Frequency dependent kinetics of the cw CO2 laser induced reaction of CF3Br

Kinetics of the decomposition of CF₃Br at 20 torr by a cw CO₂ laser have been studied over the range of laser frequencies 1043–1085 cm^-1. At constant translational temperature the change in the rate constant with laser frequency over the frequency range is a factor of 500, comparable to the effect previously observed in CF₂ClCF₂Cl and CF₃CF₂Cl. Arrhenius plots show an activation energy of 64.9 kcal/mole, independent of frequency. The laser induced optical absorption of CF₃Br exhibits a hysteresis effect at the lower laser frequencies.     

Excitation and dissociation of polyatomic molecules under the action of femtosecond infrared laser pulses

The effect of high-intensity femtosecond laser pulses (100–200 fs) in the near (0.8–1.8 μm) and medium (4.6–5.8 μm) IR ranges on the CF₂HCl, CF₃H, (CF₃)₂C=C=O, and C₄F₉COI molecules is examined. Irradiation of CF₂HCl and CF₃H molecules by 0.8-to 1.8-μm laser pulses with intensities of >40 TW/cm² (>4 × 10¹³ W/cm²) makes them dissociate to yield CF₃H and CF₄, respectively. The key mechanism of the dissociation of these molecules is field ionization and fragmentation. The excitation of the stretching vibrations of the C=O bond in the (CF₃)₂C=C=O and C₄F₉COI molecules by 4.5-to 5.8-μm femtosecond pulses produced no detectable dissociation up to a fluence of ∼0.5 J/cm² (or a intensity of ∼2.5 TW/cm²). Probable explanations of this observation are discussed. Original Russian Text ? V.M. Apatin, V.O. Kompanets, V.B. Laptev, Yu.A. Matveets, E.A. Ryabov, S.V. Chekalin, V.S. Letokhov, 2007, published in Khimicheskaya Fizika, 2007, Vol. 26, No. 4, pp. 18–25.     

Laser-pumped molecular laser

The parameters of laser-pumped molecular lasers are investigated. It is established that the energy and tuning characteristics of an NH₃ laser (E_g = 1.5 J, efficiency 20%, P_av = 20 W, radiation frequency tuning band 753–890 cm^-1) are decisively influenced by the addition of N₂. A focusing raster optical-pumping system has made it possible to obtain a specific lasing energy 12 J/liter. A CF₄ laser with lasing energy 40 mJ operates in the 612–655 cm^-1 band. Experiments on dissociation of the molecules CCl₄ and UF₆ were carried out with the aid of NH₃ and CF₄ lasers. The systems and methods of producing Raman lasers (RL) are presented. An effective RL amplifier on rotational transitions in compressed H₂, which transforms tens of beams of Nd lasers into one coherent beam of the first Stokes component with λ ? 1.13 μm at an efficiencyup to 70%, is described.     

Weakly forbidden vibration-rotation transitions ΔR ≠ 0 in CF4 laser

Known spectroscopic data on the 16 μm CF₄ laser are completely explained by considering weakly forbidden ΔR ≠ 0 vibration-rotation transitions. A rich fine structure of the v₂ + v₄ pumping band with specific spectral interval 0.01 + 0.03 cm^-1 has been experimentally observed. The components of this structure have been used for optical pumping of a CF₄ laser by a quasi-single mode continuously tunable high-pressure CO₂ laser.     

Effect of rotational relaxation on the yield of the multiphoton dissociation of CF3Cl and CF3Br under TEA-CO2 laser pulses

The yield of the multiphoton dissociation of CF₃Cl and CF₃Br induced by TEA-CO₂ laser pulses has been studied in the pressure range between 0.25 and 8 Torr, the laser wavelength being chosen so as to excite preferentially the minor isotopic components¹³CF₃Cl and¹³CF₃Br. For both compounds the dissociation probability is found either to increase almost linearly or to decrease monotonously with gas pressure, according as the laser beam is focused or unfocused, respectively. This behaviour is explained by rotational relaxation effects, and a value of 22ns·Torr for the rotational self-relaxation time of CF₃Cl is obtained.     

Transient molecular absorptions induced by the absorption of CO2 laser radiation

Laser-induced molecular absorptions are reported for the species SF₆, CF₃I, and CF₃Br. Induced absorptions are observed at frequencies ～ 40 cm^-1 lower than that of a CO₂ TEA laser which is used to vibrationally excite the molecules in a low pressure cell. The dependence of the effect on pump laser intensity indicates that the species that exhibit the induced absorption have absorbed more than one laser photon. Applications of this phenomenon with respect to laser induced isotope separation are discussed.     

