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.     

Dye laser probing of primary C2 radicals formed by IR intense field photolysis of ethylene

The C₂-radicals in the lower electronic statea ³ II _u have been studied by the method of laser-excited luminescence of products of ethylene IR photolysis. The dependence of C₂-radical concentration on IR field power density and its time evolution have been investigated. The threshold characteristics of C₂-radical occurrence and the ratio between excited and unexcited radicals have been determined. Different mechanisms for their formation are being discussed.     

Enrichment of carbon-14 by selective laser photolysis of formaldehyde

Using a frequency-doubled dye laser, simultaneous high-resolution spectra of¹⁴CH₂O and¹²CH₂O between 290 and 345 nm were measured. About 30 lines with spectral selectivities for¹⁴C≳50 were found. Photolyses on one such line at 326.94 nm of a dilute mixture of¹⁴CH₂O in natural CH₂O gave one-step enrichment factors of up to 150, with μg yields of enriched product. Since a factor of 150 in¹⁴C concentration corresponds to ∼7.2 half lives or 41,000 years, laser enrichment of archaeological samples, especially when combined with direct detection methods of¹⁴C abundance measurement, can greatly improve the range of radiocarbon dating.     

CO2 laser ablative etching of polyethylene terephthalate

Films of polyethylene terephthalate (PET) can be successfully etched with 9 m radiation from a pulsed TEA CO₂ laser. The relationship between etch depth and fluence is broadly similar to that observed for excimer laser etching but with a less well-defined threshold. Time-resolved photoacoustic measurements of stress waves generated in the interaction show that at a fluence of 1.8 J cm^–2 ablation occurs 100–200 ns after the start of the laser pulse, a time which is consistent with the rate of thermal decomposition of PET. The volatile products of ablation are carbon monoxide, carbon dioxide, methane, ethyne, ethene, benzene, ethanal, and small quantities of other products. For fluences close to and appreciably above the threshold the ablated material consists predominantly of involatile species of relatively high molecular weight, whereas at higher fluences substantial fragmentation of the polymer to small molecules occurs.School of Chemistry     

Synthesis of ethylamine with a cw tunable CO2 laser

The successful synthesis of ethylamine with a cw tunable CO₂ laser is reported. This action occurs at normal pressure (5.32×10⁴ Pa) and temperature (<100° C). No catalyst is used. The experiment shows a high directionality of this reaction. No other product except ethylamine is yielded. A possible mechanism for this reaction 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     

On-line computer controlled cw dye laser spectrometer for laser isotope separation

An on-line cw dye laser multichannel spectrometer is described, which can be controlled by FORTRAN. Automatic tuning of a dye laser to the wavelength of maximum excitation selectivity for photochemical isotope separation is achieved. Applying the system to the selective photoaddition of iodine chloride ICl to acetylene, an enrichment factor of³⁷Cl in the photoproduct C₂H₂ICl of β=100±5 was obtained. Both stable chlorine isotopes can be enriched to more than 97% purity in a single step, starting from natural samples in both cases.     

Excimer laser photolysis of molybdenum hexacarbonyl with buffer gas

We have studied the photolysis of molybdenum hexacarbonyl by focused excimer laser radiation. It was found that electronically excited Mo atoms detected in the focus region of a KrF laser are due to a direct two photon absorption transition. The upper limit of the energy for complete dissociation of Mo(CO)₆ has been derived from these results. Two photon dissociation in the gas phase should be the dominant process at metal film deposition on substrates positioned perpendicularly and near to the focus. Adding buffer gases to the organometallic vapor particle formation was observed in the whole irradiated gas volume. The analysis of scattered He-Ne laser light yields information about the density and size of these particles. Some conclusions are drawn with respect to structured metal film deposition with high spatial selectivity.Dedicated to Prof. Dr. Herbert Welling on the occasion of his 60th birthday     

The application of a high pulse repetition rate CO2 laser with high average power for isotope separation by molecular dissociation in a strong IR field

Considering a SF₆ molecule we demonstrate feasibility of using high pulse repetition rate CO₂ laser for isotope separation by selective molecular dissociation in a strong IR field. Dependences of dissociation efficiency as well as separation selectivity on pulse repetition rate up to 150 Hz are investigated. The inherent thermal effects are discussed.     

