Collision-assisted multiple step excitation of CH3F by irradiation with a TEA CO2 laser

By irradiating a mixture of CH₃F highly diluted in Ar with a TEA CO₂ laser, the v₃ overtone emission rises with a rate much larger than the “up-the-ladder” V-V pumping rate indicating that the CH₃F is excited up to the 3v₃ level almost instantly. This occurs via multiple step excitation assisted by rotational transitions in collisions.     

Vibration-vibration energy transfer iln CH3F

Previous works have reported vibration-vibration and vibration-translation transfer rates in CH₃F and CH₃FX mixtures. In this letter we report the study of the fast VV transfer rate populating the 3ν₃, ν₁ and ν₄ states of CH₃F. Gaseous CH₃F was initially excited to the ν₃ state by a TEA CO₂ laser operating on the P(20 9.6 μ line and collisional pumping to the 3ν₃, ν₁ and ν₄ states was measured by monitoring the rise time of the fluorescence at 300 cm⁻¹. The rate constant was found to be 2.2 × 10⁵ sec⁻¹ torr⁻¹.     

Study of energy transfer from laser excited CO2 to chloromethanes

Collisional energy transfer from CO₂(ν₃) to chloromethanes was studied in the temperature range 300–700 K using the laser induced fluoresce technique. Theoretical calculations using SSH and SB theories were carried out to identify the CO₂(ν₃) deactivation process. The experimental probabilities were found to decrease linearly with increasing number of substituted chlorine atoms.     

Study of near resonant energy transfer from laser excited CO2 to SO2

Collisional energy transfer from CO₂ to SO₂ was studied subsequent to pumping of CO₂ (ν₃) by a Q-switched laser. The measurements were made in the temperature range 300–800 K and in the pressure range 1–30 Torr. The fluorescence from the ν₃ level of CO₂ was monitored with the help of a Ge:Au detector at 77 K with an estimated response time of ≈2 μs. The probability of the energy transfer was found to be increasing with increasing temperature. The probable kinetic models for the V---V relaxation pathways were discussed and the experimentally measured energy transfer rate is related to the cross-over transfer processes. Theoretical calculations using both a simple SSH-breathing sphere model and the Sharma-Brau theory were carried out to evaluate the probabilities of the involved cross-over energy transfer processes and the results were compared with the experimental rates.     

CH3F absorption of the CO2-laser emission measured by the photoacoustic technique

Spectra of coincidence of CH₃F IR absorption with CO₂-laser emission at pressures of 2, 10 and 60 Torr were recorded by the use of a photoacoustic detection method in the whole range of CO₂-laser emission. The spectra show that CH₃F absorbs many CO₂-laser lines in the range 1084–1071 cm⁻¹ with the strongest absorption at 1046.80cm⁻¹, laser line P(20). Absorption is predominantly due to the fundamental of v₃, which is well spread over the whole laser emission range. The intensities of all absorptions rise with increasing pressure, but some absorptions change their relative intensity with respect to one another. In addition, the fine structure of the line spectrum, characteristic of lower pressure samples, disappears as pressure is increased.     

Vibrational energy transfer in CH2F2

Infrared fluorescence has been observed from the ν₁, ν₆, 2ν₉, ν₈ and ν₄ levels of CH₂F₂ following excitation by a 9.6 μ Q-switch CO₂ laser. All the observed states exhibit a single exponential decay rate of approximately 44 msec^?1 torr^?1. The rare gas dependence of this rate has also been measured and found to be up to 20 times slower than the rate for the pure gas. Measurements of the risetimes of the observed fluorescence signals yielded an upper limit of 5 μsec at 1 torr for the ν₁, ν₆ and ν₈ levels. The 2ν₉ and ν₄ risetimes were effectively instantaneous under the experimental conditions that prevailed. The relative magnitudes of the measured rate are discussed in terms of existing V-T/R theories and collisional energy transfer processes.     

Reactivity of electronically excited Br(2P1/2) atoms towards CH3F

The rate constants for Br(²P_1/2) and Br(²P_3/2) atoms in the reaction Br+CH₃FHBr+CH₂F in photobromination of CH₃F have been determined. Their ratio is 10^{–(2.6±0.5)} exp(10100±1000/RT) in the temperature range of 60–200 °C.

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Br(²P_1/2) Br(²P_3/2) Br+CH₃FHBr+CH₂F CH₃F. 60–200 °C 10^{–(2,6±0,5)} exp (10100±1000/RT).

     

Raman analysis of SF6 molecular beams excited by a cw CO2 laser

In a molecular beam the effects of vibrational pumping of SF₆ (ν₃ = 948 cm^?1) are studied, using a line-tunable cw CO₂ laser. Intracavity spontaneous Raman scattering is used for analysis. For excitation in the collision regime (x_E/D ≤ 1), a thermal redistribution of the ν₃ excitation over all vibrational modes is found, together with an average absorption up to six photons per molecule. The infrared absorption profile shows a red-shift of 6 cm^?1. For excitation in the relatively rare collision regime (x_E/D ? 4), a structured non-thermal ν₁ Raman spectrum is observed, especially in the case of seeded molecular beams (10% in He). The observed hot-band peaks can be explained in terms of single-photon absorptions and collision-induced near-resonant V-V energy transfer, leading to single, double and triple excitations of the ν₃ mode. The value of T_rot in the beam is found to influence sensitively the non-resonant energy-transfer rate [e.g. hν₃(948 cm^?1)+ΔE_rot → h(ν₄ + ν₆)(962 cm^?1) relative to the near-resonant transfer rate (hν₃ + hν₃ → 2hν₃ + 3.5 cm^?1)].     

