observed results can be explained satisfactorily in terms of the consecutive two-stage IRMPD process occurring in a single irradiation procedure, where the first-stage IRMPD of natural CHClF₂ produces ¹³C-enriched CHF₂I via the insertion of the initial decomposition fragment CF₂ into HI, and the second stage is the subsequent ¹³C-selective IRMPD of the CHF₂I to form a CHF₂ radical and an I atom. The CHF₂ radical reacts with HI to form CH₂F₂. Decomposition probabilities of ¹²CHClF₂ and ¹³CHClF₂ were measured as a function of laser fluence to optimize enrichment conditions. Furthermore, partial decomposition probabilities or relative production yields were measured as functions of laser line, pressure of HI, and pressure of CHClF₂. Both stages showed high ¹³C selectivities in the irradiation with the laser radiation around 1040 cm^–1 and at fluences below 4 J cm^–2. This mixture is one of the most promising chemical systems for the production of highly enriched ¹³C.     

Laser separation of oxygen isotopes by IR multiphoton dissociation of (CH3)2O

Isotopically selective (with respect to ¹⁸O) one- and two-frequency multiphoton dissociation of dimethyl ether (CH₃)₂O by pulsed TEA CO₂ laser radiation has been studied. The maximum primary selectivity, 16, is attained with the dissociation yields of the desired component (CH₃) ₂ ¹⁸ O ₁₈=5×10^–4 and 1.7×10^–2 for one- and two-frequency excitation, respectively. The dependences of MPD yields and selectivity on laser radiation frequency, (CH₃)₂O pressure, buffer gas (N₂) pressure and temperature have been measured. Multiphoton absorption coefficients have been measured and the average number of absorbed quanta calculated. The laser photon energy consumed for separating one ¹⁸O atom has been estimated: 11 and 4 keV/¹⁸O atom for one- and two-frequency excitation, respectively.     

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.On leave from the Institute of Physical and Chemical Research, Hirosawa, Wako-shi, Saitama, 351, Japan, Summer 1983Issued as N.R.C.C. No. 23638NRC/Acadia University Summer Student, 1983     

Fluence and wavelength dependence of the IRMPA and IRMPD of CDCl3 with a CO2 laser

A TEA CO₂ laser was used to study the infrared multiple-photon absorption (IRMPA) and dissociation (IRMPD) spectra of CDCl₃ in the fluence ranges 0.01–1.4 and 7–45 J/cm², respectively, for different sample pressures. Experimental results were modeled with a master equation formulation which includes rotational and anharmonic bottlenecks and collisional effects. Experimental and calculated results show that CDCl₃ has great rotational and anharmonic restrictions at the first stages of excitation. The IRMPD spectrum falls more slowly than the linear absorption spectrum at the blue wing due to intramolecular vibrational relaxation at the quasi-continuum level of excitation.     

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).     

Tritium isotope separation by CO2-laser irradiation at low temperatures

Tritium isotope separation by CO₂-laser induced multiphoton dissociation of CTF₃ is investigated. For the optimization of the performance of this working substance, trifluoromethane, the conditions to yield high-selectivity at high-operating pressure and low-critical fluence for complete dissociation are studied using our deconvolution procedure. The irradiation conditions are varied over the following ranges; wavenumber: 1052–1087 cm^–1, gas temperature: 25°C to –78°C, CHF₃ pressure: 5–205 Torr. The selectivities exceeding 10⁴ are observed for 85–205 Torr CHF₃ at –78°C by the irradiation at 1057 cm^–1.     

Laser-induced chemistry in silane-hexafluoroacetone mixtures for production of novel Si/C/F/O and C/F/O materials

Chemical reactions induced by CO₂-laser radiation in mixtures of silane and hexafluoroacetone afford various gaseous silicon- and carbon-containing compounds and result in deposition of microstructures of carbon, C/F/O and Si/C/O/F materials. These products are suggested to be formed by a variety of exothermic reactions initiated through SiH₄-photosensitized decomposition of hexafluoroacetone. Silane is shown to be a very potent reagent for the reduction of C-F bonds.     

Multiphoton absorption of broad-band CO2 laser radiation by SF6

Increase of the emission bandwidth of a high-pressure CO₂ laser up to 1.5 cm^–1 increases the multiphoton absorption cross-section of SF₆. Comparison with the previously found [9] increased absorption for shorter pulses suggests that this is also a bandwidth effect. Spectral structures as narrow as 1 cm^–1 above the 10^th absorption step are invoked to explain the observations. The temperature effect, which disappears in the broad-band case, confirms this view.     

Experimental selection of molecules for isotope separation by multiple-photon dissociation in an intense IR field

Consideration is given to selectivity estimation methods during polyatomic molecules dissociation by multiple photon absorption. Taking the CH₃NO₂ molecule as an example, the possibilities of selectivity estimations made on the electron-excited dissociation products (luminescence) are shown, as well as on the measurements of the energy absorbed in a strong IR field being transformed into heat. Some advantages of the last method of the selectivity estimation are discussed. The experiments on nitrogen-isotopes separation in the mixture of CH₃ ¹⁴NO₂ and CH₃ ¹⁵NO₂ molecules are carried out when exciting thev ₇ vibration with the isotope shift of about 7 cm⁻¹ and thev ₁₃ vibration with no isotope shift in the linear absorption spectrum. The contribution of secondary chemical reactions to the separation process is discussed.     

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     

CO2-laser isotope separation of tritium with pentafluoroethane-T (C2TF5)

Isotope separation of tritium by CO₂ laser-induced multiphoton dissociation (MPD) of C₂TF₅ is reported for the first time. The MPD spectrum obtained for C₂TF₅ comprised a broad peak at about 940 cm^–1 where C₂HF₅ was nearly transparent. The unimolecular dissociation of C₂TF₅ was induced with much lower laser fluence than that for CTF₃, another working molecule we proposed for laser isotope separation of tritium. The mechanisms and kinetics of the dissociation of C₂TF₅ and C₂HF₅ were investigated under various experimental conditions: laser frequency, pulse energy, pulse duration, tritium concentration, sample pressure, buffer gas pressure and irradiation geometry. Single-step separation factors exceeding 500 were achieved with the most efficientP(20) line in 00^o–10^o0 transition at 944.2 cm^–1.