Pulsed-laser photolysis of Mn2(CO)10: A time-resolved fluorescence study

Mn₂(CO)₁₀ was photolysed in the gas phase by the XeCl-excimer laser with fluence in the range 25–300 mJ/cm² and a dye laser. The UV/VIS emission of the products was probed on a nanosecond time scale. The emission from excited states of metal atoms was detected only. The Mn atoms are predominantly formed in their ground statea ⁶ S _{2 1/2}. The absorption of one photon and the subsequent relaxation process leads to the formation ofz ⁶ P _J ⁰ (J = 11/2, 21/2, 31/2) states and emission of photons at a wavelength of 403 nm. The formation of the excited statese ⁸ D _{5 1/2},z ⁶ F _{4 1/2} ⁰ ande ⁶ D _{4 1/2} and the subsequent emission observed at wavelengths of 357, 383 and 446 nm requires the absorption of two photons by the ground-state Mn atoms. In addition, transition from thea ⁶ D _j (J = 11/2, 21/2) lower states were observed in the wavelength-resolved Laser-Induced Fluorescence (LIF) spectra.     

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.     

Macroscopic enrichment of12C by a high-power mechanically Q-switched CO2 laser

The isotope-selective multiphoton dissociation of CHClF₂ in a multipass refocusing Herriott cell was used to enrich more than 4 moles of chlorodifluoromethane to 99.99% of¹²C isotopic purity. It is the largest isotope quantity ever separated by a laser process. A cw excited mechanically Q-switched CO₂ laser, which delivers 16 mJ pulses at 5 kHz was used. The enrichment controlled by a mass-spectrometer and guided by a PC was run with a rate of 25 g¹²C per 24 h.     

Energy and pressure dependence of the CO2 laser induced dissociation of sulfur hexafluoride

Low pressure SF₆ with its isotopes in natural abundance was irradiated by a pulsed CO₂ laser operated on theP20 line (10.6 μm band). Dissociation yields of³²SF₆ and³⁴SF₆ were measured separately. If the radiation is focussed into the cell, the dissociation yield is proportional to the 3/2 power of the laser energy, as was derived under general conditions and confirmed experimentally. The reaction probabilityP(Φ), the fraction of molecules dissociated by an energy flux Φ, was measured using parallel light. For both isotopes,P(Φ) saturates at high energy flux close toP=1. At a lower flux (2 J cm⁻²), the dissociation probability of³²SF₆ displays a threshold, whereas the dissociation probability of³⁴SF₆ is a very steep function of Φ over the whole range of fluxes.P(Φ) at the higher energy flux was measured in a cavity absorption cell, in which up to 80% of the molecules were dissociated by a single pulse. Below 0.2 mbar SF₆ the dissociation yields for both isotopes are pressure independent. Above 2 mbar the isotopic selectivity is completely lost. Addition of hydrogen always decreases the dissociation yields.     

Characterization of a Br(2P1/2)–CO2(1001-1000) transfer laser driven by photolysis of iodine monobromide

2 laser operating on the 10⁰1-10⁰0 transition at λ=4.3 μm and pumped by E –V energy transfer from Br(²P_1/2) has been demonstrated. The dynamics and performance of this device were characterized by observing the time-resolved stimulated emission and the steady-state spontaneous side fluorescence after photolysis of IBr or Br₂ by a frequency-doubled Nd:YAG laser or Ar⁺ laser, respectively. Although the E –V excitation kinetics are favorable, rapid vibrational relaxation limits laser action to CO₂ pressures of less than 1 Torr. Numerical modeling of laser pulse shapes and the dependence on IBr and CO₂ pressure and photolysis energy establish a relatively high gain of 0.33%/cm, a CO₂-pressure-dependent optical loss of 0.04–0.06%/cm, and an efficiency of 2×10^-5 4.3-μm-laser photons per incident photolysis photon. The CO₂ fluorescence after photolysis of a fixed Br₂/CO₂ gas mixture decreases as a function of photolysis time by about 30%/h, indicating the photolytic production of an important quencher. Received: 23 June 1997/Revised version: 23 September 1997     

Tunable UV laser photolysis of organometallics with product detection by laser mass spectroscopy: Trimethylaluminum

We use tunable UV laser light in the region 200–320 nm, produced by frequency doubling the output of a dye laser, for the decomposition of organometallic compounds. This method has been applied to TMA, trimethylaluminum Al(CH₃)₃. Only the TMA monomer absorbs UV light for >220 nm. TMA decomposes by one-photon absorption mainly into two channels: aluminum atoms Al plus organic fragments, and aluminummonomethyl AlCH₃ molecules plus organic fragments. The ratio [Al]/[AlCH₃] is wavelength dependent. We present a mechanism to explain the photolysis of trimethyl compounds of group III elements (Al, Ga, In).     

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.     

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.     

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.     

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.     

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.