CO2 laser-induced decomposition of ethanol

The multiple photon absorption and decomposition of ethanol irradiated by pulsed 9P18 infrared radiation (1048.7 cm^?1) from a TEA CO₂ laser has been studied in the fluence range 15 to 5 J cm^?2. The absorption cross-section is pressure-dependent due to rapid collisional rotational hole-filling. At low pressures the only important decomposition channel following absorption is molecular dehydration of ethanol to yield ethene, but at higher pressures hydrogen, methane, carbon monoxide, ethane, and ethyne are also produced. In the irradiation of pure ethanol under ‘collision-free’ conditions, thermal decomposition following collisional redistribution of energy makes only a small contribution to the overall decomposition yield at fluences above 3.5 J cm^?2 but may become more significant at lower fluences. Modelling using RRKM calculations has been employed to link measured absorbed energies to extents of decomposition of ethanol. Both these calculations and the absorption measurements indicate that at low pressures only a fraction of the irradiated ethanol molecules absorb the 9P18 radiation. © 1993 John Wiley & Sons, Inc.     

CO2 laser-induced photolysis of spirohexane: Time resolved observation of vibrationally excited 1,3-butadiene

Vibrationally excited spirohexane (SHX) generated in CO₂ laser irradiation undergoes photolysis producing ethylene, 1,3-butadiene and a C₄ compound as major products. Collisional energy pooling plays a major role in the multiphoton excitation process. Time-resolved formation of 1,3-butadiene is monitored by UV absorption from which the unimolecular rate constant for SHX dissociation is found to be 5.6 × 10⁵ s⁻¹. A red shift of 4O nm observed in the transient UV absorption spectrum has been assigned to nascent 1,3-butadiene, which suggests that vibrationally hot 1,3-butadiene molecules are formed. The effects of laser energy fluence and pressure of SF₆ as a sensitizer on dissociation yield are also investigated.     

Quantitative and separate fluence and intensity dependences of infrared multiple-photon absorption in SF6

Single-mode CO₂-laser pulses, shaped by electro-optic crystal switching, have been used to measure multiple-photon absorption cross sections in SF₆ as separate and quantitative functions of intensity and fluence at 944.2 cm^?1.     

Time resolved HF vibrational fluorescence from the IR photodissociation of SF6/H2 mixtures

Time resolved measurements of HF spontaneous emission, following the irradiation of SF₆/H₂ mixtures with the focused output from a CO₂ TEA laser, are reported. Our results indicate that F atoms are produced directly by the photodissociation process, and that these atoms have a recoil energy which is ≤500 cm^?1, and varies only slightly with the radiation intensity. Further, our results show a linear dependence of fluorescence intensity with laser energy, indicating that processes other than direct photodissociation may play a significant role in the ultimate fate of species excited by IR collisionless multiple photon absorption.     

Collisionless intramolecular energy transfer in vibrationally excited SF6

Infared-infrared double resonance measurements of the ν₃ absorption band of SF₆ using a CO₂ TEA pump laser (0.3 J/cm² maximum excitation) and a cw CO₂ probe laser have been carried out. These measurements probe both the coherent and quasi-continuum phases of the multiphoton dissociation process in SF₆. The spectral and temporal dependences of the induced absorption are obtained. The results give the first direct measurement of collisionless intramolecular vibrational energy redistribution times in a vibrationally excited complex molecule; for an excess energy of three quanta deposited in the ν₃ mode of SF₆, the ν₃ mode comes into equilibrium with all the remaining vibrational degrees of freedoom with a 3 ± 1 μs time constant.     

SF6-sensitized dissociation of UF6 with a pulsed CO2 laser

The dissociation of UF₆ sensitized by SF₆ excited with a pulsed CO₂ laser in the presence of H₂ and CO as scavengers has been investigated. In the SF₆-UF₆-H₂ system the dissociation yields have been determined as a function of the laser frequency, the fluence, and H₂ partial pressure. A maximum dissociation yield has been found at a laser frequency of 935 cm^?1. No obvious dissociation of UF₆ was observed in the UF₆-SF₆ system without F-atom scavengers.     

