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.     

CO2-laser-induced multiphoton absorption of CF2Cl2

Energy absorption by CF₂Cl₂ from an intense TEA CO₂ laser pulse is measured as a function of the pressure of CF₂Cl₂ and the pressure of Ar bath gas for different laser energy fluences. The fraction of the molecules excited by the laser field and the average energy of the molecular ensemble are determined by a simple experimental method.     

Oxygen isotope separation utilizing two-frequency infrared multiphoton dissociation of 2,3-dihydropyran

Oxygen isotope separation has been examined by utilizing the two-frequency infrared multiphoton dissociation (IRMPD) of 2,3-dihydropyran (DHP). The two-frequency IRMPD reduces the required laser fluences to those lower than the damage threshold of optical windows. For example, dissociation probability of DHP containing ¹⁸O (D(¹⁸O)) and enrichment factor (S) were obtained to be 1.6×10⁻³/pulse and 316, respectively, by the simultaneous irradiation with 1052.2 cm⁻¹ photons at 0.45 J/cm² and 1031.5 cm⁻¹ photons at 1.06 J/cm². These are comparable with D(¹⁸O)=2.2×10⁻³/pulse and S=391 obtained by the single-frequency irradiation of 1033.5 cm⁻¹ photons at 2.2 J/cm². Therefore, the production rate of an ¹⁸O enriched dissociation product has been increased to four times or more, compared with the single-frequency IRMPD, and this two-frequency method would promise a practical large scale separation.     

T/D isotope selectivity in CO2 laser induced multiple-frequency multiphoton dissociation of trifluoromethane-T

Isotope-selective multiphoton dissociation of CTF₃ in the presence of CDF₃ by TEA CO₂ laser is studied. The highest T/D selectivity of 58 was observed at a sample pressure of 2 Torr in the presence of 20 Torr of argon on excitation by the 9P(24) CO₂ laser line. The effect of multiple-frequency irradiation on selectivity is studied in the P and Q branches of the v ₂ absorption band of CTF₃. No improvement in the selectivity is noticed on going from single- to multiple-frequency irradiation. These results are compared with previous multiple-frequency work on the CTF₃/CHF₃ system, and explained in terms of strong spectroscopic interference from ¹³CDF₃ in the present case.     

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     

Chemical etching of silicon by CO2-laser-induced dissociation of NF3

The pulsed infrared laser dissociation of NF₃ is reported for the first time, and is used to investigate silicon etching. The role played by collision-enhanced multiple-photon absorption and dissociation is considered, with data on the nonlinear decrease of the absorption cross-section with increasing pulse energy and increasing pressure presented. Using an experimental arrangement in which the laser beam is focussed parallel to the surface, the dissociation process induces spontaneous etching of silicon. Fluorinecontaining radicals diffuse from the focal volume to the surface where a heterogeneous chemical reaction occurs. Etching was monitored by use of a quartz-crystal microbalance upon which a thin film of amorphous silicon was deposited. For a surface with no previous exposure to the photolysis products, dissociation causes the formation of a surface layer prior to the onset of etching. X-ray photoelectron spectroscopy demonstrates this to be a fluorosilyl layer possessing a significant concentration of SiF₃ and SiF₄. In contrast, a surface already thickly fluorinated does not form a thicker layer once laser pulsing commences again. In this case, etching starts immediately with the first pulse. The etch yield dependencies on several parameters were obtained using silicon samples possessing a thick fluorosilyl surface layer. These parameters are NF₃ pressure, laser wavenumber, pulse energy, buffer gas pressure, and perpendicular distance from focal volume to surface. Modeling of the etch yield variation with perpendicular distance shows the time-integrated flux of radicals impinging on the surface to be inversely proportional to the distance. Attempts at etching SiO₂ under identical conditions were unsuccessful despite the evidence that thin native oxide films are removed during silicon etching.     

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.     

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).     

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.     

Laser isotope separation of tritium using CF3CTBrF/CF3CHBrF

A new working molecule 2-bromo-1,1,1,2-tetrafluoroethane is reported for laser isotope separation of tritium. The multiphoton dissociation rate of CF₃CTBrF is studied as a function of irradiation wavenumber, using a CO₂ or NH₃ laser. In a tightly focused irradiation geometry where the dissociation saturates both for CF₃CTBrF and CF₃CHBrF, their threshold fluences of dissociation and the geometrically biased selectivity are measured near the maximum of the dissociation rate of CF₃CTBrF. When the irradiation geometry is optimized, a high selectivity exceeding 2700 is obtained.     

