Practical separation of silicon isotopes by IRMPD of Si2F6

Large-scale silicon isotope separation based on the IRMPD of natural Si₂F₆ has been carried out using a commercially available high power CO₂ TEA laser and a flow reaction system. The decomposition product SiF₄ containing 19–33% of ³⁰Si was obtained at a production rate of 1.5×10^–2–2.6×10^–2 mol·h^–1, depending on experimental parameters such as laser wavelength, laser fluence, pressure, and flow rate. SiF₄ containing 12% of ²⁹Si was obtained under slightly different conditions, i.e., at a shorter wavelength than that for ³⁰Si. When 39% of Si₂F₆ was decomposed at a slow flow rate, residual Si₂F₆ was found to have 99.7% of ²⁸Si. The production rate was 4.2×10^–2 mol·h^–1.     

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%.     

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     

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.     

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.     

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.     

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.     

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     

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.     

Contribution to time-resolved enhanced chemical etching and simultaneous annealing of ion implantation amorphized silicon under intense laser irradiation

Surfaces of single-crystal silicon wafers are amorphized by high-dose phosphorous ion implantation. These surfaces of the wafers, immersed in concentrated KOH, are laser-chemically etched by pulse irradiation of a ruby laser. Simultaneously, the remaining parts of the amorphous layer are annealed. The time dependence of the etching process enhanced during pulse irradiation is recorded and analysed. Reasons for the etching rates which differ between amorphous and single-crystal silicon are given on the basis of experimental and numerical results.     

Lead films produced by laser vapor deposition

The deposition of lead film on quartz substrates by photolysing tetraethyl lead vapor with a laser is reported. The dependence of the deposition rate on the pressure of tetraethyl lead, on the light intensity and on the pressure of the buffer gas is measured. Deposition phenomena at low light intensity are discussed.     

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.     

Influence of electric field on heterogeneous reactions stimulated by laser light

We present experimental data for vanadium and copper oxidation by cw CO₂ laser light in an external electric field. Direct influence of the external field on the crystallisation process has been demonstrated in both vanadium and copper cases; the oxidation rate and surface morphology depend sensitively on the sign and strength of the external field.     

Geometrical effects on the isotopically selective photodissociation of SF6 under CO2 laser pulses

A linear relationship is found between the³⁴S isotopic enrichment factor per pulse and the focal distance of the lens used to concentrate the laser beam. From this, one derives a threshold power of 26 MW/cm² for photodissociation, a mean absorption of 100 photons of 10.59 μm wavelength per³²SF₆ dissociation, and a dependence of the enrichment factor on the 3/2 power of the laser pulse energy.     

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.     

UV desorption and photochemistry of dimethylgold hexafluoroacetylacetonate adsorbed on a quartz substrate

The photodesorption and photodecomposition pathways of dimethylgold hexafluoroacetylacetonate, DMG (hfac), adsorbed on a cooled quartz substrate is reported for 222-nm KrCl excimer laser radiation. The time-of-flight (TOF) of neutral photoproducts, desorbed from the surface of the gold film formed during the experiment, were analyzed under collisionless conditions by a differentially-pumped mass spectrometer. Extensive dissociation of adsorbed DMG (hfac) into DMG and the hfac ligand was observed. The ligand was found to recombine with a CH₃ radical on the surface. Translational energy distributions for the detected species were obtained by deconvoluting the TOF curves into a self-consistent set of Maxwell-Boltzmann distributions for the desorbed parent molecule, laser-induced decomposition products, and surface recombination reaction products. The implications of these results for the mechanistic details of the low-pressure, laser-assisted organometallic deposition of DMG(hfac) are discussed.ONT/NRL Research Associate (Nov. 1987-Oct. 1988)NRC/NRL Cooperative Research Associate     

Dopant-enhanced ablation of nitrocellulose by a nitrogen laser

The photoetching behavior of pure nitrocellulose and of nitrocellulose dyed with stilbene-420, coumarin-120 and rhodamine 6G by 337 nm nitrogen laser pulses has been studied. Ablation with a low power nitrogen laser is hereby reported for the first time. A two step photochemical mechanism is proposed to account for the ablation of the pure material. With the addition of dyes strongly absorbing at 337 nm the photoetching rate of nitrocellulose can be increased significantly. This increase is proportional to the molar extinction coefficient of the dye at 337 nm and its concentration in the polymer. The photoetching mechanism and the energy transfer processes from the dye to the polymer are discussed in detail.     

PET surface modifications by treatment with monochromatic excimer UV lamps

Received: 11 August 1997/Accepted: 14 August 1997     

The role of excited species in UV-laser materials ablation

The stability of a planar surface upon pulsed UV-laser irradiation is studied with special emphasis on polymer ablation. Here, we consider a two-level system in which the excitation energy is dissipated via stimulated emission, non-radiative transitions, and activated desorption of excited species. With thermal relaxation times t _T10^–10 s the ablation front turns out to become stable. This could explain the smooth surfaces obtained after pulsed UV-laser ablation of pure and stress free organic polymers. The situation is quite different for materials, for example metals, where fast thermal relaxation of the excitation energy within times, typically, t _T<10^–11 s, gives rise to instabilities which result in surface roughening.On leave from the Institute of General Physics, Russian Academy of Sciences, 117942 Moscow, RussiaOn leave from the Institute of Applied Physics, Russian Academy of Sciences, 603600 Nishnii Novgorod, Russia     

The role of excited species in ultraviolet-laser materials ablation III. Non-stationary ablation of organic polymers

The role of non-stationary effects in nano-second ultraviolet (UV) excimer-laser ablation of organic polymers is discussed. The model includes reversible changes in absorption related to darkening and bleaching effects. Comparison of calculations and experimental data for polyimide demonstrates that the photophysical model describes the ablation kinetics quite well.