Time-resolved investigation of the de-excitation of xenon excimer by SF6 and N2 excited with synchrotron radiation

An attempt has been made 'o measure the rate constant for the de-excitation of a state-specified excimer using the pulse character of synchrotron radiation. The rate constants have been obtained for de-excitation of a vibrationally relaxed excimer Xe₂^*(O_u⁺; low v) by SF₆ and N₂ as 9 × 10^?10 and 7 × 10^?12 cm³ molecule^?1 s^?1 respectively.     

A spectrokinetic study of CH2I and CH2IO2 radicals

The UV absorption spectrum and kinetics of CH₂I and CH₂IO₂ radicals have been studied in the gasphase at 295 K using a pulse radiolysis UV absorption spectroscopic technique. UV absorption spectra of CH₂I and CH₂IO₂ radicals were quantified in the range 220–400 nm. The spectrum of CH₂I has absorption maxima at 280 nm and 337.5 nm. The absorption cross-section for the CH₂I radicals at 337.5 nm was (4.1 ± 0.9) × 10^?18 cm² molecule^?1. The UV spectrum of CH₂IO₂ radicals is broad. The absorption cross-section at 370 nm was (2.1 ± 0.5) × 10^?18 cm² molecule^?1. The rate constant for the self reaction of CH₂I radicals, k = 4 × 10^?11 cm³ molecule^?1 s^?1 at 1000 mbar total pressure of SF₆, was derived by kinetic modelling of experimental absorbance transients. The observed self-reaction rate constant for CH₂IO₂ radicals was estimated also by modelling to k = 9 × 10^?11 cm³ molecule^?1 s^?1. As part of this work a rate constant of (2.0 ± 0.3) × 10^?10 cm³ molecule^?1 s^?1 was measured for the reaction of F atoms with CH₃I. The branching ratios of this reaction for abstraction of an I atom and a H atom were determined to (64 ± 6)% and (36 ± 6)%, respectively. © 1994 John Wiley & Sons, Inc.     

IR-laser-induced sensitized organic reactions: An example of rate control by vibrational energy transfer

The CO₂-laser-induced decomposition of cyclohexene has been studied in the presence of SiF₄, SF₆ and C₆F₆ as sensitizers. The pressure range investigated is between 10 and 30 Torr at laser fluences between 0.20 and 1.20 J cm⁻². At the same time measurements of the IR multiphoton absorption of the three sensitizers were also performed.The results have been analyzed within the framework of a kinetic model based on the assumption that the rate is controlled by vibrational energy transfer from the multiphoton-excited sensitizer molecules to the reactant. Evidence is presented to show that these sensitized reactions proceed under non-equilibrium conditions.     

Spectrokinetic study of SF5 and SF5O2 radicals and the reaction of SF5O2 with NO

UV spectra of SF₅ and SF₅O₂ radicals in the gas phase at 295 K have been quantified using a pulse radiolysis UV absorption technique. The absorption spectrum of SF₅ was quantified from 220 to 240 nm. The absorption cross section at 220 nm was (5.5 ± 1.7) × 10⁻¹⁹ cm². When SF₅ was produced in the presence of O₂ an equilibrium between SF₅, O₂, and SF₅O₂ was established. The rate constant for the reaction of SF₅ radicals with O₂ was (8 ± 2) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹. The decomposition rate constant for SF₅O₂ was (1.0 ± 0.5) × 10⁵ s⁻¹, giving an equilibrium constant of K_eq = [SF₅O₂]/[SF₅][O₂] = (8.0 ± 4.5) × 10⁻¹⁸ cm³ molecule⁻¹. The SF₅ O₂ bond strength is (13.7 ± 2.0) kcal mol⁻¹. The SF₅O₂ spectrum was broad with no fine structure and similar to the UV spectra of alkyl peroxy radicals. The absorption cross section at 230 nm was found to (3.7 ± 0.9) × 10⁻¹⁸ cm². The rate constant of the reaction of SF₅O₂ with NO was measured to (1.1 ± 0.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ by monitoring the kinetics of NO₂ formation at 400 nm. The rate constant for the reaction of F atoms with SF₄ was measured by two relative methods to be (1.3 ± 0.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. © 1994 John Wiley & Sons, Inc.     