Design and the operational characteristics of a 16 µ CF4 laser

The design and the operational characteristics of a CO₂ laser pumped CF₄ laser developed at BARC are reported. Output energies of up to 20 mJ have been obtained at 615 cm⁻¹ with an absorbed energy conversion efficiency of 10%.     

High repetition rate CH4 laser

A 16-m CF₄ laser oscillator has operated at 1 kHz in a cooled static cell. Pump energies required from the low pressure, Q-switched, cw discharge CO₂ laser were as low as 60 J. The laser cavity employed a multiple-pass off-axis path resonator in a ring configuration. CF₄ laser power at 615 cm^–1 and a 1-kHz repetition rate exceeded 300 W.     

Infrared multiple photon decomposition of CHBrF2 induced by a free-electron laser

2 . The laser generates an intense infrared macropulse with a duration of 17 μs; the macropulse consists of a train of 380 micropulses, each of which has a duration of a few picoseconds. The fluence of a macropulse was estimated to be about 16 Jcm^-2 at a beam waist. Peak wavelengths were set in the range of 9–10 μm. The macropulse induced the IRMPD of 1 and 5 Torr CHBrF₂; most of molecules in the focal region seemed to decompose at a wavelength of 9.3 μm. The mechanism is the initial decomposition of CHBrF₂ to CF₂ and HBr, followed by the dimerization of CF₂ to form C₂F₄. The decomposition was found to be isotopically selective at 9.7 μm; the final product C₂F₄ had a ¹³C atomic fraction of 6%. Th e addition of CO₂ to CHBrF₂ significantly decreased the yield of C₂F₄. vibrationally excited CHBrF₂ molecules produced by laser pulses were efficiently deactivated by CO₂ molecules. Received: 7 October 1996     

cw far-infrared lasing in15NH3, stark resonantly pumped by a CO2 or N2O laser

CF₂ClCF₂Cl and ethyl acetate have absorption bands of similar width centered at the same frequency. Kinetics of the decompositions of these compounds by a cw CO₂ laser have been studied over a range of laser frequencies extending to 25 cm^?1 below band center. At constant translational temperature and pressure, the CF₂ClCF₂Cl rate constant changes by more than 200 with frequency, while the corresponding change for ethyl acetate is at most 3. The effect of laser frequency increases with increasing CF₂ClCF₂Cl pressure, while the reverse is true in ethyl acetate. Arrhenius plots show activation energies independent of both frequency and pressure.     

Absorption of CO 2 laser emission by freon-12

The infrared (IR) absorption of freon-12 (CF₂Cl₂) was studied in the emission range of a 3-W tunable CW CO₂ laser by using a brass cell with KBr windows that was located outside the laser resonator. The results show that CF₂Cl₂ absorbs all CO₂ laser emission lines in the ranges of 1073–1083 cm^-1 and 937–943 cm^-1. The most strongly absorbed laser line was 10P (28) ( 937.21 cm^-1). Absorption coefficient values were obtained for all available wavelengths of the CO₂ laser as the CF₂Cl₂ pressure was varied from 5 to 1000 mbar. By using the HITRAN database for freon-12, the absorption coefficients were calculated at the 10P (28) and 9R (28) lines as functions of the gas pressure and compared with the experimental values. The calculated results are in reasonable agreement with the experiment. PACS 33.20.Ea; 42.55.-f; 42.55.Lt     

Features of kinetic processes in the active medium of a pulsed chemical oxygen-iodine laser

Conclusion It can be concluded from our experiments and calculations that the product CF₃O₂ of the interaction between the CF₃ radical and the O₂ molecule quenches the oxygen O₂(¹) more strongly. At low chlorine admixture density in the singlet-oxygen stream this output energy of the oxygen-iodine laser with CF₃I as the atomic iodine donor is lower compared with CH₃I. The rate constant of quenching singlet oxygen by CF₃O₂ molecules is (3–5)·10^–11 cm³·sec^–1. It would be possible to decrease the influence of CF₃O₂ by adding to the initial O₂ ^*–O₂–CF₃I–Ar active mixture some other substance causing the CF₃ radicals to enter in a chemical reaction with a shorter characteristic time than that for CF₃O₂ formation. Of course, neither the initial substance nor the reaction products should quench O₂ ^* noticeably. This role can be possibly assumed by the NO molecule.The influence of the chlorine additive on the output energy of a laser with CH₃I and CF₃I differs greatly. The choice of the chlorine donor must therefore be determined by the amount of this additive. CH₃I is preferable if the chlorine is fully utilized in the singlet-oxygen laser, and CF₃I in the opposite case.Quantum Radiophysics Division, Lebdev Physics Institute. Translation of Preprint No. 21 of the Lebedev Physics Institute, Moscow, 1991.     

