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.     

Self-focussing of CO2 laser radiation in SF6

Self-focussing of high-power TEA CO₂ laser pulses for a number of 10 m P-band lines is reported in SF₆ molecular gas. Application of this effect to estimating the intensity-dependent refractive index of the gas is demonstrated.     

Self-focusing and defocusing of TEA CO2 laser radiation in NH3

We report on the observation of self-focusing and defocusing of TEA CO₂-laser pulses in NH₃. Self-focusing of the 9R(30) CO₂ laser line resulted in a significant spatial narrowing of the laser beam after propagating through a 100 cm long cell containing NH₃ at a relatively low pressure of 1 Torr. Spatial broadening, characteristics of self-defocusing, has been observed for the 9R(16) CO₂-laser line.     

The effect of mid-infrared and far-infrared emission,generated at NH3 excitation by intense radiation of a TEA CO2 laser,on ammonia absorption

The effect of Mid-InfraRed (MIR) ( 12 m) and Far-InfraRed (FIR) ( 100 m) emission from excited ammonia on the absorption of intense radiation of a TEA CO₂ laser has been studied experimentally under collisional and collisionless excitation conditions with ammonia pressures from 0.5 to 0.03 Torr. The energy of MIR and FIR emission was studied as a function of NH₃ pressure and laser energy fluence. Particular emphasis was given to the kinetics of MIR and FIR emission generation at different NH₃ pressures and to the measurement of the time delay of re-emitted pulses relative to the exciting CO₂ laser pulse. It has been found that the re-emission in the MIR range is highly collisional in nature. The intensity of MIR emission drops sharply (asp ³) with decreasing NH₃ pressure and its delay time relative to the exciting laser pulse increases. At the same time, re-emission in the FIR range (in the case of resonant excitation of NH3 at the 9R (30) line of CO₂ laser) is observed during an exciting pulse up top < 0.03 Torr. When binding the rotational sub-levels of a molecule with transitions, FIR emission acts as rotational relaxation and thus leads to an increase in NH₃ IR absorption even at collisionless excitation.     

Molecular dissociation of SF6 by ultra-short CO2 laser pulses

We have measured the absolute dissociation probability of SF₆ gas irradiated by CO₂ laser pulses varying in duration from 0.5 nsec to 100 nsec. We find a threshold for dissociation of ? 1.4 J/cm², independent of pulse duration. For fixed energy density, the dissociation probability increases as the pulse duration decreases, but not nearly to the degree expected from theory. We have also determined the dissociation probability in the ν₂ + ν₆ combination band and find it to be 10³ times less than in the ν₃ band, in contradiction to some conclusions recently reported by Ambartzumian et al. The dissociation mechanism takes place without collisions, and if collisions occur, they tend to be detrimental to the dissociation rate.     

Self-diffraction of CO2-laser radiation in SF6

Self-diffraction of CO₂-laser radiation (=10.51 m) has been observed in gaseous SF₆. Different orders of self-diffraction have been studied, up to the third. The amplification due to degenerate resonant multiphoton parametric processes has been measured to be as large as eight. The experimental estimates have been derived for the effective nonlinear susceptibilities (coefficients of nonlinear interaction) of the third, fifth, and seventh orders. The self-diffraction may essentially affect nonlinear processes including optical phase conjugation.     

Strengths of the SF6 transitions pumped by a CO2 laser

The absolute intensities of the SF₆ transitions from the ground state to ν₃=1 that fall within ±1.5 GHz of the CO₂ P(14), P(18) and P(20) laser lines have been calculated. Good agreement has been found between these theoretical results and the available experimental values, especially for the firm assignments of R(28) A⁰₂ and P(33) A¹₂ for SF₆ absorption nearest the CO₂ P(14) and P(18) laser emissions, respectively.     

Identification of the SF6 transitions pumped by a CO2 laser

The quantum numbers (J values and octahedral symmetry types) of the SF₆ transitions from the ground state to v₃ = 1 that fall within ± 1.5 G Hz of the CO₂ P(14), P(18), and P(20) laser lines have been assigned. The SF₆ absorptions nearest these three laser frequencies are R(28) A⁰₂, P(33) A¹₂, and an F₂ component of P(59) or P(60), respectively.     

Isotope separation of32SF6 and12CF3I by a Q-switched CO2 laser

In the paper, polarization beam splitters are studied. Our theoretical study reveals that the coupling length and modulation period of a polarization beam splitter made from the depressed cladding fiber are shorter compared with that made from the matched fiber. Thus the former is characterized by a short interaction length. On the other hand, a polarization beam splitter made from the raised cladding fiber has a longer coupling length and a longer modulation period than those made from the matched fiber.     

Nonlinear absorption of CO2 laser radiation by nonequilibrium carriers in germanium

Nonlinear absorption of CO₂ laser pulses at the multi-MW level has been studied in n-type germanium at room temperature. This nonlinear absorption limits the level of CO₂ laser intensity that can be transmitted through an optical grade germanium crystal. The observed nonlinearity may be interpreted in terms of nonequilibrium electron-hole pairs generated by hot electrons created by the intense laser field. The number of nonequilibrium carriers generated is measured by photoconductivity experiments, and the results are correlated to the absorption measurements.     