Mechanism of alkyl halide photolysis by a pulsed CO2 tea laser

A CO₂ TEA laser has been used to photolyze dilute samples of various alkyl halides in helium. The mechanism of the high intensity infrared photolysis of these molecules involves the decomposition of molecules into molecular or radical fragments. The reaction pathway is always dissociation into the lowest thermal dissociation channel(s) of the molecule photolyzed. Products from the initial process are also photo-dissociated by the laser pulse and their decomposition pathways are similarly governed. The molecules being photolyzed are not thermally equilibrated with the bath or with each other and the molecular-specific nature of the laser excitation is demonstrated.     

32SF6 dissociation rate during irradiation by a tea CO2 laser

A new experimental method is presented for measuring the multiple-photon dissociation rate of SF₆. It appears that there is a reverse process which associates the photofragments into SF₆. The measured dependence of the dissociation probability versus laser flux seems to agree qualitatively with theoretical models.     

Die gas-chromatographische Analyse von Fluormethanen (CF4, CHF3, CH2F2, CH3F)

Ohne Zusammenfassung     

The kinetics of vibrational energy transfer and relaxation processes in matrix isolated CH3F

Subsequent to exciting v₃ by a CO₂ laser pulse, fluorescence has been detected from 2v₃ of CH₃F trapped in rare gas matrices. 2v₃ is activated by V-V energy transfer and deactivates at twice the rate of v₃ relaxation. A kinetic model is presented to interpret these observations.     

Binding energies of CH3F and CH2F2 calculated by the overlapping sphere SCF MS Xα method

Multiple scattering Xα calculations with overlapping spheres were performed on CH₃F and CH₂F₂ for all-valence region ionization potentials. Comparison with previous Xα calculations shows a substantial improvement.     

Rotation-vibration-translation energy transfer in laser excited Li2 (B1Πu)

Using the method of laser fluorescence, inelastic collisions with rare gas atoms of electronically excited ⁷Li₂ molecules in the υ = 2 and 4 levels were studied. Vibrational transitions ranging from Δ = +2 to ?4 were observed. The simultaneous rotational transitions were completely resolved, and detailed rate constants k_{Δυ, ΔJ} for specific collision- induced quantum jumps Δυ, ΔJ were determined. The effect of secondary rotational relaxation was eliminated by an extrapolation to zero pressure. By integration over ΔJ, rate constants k_Δυ, were found. They are, within the error limits, independent of the collision partner and on the initial υ (2 or 4) and depend rather weakly on Δυ. These findings are compared with theoretical results from various methods, generally based on a collinear collision model. The apparent disagreement in all respects suggests strongly the importance of rotational degrees of freedom in the collision. Experimental evidence for this is the large amount of V — R transfer observed, which about equals the V — T transfer. The mean cross sections σ(Δυ) for specific vibrational transitions Δυ range between 6 and 15 A², among the largest ever observed.     

Molecular beams of CF3Br probed by spontaneous raman effect and excited with a CO2 laser

The state population of CF₃Br is found to be entirely non-thermal under certain molecular beam conditions; the various vibrational modes show distributions which can be described using mode-temperatures differing by as much as a factor of 1.7. Considerable vibrational excitation (ν₁, ν₂ + ν₃) was produced with a focused cw CO₂ laser. A structured excitation spectrum was observed.     

Slow energy transfer between vibrational models of CH2Cl2, CD2Cl2, CH2ClBr and CH2Br2

Inefficient vibrational energy exchange between the lowest vibrational mode and the higher lying vibrational modes of CH₂Cl₂, CD₂Cl₂, CH₂ClBr and CH₂Br₂ was investigated by ultrasonic absorption experiments. Breathing sphere theory is used to interpret the data available for VV and VR, T transfer in methylene halides.     

Fluorine core esca linewidths in CH3F and CF4

Ab initio calculations within the Hartree-Fock formation have been carried out on potential energy surfaces of the ground and the F_1s hole states of CH₃F and CF₄ in order to investigate linewidths of their ESCA spectra. The calculations show that potential energy surfaces of both hole states have dissociative character and can be approximated by straight lines in the region of interest. A simple formula for the ESCA fwhm linewidth is derived which yields results in good agreement with experiment. Theoretically derived relaxation and Koopmans' energies have been investigated as a function of geometry.     

Free energy of interchange between COO and CH3 or CH2 groups

The liquid—solid equilibrium temperatures in the binary systems between n-octacosane and methyl or ethyl octadecanoate are determined in order to obtain the interchange parameters between CH₃ or CH₂ and COO groups by means of the statistics of group interaction.     

Vibrational energy transfer in CH3I

Previous works have reported vibration—vibration and vibration—translation transfer rates in the methyl halides. Using the technique of laser induced infrared fluorescence we have studied energy transfer in the concluding member of this series, CH₃I. Following excitation by resonant lines of a Q-switch CO₂ laser, infrared fluorescence has been observed from the v₂, v₅ as well as the 2v₅, v₁, v₄ vibrational energy levels of CH₃I. All the observed states exhibit a single exponential decay rate of 23 ± 2 ms^?1 torr^?1. Measurements have also been made on deactivation of the various modes by rare gases. The risetime of the v₂, v₅ levels was found to be approximately 101 ± 20 ms^?1 torr^?1, while that of the 2v₅, v₁, v₄ levels was approximately 225 ± 45 ms^?1 torr^?1. Fluorescence was not detected from the v₃ level. These results are discussed in terms of SSH type theoretical calculations, and comparison is made with the results obtained for other members of the methyl halide series, namely CH₃F, CH₃Cl and CH₃Br.     

Reactivity of Cl(2P1/2) atoms towards CH3F

The photochlorination of methyl fluoride has been studied at low pressures. Excited (²P_1/2) chlorine atoms have been found to participate in the reaction. The ratio of the excited and ground state rate constants has been estimated at room temperature.

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. (²P_1/2) . .