Relevance of polarization phenomena in an optically pumped CF4 laser

Some applications of polarization phenomena in an optically pumped CF₄ lasers are reported. It is shown that the performance of a CF₄ laser can be greatly improved when the polarization characteristics of its optical cavity are chosen according to the types of transitions involved in the optical pumping and emission processes. It is also shown that polarization phenomena can be used to obtain an indication of the type of pumping and lasing transitions.     

Long lifetime operation of an ArF-excimer laser

An experimental investigation to improve the lifetime of a discharge-excited ArF-excimer laser is presented. The three dominant factors restricting its lifetime are CF₄ generation in the laser gas, color-center formation in the optics and input power density reduction due to electrode ablation. Copper electrodes were superior to nickel electrodes in regard to electrode ablation. A gas lifetime of more than 10⁹ shots (about one month at 400 Hz) is shown for an ArF-excimer laser with a liquid-nitrogen trap and high-temperature zirconium alloy trap.     

Quantitative infrared photochemistry with CO2 laser of different temporal shape: Dissociation of CF3I with nanosecond pulses

Infrared laser pulses of different temporal pulse shape with a length of 2–50 ns and a nominal power of 100–1000 MW are generated by a TEA CO₂ laser oscillator combination using saturable intracavity absorbers and a fast CdTe electro-optical switch. The multiphoton dissociation of CF₃I + nhv → CF₃ + I at 1074.6 cm⁻¹ and p(CF₃I) = 10 Pa was studied using these pulses. The product yields and the absolute rate constants were determined for different temporal pulse profiles, showing clear intensity effects at low intensities and a transition to a linear intensity regime at higher intensities.     

The photodissociation of UF6 using infrared lasers

The ir collisionless multiple photon absorption (cmpa) photodissociation of UF₆ is reported. Single frequency photodissociation is a accomplished with the focused output from a pulsed CF₄ laser operating at 615 cm^-1. When focusing the CF₄ laser output with a 7.5 cm f.1. lens, photodissociation is observed at laser energies as low as 5 mJ. By using a CO₂ laser (0.7 J, 1077 cm^-1) in concert with the CF₄ laser, significant enhancement (factors of 10–100) of the photodissociation yield is obtained at low CF₄ laser energies. Electronic emission is observed from the focal region, but only when both lasers are present.     

Reactions induced in (CF3I) n clusters by femtosecond UV laser pulses

The excitation and ionization of CF₃I molecules and their clusters by femtosecond UV laser pulses is studied. It is concluded that the types of excitation of free CF₃I molecules and their clusters by femtosecond UV laser pulses are different. The composition and kinetic energy of ion products observed upon the ionization of (CF₃I) _n clusters by femtosecond pulses are found to differ considerably from those obtained upon ionization by nanosecond pulses. It is shown that the molecular I ₂ ⁺ ion is produced in reactions induced in (CF₃I) _n clusters by UV radiation. Using the pump-probe method, we found the two channels of producing I ₂ ⁺ ions with characteristic times ??₁ ?? 1 ps and ??₂ ?? 7 ps. A model of the reactions under study proposed in the paper is consistent with our experimental results.     

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.     

Improved isotope separation in laser dissociation of trifluoromethyl halides: Scavenging of CF3 with HI

The efficiency of¹³C isotope separation by multiphoton dissociation using a CO₂ laser has been shown to increase by ∼33% upon addition of small amounts of HI to the starting mixture. The reason for this increase is that the CF₃ radicals are scavenged by HI more rapidly than they recombine with free iodine atoms to reform the starting material. For Torr and for 0.5J/cm² roughly 65% of the CF₃ and I radicals reform the starting material in the absence of HI. The results of this study indicate that at −80°C the rate constants for CF₃+CF₃→C₂F₆ and CF₃+I→CF₃I are about equal and are roughly 8 times higher than that for CF₃+HI→CF₃H+I.     

Shorter dye laser wavelengths from substitutedp-terphenyls

The spectral and laser characteristics of 14 CH₃-, CF₃-, and F-substitutedp-terphenyls have been investigated. 2,2″-dimethyl-p-terphenyl in cyclohexane solution exhibits a shortest tuned laser wavelength of 311.2 nm, thus extending the range of dye laser emission by 1000 cm⁻¹ in the uv. Peak output powers of 1.5 MW and 14% conversion efficiency at 332 nm were obtained under KrF-laser pumping in an untuned cavity.