Parametric study of a UV preionized CO2 laser

Small signal gain measurements of a uv preionized TEA CO₂ laser system are described. It is shown that the relative timing of preionization with respect to the main discharge determines the amount of electrical energy that can be deposited in the gas prior to arc formation.     

Pure MW Raman laser emission in superradiant mode in ammonia by pumping with 9P20 CO2 laser line

In this work, clear FIR Raman emission in ammonia was observed when a 9P20 CO₂ line was used as a pump. We have noted an intense pump depletion and strong superradiant FIR and MIR emissions. The time history of these radiations has also been recorded. The intense MIR radiation is a new line observed in the ammonia laser.     

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.     

Simultaneous measurements of absorption and vibrational relaxation time in CF2HCl molecules

Summary We describe in this paper a simple interferometric technique which allows a direct and simultaneous determination of energy absorption and of the vibrational-translational relaxation time of polyatomic molecules strongly excited by reasonant, infrared (10.6 μm) laser radiation. In particular, we have applied this method to the study of freon-22 (CF₂HCl), a medium-size molecule that, apart from its potential application in¹³C isotope separation, shows interesting absorption features in the 10 μm region. The results are also compared with our previous findings in SF₆. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

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.     

Formation and measurement of cluster beams at gasdynamic cooling of CF2HCl molecules

The clustering of CF₂HCl (Freon-22 refrigerant) molecules is found to take place when these molecules are dynamically cooled in a pulsed supersonic beam. A method of CF₂HCl cluster beam diagnostics is developed that combines UV multiphoton ionization, time-of-flight mass spectroscopy, and cluster IR photodissociation. The velocity of directed motion of (CH₂HCl)_n clusters, as well as the longitudinal and transverse velocities of their thermal motion, are measured at different stagnation pressures P ₀. The cluster mean size and the degree of clustering in the flow are estimated depending on supersonic flow conditions.     

Multiphoton fragmentation and ionization of CF2HCl molecules and clusters by UV radiation

The results of experimental studies of multiphoton ionization of CF₂HCl molecules and clusters by UV laser radiation in the wavelength range 217–236 nm are reported. In the case of molecules, the main reaction products are CF₂H⁺ and CF⁺ ions as well as atomic chlorine. It is found that the spectra of the products of ionization of free molecules and molecules condensed into clusters differ qualitatively: multiphoton ionization of clusters does not yield CF₂H⁺ ions. The dependences of the ion yield on the intensity of laser radiation and its wavelength are measured. The effect of a constant electric field and the radiation spectral width on the multiphoton ionization process is demonstrated. The shape of the velocity distributions is determined for a number of products. A strong anisotropy is detected in the reaction of formation of CF₂H⁺ ions. Possible mechanisms for these processes are discussed.     

Laser-induced desorption of polyatomic molecules with a CO2 laser

Laser-induced thermal desorption (LITD) has been increasingly employed as a tool for investigating surface processes. In LITD, a pulsed laser beam that is focused onto a surface induces a rapid temperature rise that causes desorption. In spite of the success enjoyed by the CO₂ laser in studies of diatomic molecules, its use with polyatomic molecules is shown to be severely limited by laser-induced dissociation. In desorption experiments with CH₃OH, HCOOH, CH₃NH₂ and NH₃ dissociation occurs only when the laser frequency coincides with an infrared absorption band of the molecule. Fragmentation may take place either on the surface or in the dense gas phase present above the surface during the laser pulse.     