High-pressure range of the recombination O + O2 → O3

The recombination reaction O + O₂ → O₃ was studied by laser flash photolysis of pure O₂ in the pressure range 3–20 atm, and of N₂O? O₂ mixtures in the bath gases Ar, N₂, (CO₂, and SF₆) in the pressure range 3–200 atm. Fall-off curves of the reaction have been derived. Low-pressure rate coefficients were found to agree well with literature data. A high-pressure rate coefficient of k_∞ = (2.8 ± 1.0) × 10^?12 cm³ molecule^?1 s^?1 was obtained by extrapolation.     

Measurement of strong nonlinear absorption in stilbene-chloroform solutions,explained by the superposition of two-photon absorption and one-photon absorption from the excited state

A strong nonlinear absorption is measured when ruby laser light passes through a solution of stilbene in chloroform. The experimental results are interpreted by the use of the following model: two-photon absorption (TPA) is followed by one-photon absorption (OPA) by the excited molecules. The solution of the corresponding system of rate equations and fitting of parameters to the experimental curve give the TPA cross section σ⁽²⁾ = 8 × 10^?49 cm⁴ sec/photon and the product σ⁽¹⁾ τ₁₀ = 3 × 10^?26 cm² sec. Measurements at other frequencies are in agreement with this model.     

Excitation of H2 using continuously tunable coherent XUV radiation (97.3–102.3 nm)

Broady tunable EUV radiation from 97.3 to 102.3 nm has been generated by frequency tripling the second-harmonic output of a Nd:YAG-pumped dye laser in a pulsed argon source. About 7 × 10⁹ photons per shot in a 1.7 cm^?1 bandwidth are produced, corresponding to a conversion efficiency of ≈10^?6. Four bands of the Lyman system and two bands of the Werner system of molecular hydrogen were excited. Detection of H₂ densities of ≈2 × 10⁸ molecules/cm³ in a ω″, J″, level was accomplished using laser-induced fluorescence.     

Optical spectroscopy of low-phonon Ho3+ doped BaY2F8 single crystal

The Ho:BaY₂F₈ crystal was grown by Czochralski method. The crystal phase structure and absorption spectra were tested, the absorption peak exists near 899 nm, the absorption cross section was 1.27 × 10^?21 cm². The emission spectra of crystals in the vicinity of 2 and 3.9 μm were measured, the 2 μm near infrared light induced by ⁵I₇ → ⁵I₈ transition of Ho3+ ions was observed, as well as the fluorescence output at 3.9 μm (⁵I₅ → ⁵I₆), emission cross section at 3.9 μm was calculated to be 0.86 × 10^?21 cm². We suppose that the Ho:BaY₂F₈ crystal has a large application prospect for the 2–4 μm wavelength near infrared laser.     

Mechanisms of the IR laser initiation of combustion in a supersonic H2/O3/O2 flow

The ignition of a supersonic H₂/O₃/O₂ flow by the λ_I = 9.7 μm laser excitation of asymmetric vibrations in O₃ molecules is considered. The O₃ molecules vibrationally excited by laser radiation dissociate more readily to generate active O atoms, thus accelerating the ignition/combustion process. Even at a low input of radiation energy (～7 × 10^?4 J/cm³), combustion in the supersonic flow can be initiated at a short distance (<1 m) from the irradiation zone and at a low gas temperature (～300 K).     

