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.     

Direct measurement of lifetimes of infrared multiple-photon dissociated molecules

A beam of mass selected SF ₅ ⁺ ions is crossed with a pulsed CO₂ laser beam. The distribution of lifetimes of infrared multiple-photon pumped SF ₅ ⁺ ion-molecules prior to dissociation into SF ₄ ⁺ +F has been measured directly using a time-of-flight ion-beam technique. The data provide new information on the distribution of the total internal energy of molecules excited by absorption of infrared radiation.     

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.     

Efficient laser isotope separation of 13C using novel linear multi-pass cavity

¹³C isotope has been separated in the form of enriched product C₂F₄ by selective multi-photon dissociation (MPD) of Freon-22 (CHClF₂) using the 9P(26) laser line of a transversely excited atmospheric CO₂ laser. The non-linearity factor, γ, that determines the dependence of the yield of ¹³C isotope on the fluence (J/cm²) has been determined for various laser rotational lines (9P(20)–9P(26)) and the advantage of a lower γ in the case of 9P(26) is highlighted for macroscopic production of ¹³C isotope. It is also shown that a higher value of the optimum fluence at 9P(26) not only results in a higher enrichment efficiency but in a relatively lower value of γ also. The laser pulse energy is efficiently utilized for selective MPD of Freon-22 by focusing the pulse energy repeatedly with the help of a novel linear multi-pass cavity (LMPC). The novelty of this optical arrangement lies in its ability to maintain the laser fluence around an optimum value for a desired enrichment of ¹³C in the product. This also ensures a higher quantity of enriched product because of the higher reaction volume. The advantage of the LMPC over the conventionally used Herriott multi-pass cell has also been presented. The gain in reaction volume in the present optical cavity having 20 passes with a constant fluence in each pass is as high as 12. Isotope-selective MPD of Freon in a LMPC with constant fluence in each pass showed a distinct advantage in energy utilization to separate ¹³C isotope over the gradually reducing fluence case.This revised version was published online in August 2005 with a corrected cover date.     

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.     

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).     

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     

Laser isotope separation of 13C: A comparative study

IR laser chemistry of (CF₃Br/Cl₂) mixture and neat CF₂HCl are examined in the context of ¹³C enrichment. Decomposition extent, enrichment factor and energy absorbed are measured for both systems at their respective optimum conditions. A direct comparison is obtained by keeping extraneous factors such as laser, its pulse duration, cell, irradiation geometry etc. the same. The halogen scavenged CF₃Br MPD requires lower fluence compared to neat CF₂HCl irradiation. Overall throughput for a product with 60–65% ¹³C content in a single stage is the same for both systems requiring a similar amount of energy. However, at lower enrichment levels, CF₂HCl MPD is better than (CF₃Br/Cl₂) photolysis in terms of both product yield and energy absorption.     

Infrared multiple-photon dissociation of CDCl3 induced by a TEA CO2 laser

Infrared multiple-photon dissociation (IRMPD) of CDCl₃ was studied using a tunable TEA CO₂ laser. Effects of number of irradiation pulses, wavelength and energy fluence as well as of sample pressure on the reaction yield are reported.     

Determination of diffusion coefficients of C2 radicals by laser induced fluorescence spectroscopy

Diffusion coefficients of C₂ radicals in Ar and H₂ were determined experimentally for the first time. The C₂ radicals were generated by a multi-photon laser photolysis and the spatial behaviour was observed by laser-induced fluorescence spectroscopy. Spatial distributions measured were compared with a theory to determine diffusion coefficients. It is shown that the present technique is suitable to measure diffusion coefficients of short-lived radicals that are difficult to measure by any other means.     

N2 production in e-beam pumped XeF lasers containing NF3

The rate of build-up of N₂ was measured in electron-beam-irradiated Ne/Xe/NF₃ mixtures using mass spectroscopy. the amount of N₂ produced indicated that N₂ is the primary nitrogen bearing stable species created in these mixtures. The rate constant for dissociative electron attachment to the NF₂ fragments produced in electron attachment to NF₃ is estimated to be 5×10^–8 cm³/s in order to explain the amount of N₂ produced.This work was supported by DARPA under Contract No. DAA01-82-C-A125 and monitored by MICOM     

Isotopically selective infrared multiphoton dissociation of 2-chloro-1,1,1-trifluoroethane: 13C selectivity and mechanism

