Multiphoton Dissociation Of SF6 Research Articles

Improved separation of the rare sulfur isotopes by infrared multiphoton dissociation of SF6

The dissociation probabilities of32SF6 and some of34SF6 have been measured at a large number of CO2 laser lines both at room temperature and at 140 K. The longwavelength wing of this dissociation spectrum is exponential in the wavenumber. Its logarithmic slope is proportional to the inverse temperature. Selectivities are high enough at 140 K, that the photons are consumed only for the rare isotope in the case of34SF6 and nearly so for36SF6. For33SF6 further improvement of the selectivity would be desirable.

Investigation of the process of formation of oxygen-containing products as a result of multiphoton dissociation of SF6by radiation from a high-power pulsed CO2laser

The relative yields of sulfur oxyfluorides SOF4, SOF2, and SO2F2 formed by multiphoton dissociation of SF6 in Ar:SF6 = 1:5 and Ar:SF6:O2 = 1:5:50 mixtures were determined at a partial SF6 pressure of 0.2 Torr for various energy densities of CO2 laser radiation. It was found that the addition of 2 Torr O2 to the Ar–SF6 mixture enhanced selective dissociation of SF6 and increased 20-fold the coefficient representing separation of the 32S and 34S isotopes in SOF4, and also altered the relative yields of all the sulfur oxyfluorides. The results obtained were used to propose a chemical reaction scheme occurring in the system in the presence of oxygen in the Ar–SF6 mixture and also in the absence of oxygen. This scheme accounted for the appearance of the products and for the changes in the relative yields of SOF4, SOF2, and SO2F2 on addition of oxygen.

The temperature dependence of hydrogen abstraction reactions: F+HCl, F+HBr, F+DBr, and F+HI

A laser photolysis/infrared fluorescence technique has been used to study the temperature dependence of the F+HCℓ, F+HBr, F+DBr, and F+HI absolute rate constants in the range from 195 to 373 °K. Multiphoton dissociation of SF6 with a TEA CO2 laser was used to provide a transient concentration of fluorine atoms. Their subsequent reaction with hydrogen donors was monitored by observing the appearance rate of HF(v?1) through its infrared fluorescence. The observed rate constants show a marked nonlinear Arrhenius behavior which is consistent with a model in which reaction takes place both vai direct attack by the fluorine atom at the hydrogen end of HX and via formation of a collision complex FXH which rearranges to FHX and then dissociates to HF and X products.

Multiphoton dissociation of SF6 by a molecular beam method

The dynamics of infrared multiphoton excitation and dissociation of SF6 has been investigated under collision-free conditions by a crossed laser–molecular beam method. In order to understand the excitation mechanism and to elucidate the requirements of laser intensity and energy fluence, a series of experiments have been carried out to measure the dissociation yield dependences on energy fluence, vibrational temperature of SF6, the pulse duration of the CO2 laser, and the frequency in both one and two laser experiments. Translational energy distributions of the primary dissociation product SF5, measured by time-of-flight and angular distributions and the dissociation lifetime of excited SF6 as inferred from the observation of secondary dissociation of SF5 into SF4 and F during the laser pulse suggest that the dynamics of dissociation of excited molecules is dominated by complete energy randomization and rapid intramolecular energy transfer and can be adequately described by RRKM theory. An improved phenomenological model including the initial intensity dependent excitation, a rate equation describing the absorption and stimulated emission of single photons, and the unimolecular dissociation of excited molecules is constructed based on available experimental results. Our studies show that although the energy fluence of the laser determines the dissociation yield of molecules in the quasicontinuum, the role played by the intensity of the laser in multiphoton dissociation is more significant than just that of overcoming the intensity dependent absorption in the lowest levels. Once molecules are excited beyond the dissociation energy, the average level of excitation of the dissociating molecules will be significantly influenced by the laser intensity for a given energy fluence when the rate of decomposition starts to compete with the rate of up-excitation.

The temperature dependence of absolute rate constants for the F+H2 and F+D2 reactions

Multiphoton dissociation of SF6 has been used to generate a transient concentration of fluorine atoms in mixtures with argon and H2 or D2. Absolute rate constants for the F+H2 and F+D2 reactions have been determined as a function of temperature by monitoring the appearance rate of HF or DF product chemiluminescence. In the temperature range from 190 °K to 373 °K the results are fit by the expressions kH=1.0×10−10 exp(−(860±100)/RT) and kD=9.1×10−11 exp(−(1100±100)/RT), both in cm3 molecule−1 sec−1. These values are in rough agreement with those obtained recently using a similar technique. The value of the isotope effect kH/kD is in good agreement with two previously determined values.

Collision effects in the multiphoton dissociation of SF6

Abstract Measurements of multiphoton dissociation (MPD) of 32 SF 6 by irradiation with a single TEA CO 2 laser pulse as a function of gas pressure are reported. Superexcitation conditions and pulses of different lengths were used to investigate collision effects on the MPD of SF 6 . In addition the energy of the pulses transmitted through the absorption cell was measured to distinguish between optical absorption and collision effects on the MPD. The results using laser pulses of 8 J with a duration of 150 ns show that at low gas pressures there is an almost constant MPD yield where 15% of the molecules is dissociated in a single pulse, whereas at pressures higher than 0.1 Torr a strong decrease of the MPD yield is found. The transmitted pulse energy in this case follows the MPD behavior as a function of pressure. The results with the stronger laser pulses of 30 J and duration of ≈ 1 μs show a pressure dependence of the MPD similar to that obtained with the weaker pulses, but with 100% yield at the low gas pressures. The transmitted pulse energy, however, was found to be almost constant at pressures up to 0.3 Torr. The present results indicate that collisions with photofragments provide the most likely channel for the observed decrease of the MPD yield versus pressure.