Laboratory scale-up of two-stage laser chemistry separation of13C from CF2HCl

The laboratory scale-up of a two-stage laser enrichment process for carbon isotopes, involving infrared multiphoton dissociation of freon-22, is described. Unmodified commercial equipment and materials were used. An initial study of the effect of fluence, laser frequency, freon-22 pressure and pressure of argon, nitrogen and trifluoromethyl chloride was made in short irradiation cells (constant fluence) in order to define optimum process parameters. The process was then scaled to higher throughput in longer cells (1–5 m) in which compensation for beam-energy depletion by absorption was made by reduction in the beam area by focussing. From the scale-up experiments, measurements of yield and enrichment of the tetrafluoroethylene product gave demonstrated production rates. These, coupled with measurements of the absorption, allowed extrapolation to production rates assuming total utilization of the available output energy.Using a 100 W TEA CO₂ laser (10 J, 10 Hz) we have demonstrated production rates of 0.20 g h^–1 carbon-12 at 99.99% carbon-12, 11 mg h^–1 carbon-13 at 72% carbon-13 and 2 kg per annum carbon-13 at 50%. Energy absorption measurements imply a capability to produce 3 kg per annum carbon-13 at over 95% carbon-13 in a two-stage process.The apparatus was used to produce gram quantities of carbon-13 depleted freon-22 (99.99% carbon-12). A comparison of the infrared multiphoton dissociation of this material with that of natural freon-22 (1.11% carbon-13) showed that under the conditions required to give selective dissociation of¹³CF₂HCl that¹²CF₂HCl was excited as a result of a dominantly radiative interaction and that collisional transfer from¹³CF₂HCl molecules played a minor role.Issued as NRCC 23639Summer Student 1983, Acadia University, N.S., Canada     

Theoretical analysis of the multi-rotational line TEA CO2 laser

A theoretical model for the multiline TEA CO₂ laser has been developed which takes into account the overlap of the P(20) line of (00⁰1) (10⁰0) regular band transition with the R(23) line of (01¹1) (11¹0) hot band transition. The model is used for the study of the influence of different parameters like laser gas temperature, gas mixture and pump rates on laser intensities of different rotational lines. This study explains the experimental results of the high power multiline TEA CO₂ laser very well.     

The frequency,fluence and pressure dependence of the absorption of pulsed CO2-laser radiation by CF2HCl

Absorption measurements in the CF₂HCl fundamental vibrational band v₃, and combination band v₅ + v₉, for laser energy fluence between 0.1–1.2 cm⁻² and 1–100 Torr gas pressure, are reported. Three distinct regions are experimentally observed for the fluence dependence of the absorption cross section σ and the average number of absorbed photons <n>. Adding Ar buffer gas to CF₂HCl enhances the absorption efficiency only in the v₃ band.     

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.     

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.     

Rotational relaxation studies in CO2 molecules by means of a multiline tea CO2 laser system

If a CO₂ laser amplifier is injected by a single-frequency oscillator, the efficiency is limited. A multiline pulsed oscillator has been developed to overcome this limitation.     

A study of the methanol laser with a 500 MHz tunable CO2 pump laser

The absorption spectrum of CH₃OH is measured optoacoustically over a 500 MHz tuning range in 84 different CO₂ laser lines, and 19 assigned CO-stretch absorptions are used for establishing the linearity of the measurements over two decades of absorption coefficient. 40 new FIR laser lines originating from large offset absorption, are reported in the wavelength range between 40 and 400 m, and their wavelengths are given with a relative accuracy of 5×10^–5. 30 lines are assigned, involving CO-stretch transitions in torsionaln=0, 1, and 2 states, as well as transitions inn=3 of the vibrational ground state, pumped by transitions to Fermi interacting states.Work supported by the Danish National Science Research Council grant No. 11-5375     

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.     

Study of the parameters of the collisionally broadened R22 absorption line of the 1000-0001 transition in CO2 molecules: I. Experiment

Unsaturated absorption coefficients in pure carbon dioxide at pressures of 1 and 100 Torr and in CO₂-N₂ and CO₂-He binary mixtures at a pressure of 100 Torr have been measured in the temperature range of 296?C700 K using a frequency-stabilized tunable CO₂ laser. The collisional self-broadening coefficient, the relative coefficients of collisional broadening caused by N₂ and He buffer gases, and their temperature dependences have been determined for the R22 absorption line of the 10⁰0?C00⁰1 transition in CO₂ molecules.     

Collisionless multiphoton ionization of water molecules by a TEACO2 laser

Collisionless ionization of water molecules into fragments of H⁺, H₂⁺, O⁺, OH⁺, H₂O⁺ were observed using a high power TEACO₂ laser. The mechanism could be related to that of selective dissociation of complex molecules by the same laser.