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.     

Separation of tritium from deuterium by pulsed NH3 laser-selective multiple photon dissociation of CTCl3

The multiple photon dissociation of ppm level CTCl₃ in CDCl₃ and the selectivity of T/D separation were investigated using a pulsed ammonia laser. The effect of laser frequency, fluence and buffer gas pressure on the dissociation rate and isotopic selectivity were studied. The depletion of CDCl₃ was not observed within experimental errors. A lower limit of single step selectivity factor was found to be >5000 at 133 Pa substrate pressure.     

Selectivity enhancement in tritium isotope separation by multiple frequency multiple photon dissociation of the CTF3/CHF3 System

Isotope separation of tritium by multiple photon dissociation process in multiple frequency fields of a TEA-CO₂ laser is reported for the first time. A ten-fold improvement in the bulk selectivity was obtained in 8.5 Torr CTF₃/CHF₃ in the presence of buffer gas at room temperature using 9R(8) to 9R(14) CO₂ laser lines compared to single frequency excitation. Investigations of various process parameters such as exciting laser frequencies, pulse energy, sample and buffer gas pressure indicate that this is a promising technique for the separation of tritium.     

Infrared multiphoton dissociation of13CF3Cl induced by CO2 laser pulses

Selective infrared multiphoton dissociation of¹³CF₃Cl induced by CO₂ laser pulses adjusted on = 1071.9 cm^–1 has been studied in the energy rangeE between 0.5 and 2 J per laser pulse or fluence range between 5 and 25 J per cm², and in the pressure range between 0.10 and 60 Torr. The effect of these parameters on the isotopic selectivity of the dissociation gives information on the rotational relaxation constants. As for the dissociation probabilities, they vary exponentially withE ^–1. The applicability of such an Arrhenius-type relation is discussed and semi-quantitatively justified.     

Laser multiphoton mass spectroscopy of organometallic compounds: State selective and mass resolved detection of neutral fragment tellurium atoms from C2H5TeC2H5

Typical features in laser multiphoton ionization of organometallic compounds are well evident in the case of the diethyltelluride C₂H₅TeC₂H₅ molecule. The use of a tunable dye laser coupled with time-of-flight (TOF) mass spectroscopy has allowed to establish that a large amount of tellurium is eliminated from the parent molecule as a neutral atom either in its ground or low excited states. Sharp two- and three-photon atomic Te resonances, which give origin to extraproduction of Te⁺ ions, have been identified in the optical spectra measured by varying the laser wavelength.     

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     

Investigations of multiphoton selective dissociation of (CH3)2O in the field of a pulsed CO2 laser

Experimental investigations of multiphoton selective dissociation of (CH₃)₂O induced by a pulsed CO₂ laser have been conducted. Separation of H, C, and O isotopes was performed in enriched mixtures and in samples with the natural abundance. The following coefficients of selectivity have been obtained:K _D/K_H=4.0,K ₁₃/K₁₂=1.7, andK ₁₈/K₁₆≧1.6. We studied the dependences of the selectivity coefficient on ether pressure, on the laser energy and frequency as well as the influence of secondary chemical reactions on the dissociation selectivity. Estimations made by using the RRKM theory have indicated that ether molecules that decompose have an average excitation energy above the dissociation threshold of ∼1.5 kcal/mole.     

Strong isotope effects on the charge transfer in slow collisions of He2+ with atomic hydrogen, deuterium, and tritium

Probabilities and cross sections for charge transfer by He2+ impact on atomic hydrogen (H), deuterium (D), and tritium (T) at low collision energies are calculated. The results are obtained using an ab initio theory, which solves the time-dependent Schr?dinger equation. For the H target, excellent agreement is achieved between the present and previous results. Differences by orders of magnitude are observed between the cross sections for H, D, and T. A method is introduced to separate the contributions of charge-transfer mechanisms due to radial and rotational coupling. The large differences observed for H, D, and T are attributed to isotope effects in the rotational coupling mechanism.