The electron swarm method as a tool to investigate multi-body electron attachment processes

The electron swarm method has been used for the investigation of multi-body thermal electron attachment process. Ethylene, carbon dioxide and their mixtures were used as carrier gases. The rate constant for the reaction of thermal electrons with SF₆ has been measured and found to be (2.5±0.2) × 10^-7 cm³·mol^-1·s^-1. The mechanism and kinetics of thermal electron capture by N₂O was investigated in the mixtures with C₂H₄ and CO₂. A mechanism involving neutral van der Waals molecules (N₂O·CO₂) has been proposed and the rate constants calculated to be 1.9 × 10^-10 cm³·mol^-1·s^-1 for the formation of (N₂O·CO₂) and 1.8 × 10^-30 cm⁶·mol^-2·s^-1 for the three-body reaction with CO₂ as a stabilizing agent.     

Infrared laser-induced decomposition of 2,2-dimethyloxetane

The unimolecular decomposition of 2,2 dimethyloxetance to give either isobutence and formaldehyde or ethene and acetone induced by a pulsed CO₂ laser has been investigated. Absorption characteristics and fractional decomposition have been studied as a function of laser fluence, irradiation frequency, reactant pressure, and added inert bath gas. Both absorption cross section and fractional decomposition are approximately independent of pressure of 2,2-dimethyloxetane below 50 times; 10^?3 torr and increase with pressure at higher pressures of 2,2-dimethyloxetane. At pressures sufficiently low that collisions are negligible during the laser pulse, added inert gases reduce the amount of decomposition. Calculations of the fractional decomposition have been carried out based on RRKM theory and assuming either a Boltzmann or a Poisson intermolecular energy distribution. Master equation calculations of both absorption and decomposition for 10R20 irradiation have also been performed. Agreement between observed and calculated results for 10R20 irradiation could be obtained only by assuming that most, but not all, of the molecules in the irradiated volume absorb the laser radiation. Differences between the absorptions of the 10R20 and 9P20 lines and in the resulting extents of decomposition indicate that the fraction of irradiated molecules which absorbs 9P20 radiation is smaller than the fraction which absorbs 10R20 radiation.     

Photodissociation of the dimer ions Ar+2, Ne+2, and (CO2)+2

The tandem quadrupole photodissociation mass spectrometer has been used to study photodissociation reactions of Ar⁺₂, Ne⁺₂, and (CO₂)⁺₂. The cross sections for photodissociation of Ar⁺₂ exhibited a strong dependence on ion source pressure, varying from 2 × 10 ^?18cm² at 0.1 torr to 6 × 10^?19cm² at 0.5 torr. A large photodissociation cross section (2 × 10^?17cm² for the reaction (CO₂)⁺₂ → CO⁺₂+ CO₂ was observed at the red end of the visible spectrum (580–620 nm) suggesting that this may be an important reaction in CO₂ rich planetary ionspheres such as that of Mars.     

The direct observation of a highly mobile positive ion in nanosecond pulse irradiated liquid cyclohexane

The electrical conductivity induced by pulse irradiation of liquid cyclohexane has been studied by means of microwave absorption. The conductivity in pure cyclohexane, due principally to the excess electron, is reduced to less than 10% of the initial value on addition of 5 × 10^?4 M of the electron scavenger SF₆. The conductivity remaining after addition of SF₆ is however more than an order of magnitude larger than expected for massive ions in cyclohexane and, since it is almost completely removed by the addition of 4 × 10^?3 M of the positive ion scavenger NH₃, is attributed mainly to the high mobility of the positive hole in this liquid. The ratio of the electron to hole mobility is determined to be 15. The mean lifetime of the hole under the present conditions is 86 ns. The rate constant for reaction of the hole with NH₃ is determined to be 1.8 × 10¹¹ M^?1 s^?1. From the conductivity remaining after removal of both the electron and the hole the sum of the mobilities of the resulting molecular ions is determined to be 8.4 × 10^?4 cm² V^?1 s^?1.