13 C-selective infrared multiphoton dissociation of CF₃CH₂Cl has been studied by analyzing the distribution of ¹³C concentrations of the main products CF₂=CHCl, CF₂=CH₂, CF₂=CHF, C₂F₆, and the trace products CF₃CH₂CF₃ and CF₃CH=CHF₃. The mechanism mainly concerns the dissociation of energized CF₃CH₂Cl, the collisional stabilization of excited CF₃CH and CF₃CH₂ and the recombination of the nascent radicals. No significant radical–molecule reactions degrade the intrinsic ¹³C dissociation selectivity. High ¹³C production yield and ¹³C concentration can be attained at a laser fluence of 1.6 J/cm². Such low fluence can be used to improve focus condition and enhance photon utilization efficiency for practicable ¹³C separation. Received: 10 March 1998/Revised version: 17 September 1998     

LIF diagnostics of C2 radical behaviour in a laser CVD environment

A laser-induced fluorescence (LIF) system was developed to diagnose the reaction and transport of radicals in the ArF-laser-assisted CVD environment. The C₂(a ³ II _u) radicals were produced by the multi-photon dissociation of C₂H₂. The transport of the radical was directly measured by LIF. The disappearance rate of the radical in C₂H₂ was also determined. By using the values determined, the in-flux of the C₂ radicals onto a substrate was investigated, based on a diffusion model.     

Narrow resonances (Δv=2·10−2 cm−1) of multiple-photon IR absorption in ethylene

Narrow peaks of multiple-photon absorption in thev ₇ band of ethylene with the spectral width 0.02 cm⁻¹ and the contrast factor of up to 10² have been observed at the intensity of laser radiation 0.04 and 0.6 MW/cm². The multiple-photon spectra of ethylene in intense IR field have been studied with the use of a quasi-single-mode continuously tunable CO₂ laser. The results of the experiments are interpreted within the model of spepwise molecular excitation due to weak transitions.     

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.     

Large area excimer laser induced deposition of titanium from titanium tetrachloride

Large area excimer laser induced deposition of titanium on fused silica from TiCl₄ is studied with an emphasis on process modeling. We show that several TiCl₄ monolayers can be adsorbed if the surface is adequately prepared and that the Ti thin film growth occurs through the photodecomposition of this adsorbed TiCl₄ layer. We propose two growth regimes. During an initiation phase, up to 3 nm in thickness, the adsorbed layer is photochemically decomposed giving a growth rate of 0.015 nm/pulse. In a second phase, the deposition rate increases to between 2 and 7 nm/pulse due to the laser heating of the preceding photochemically deposited titanium film. Between consecutive pulses, TiCl₄ molecules primarily from the adsorbed layer diffuse to the reaction zone leading to a new adsorbed layer ready to be transformed to solid titanium.     

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%.     

ArF laser CVD of hydrogenated amorphous silicon: The role of buffer gases

ArF laser-induced CVD has been employed to generate hydrogenated amorphous silicon (a-Si:H) from Si₂H₆ gas dilute with He, Ar, or H₂. The formation of amorphous films or powder is found to depend critically on the kind of buffer gas, the stationary total and partial gas pressures, and the substrate temperature. These dependences have been investigated in the 1–5 Torr pressure and 100–400 °C temperature ranges. They are semiquantitatively discussed in terms of ArF laser photolysis of disilane, gas heating by heat flow from the substrate and laser irradiation, diffusion, and gas phase polymerization. Furthermore, photo ionization has been observed but found irrelevant for the a-Si:H layer properties. The photo and dark conductivities ( _ph, _d) of the semiconductor layers are determined by the substrate temperature. The _ph values range between 10^–7 and 10^{–4 –1} cm^–1 and the _d values between 10^–11 and 10^{–8 –1} cm^–1. The maximum ratio _ph/ _d amounts to 4×10⁴. The layers are further characterized by their optical band gap and activation energy. The layer properties are compared to literature values of amorphous films prepared by various photo, HOMO, and plasma CVD methods.     

Laser FM spectroscopy with photochemical modulation

We have combined the techniques of frequency-modulation spectroscopy (FMS) and photochemical-modulation spectroscopy to carry out high-resolution, high-sensitivity absorption measurements on the formyl and amino radicals. Using the (0,9⁰, 0)-(0,0¹, 0) band of theÃ,² A- ,² A transition of HCO at 614 nm, we obtained a sensitivity limit for absorption of 1.5×10^–6. Reconstructed spectra of several HCO lines are presented.     

Carbon-13 enrichment by IR laser chemistry of CHF3-Cl2

IR laser chemistry of CHF₃ is investigated in both neat form and in the presence of Cl₂ for carbon-13 enrichment. Infrared multiple-photon dissociation of CHF₃ is an order of magnitude more efficient in the scavenged system compared to the neat case. The photolysis of CHF₃/Cl₂ mixture results in two products, viz., CF₂Cl₂ and C₂F₄Cl₂ but with different enrichment factors. The parametric studies show that C₂F₄Cl₂ arises due to MPD of CF₂Cl₂ in secondary photolysis.