A shock-tube study of NH3 and NH3/H2 oxidation using laser absorption of NH3 and H2O

Ammonia (NH₃) is an attractive carbon-free fuel, yet its low reactivity presents many challenges for direct use in combustion applications. These combustion challenges could be resolved by mixing NH₃ with more reactive fuels such as hydrogen (H₂). To further contribute to NH₃ and NH₃/H₂ kinetics—which arguably still requires much improvement—new experiments were conducted over a wide range of temperatures (1474–2307 K), near-atmospheric pressure, several NH₃/O₂ mixtures (equivalence ratios varying from 0.56 to 2.07), and near-stoichiometric NH₃/H₂/O₂ mixtures with NH₃:H₂ ratios of 80:20 and 50:50. During these experiments, laser absorption diagnostics near 10.4 µm and 7.4 µm were simultaneously employed to measure NH₃ and H₂O time histories, respectively. Characteristic parameters, such as NH₃ half-life time and H₂O induction delay time, were extracted from the time-history profiles, and these parameters present stringent speciation targets for mechanism validation. After an assessment of most modern kinetics models, three, most accurate, mechanisms were compared against the experimental results. Only one model was able to partially reproduce the pure NH₃ experiments, yet none of the models were capable of predicting the NH₃/H₂ experiments. Reaction pathway analysis showed that NH₃ oxidation proceeds via forming NH₂ then followed three different routes to form N₂. Importantly, the models considered showed different levels of importance for each route. Sensitivity analysis showed that the NH₃/H₂ experiment is mostly sensitive to NH₃+OH⇄NH₂+H₂O. Interestingly, this reaction showed no sensitivity for the NH₃/O₂ experiments. Overall, the models exhibited significantly slower reactivity than the NH₃/H₂ experiments, and the kinetics analysis showed that the start of this reactivity is governed by the levels of H-atoms in the early stages of the experiments. At these early stages of the experiments, propagation and branching reactions in the H₂/O₂ system are the main contributors to generating H radicals, along with the reaction NH₃+H⇄NH₂+H₂ which proceeds in its reverse direction.     

Collisionless dissociation of SF6 using two resonant frequency CO2 laser fields

The collisionless dissociation of SF₆ has been studied using simultaneous irradiation by two frequencies from a CO₂ laser which are both nearly resonant with the SF_6v3 absorption band. It was found that the dissociation was enhanced, and occurred over a wider frequency range, than for single frequency dissociation. No threshold effect was observed for a weak resonant and a much higher energy field pumping slightly off-resonance. For such two frequency irradiation, the peak in the dissociation curve was found to be shifted to lower frequencies with respect to that for single frequency dissociation.     

Simultaneous sensing of temperature,CO, and CO2 in a scramjet combustor using quantum cascade laser absorption spectroscopy

A mid-infrared laser absorption sensor was developed for gas temperature and carbon oxide (CO, CO₂) concentrations in high-enthalpy, hydrocarbon combustion flows. This diagnostic enables non-intrusive, in situ measurements in harsh environments produced by hypersonic propulsion ground test facilities. The sensing system utilizes tunable quantum cascade lasers capable of probing the fundamental mid-infrared absorption bands of CO and CO₂ in the 4–5 µm wavelength domain. A scanned-wavelength direct absorption technique was employed with two lasers, one dedicated to each species, free-space fiber-coupled using a bifurcated hollow-core fiber for remote light delivery on a single line of sight. Scanned-wavelength modulation spectroscopy with second-harmonic detection was utilized to extend the dynamic range of the CO measurement. The diagnostic was field-tested on a direct-connect scramjet combustor for ethylene–air combustion. Simultaneous, laser-based measurements of carbon monoxide and carbon dioxide provide a basis for evaluating combustion completion or efficiency with temporal and spatial resolution in practical hydrocarbon-fueled engines.     

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.     

Quenching of infrared emission from CO2 by near-resonant vibrational interactions with SF6, BCl3, and PF5

The transfer of substantial amounts of vibrational energy from CO₂ to specific complex molecules has been observed in flow-tube experiments by monitoring the spectra from the 4·3 μm fundamental bands and the 2·7 μm mixed-mode Fermi-resonance bands of CO₂. Small amounts of SF₆, BCl₃, and PF₅ were found to reduce the intensities of these emissions significantly. Quenching cross-sections were calculated using a non-equilibrium analysis.     

Multiple absorption and dissociation of trifluoromethylhypofluorite under the effect of pulsed CO2 laser radiation

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 5, pp. 753–758, May, 1991.     

Isotopically selective dissociation of CCl4 molecules by NH3 laser radiation

The voltage-current characteristics of a UV preionised CO₂ laser operating at pressures ?13 atmospheres have been investigated and the quasi-steady glow and breakdown fields measured. The discharge resistivity has been found to be a function of the applied excitation field and a simple model has been developed to explain the results obtained.     

Detection of ozone by differential absorption using CO2 laser

The differential absorption method with the conventional CO₂ laser is discussed for the detection and monitoring of ozone in ambient atmosphere. By using the P(14) line in the (00°1 to 02°0) and the R(16) line in the (00°1 to 10°0) band, the measurement of ozone was made in a field. As an experimental result, the minimum detectable concentration of 0.15 ppm was obtained with the system, and the concentration of 0.2 ppm was measured in a photo-chemical smog.     

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.