John L. Lyman

Isotopic enrichment of SF6 in S34 by multiple absorption of CO2 laser radiation

An increase in the ratio of S34/S32 of 3300% over its natural ratio has been obtained by irradiating SF6 in the presence of H2 with an intensely focused CO2 laser. The quantities of material separated are on the order of 100 μg.

A model for unimolecular reaction of sulfur hexafluoride

The thermal dissociation data for SF6 has been re‐evaluated using RRKM unimolecular reaction rate theory and was found to be consistent with a higher value for the sulfur–fluorine bond strength (92 kcal/mole). A model based on a rate equation formalism has been developed that includes unimolecular dissociation, collisional energy transfer (V–T processes), and absorption and emission of laser radiation. The model was designed to test theories of absorption of laser radiation for laser‐induced chemical reaction in SF6 that are amenable to a rate equation formalism. It was found that the thermal reaction and vibrational relaxation data could be adequately reproduced. Sample calculations of laser‐induced chemical reactions are performed.

The temperature-dependent absorption spectrum of the v3 band of SF6 at 10·6 μm☆

Abstract Mixtures of SF 6 diluted by argon were heated behind an incident shock front in the range 400 to 1500K. Absorption measurements at 11 wavelengths near 10·6 μm were made on the post-shock, equilibrated gas using a CO 2 laser. A band contour model, which makes use of known spectroscopic constantd and the experimental results, satisfactorily accounts for the observed temperature dependence of the absorption coefficient. We are able to estimate the identity of the transitions responsible for absorption of the laser radiation.

Enrichment of boron, carbon, and silicon isotopes by multiple‐photon absorption of 10.6‐μm laser radiation

The polyatomic molecules SF6, BCl3, CF2Cl2, and SiF4 have been irradiated in the presence of H2 or N2 using intense focused CO2 laser pulses. All four of the polyatomic molecules underwent isotopically selective reactions showing depletion of the isotopic forms irradiated. The best enrichment was achieved with SF6; however, all systems showed similar behavior with regard to pressure and irradiation time. Both BCl3 and SiF4 showed evidence of isotopic rescrambling reactions that occurred on about a 1‐h time scale. The mechanisms of the absorption process are believed to be enabled by the large density of states in the vibrational manifold of polyatomic molecules.

Laser induced dissociation of N2F4

Abstract A pulsed CO2 laser has been used to drive the dissociation of N2F4 by depositing energy directly in the vibrational degrees of freedom. It is observed that dissociation proceeds at an initial rate that far exceeds the measured thermal rate.

Thermochemical functions for gas-phase, 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), its condensed phases, and its larger reaction products

Abstract 1,3,5,7-Tetranitro-1,3,5,7-tetraazacyclooctane (HMX) is a major component of many explosives and propellants. Chemical kinetic simulations of the reactions of HMX require knowledge of its enthalpy, entropy, and heat capacity in the gas-phase and condensed phases. While some experimental measurements are available, most available thermochemical information has been obtained by analogy with the smaller species, RDX, and its reaction products. Using the Hartree-Fock method with the 6-31G∗ basis set we have calculated structures, vibrational frequencies, and bond strengths for the isolated HMX molecule and its two major reaction products. This, with some available experimental data, allows us to calculate the relevant thermochemical parameters for these species. With this information and some published experimental measurements of sublimation pressure, phase change enthalpies, and heat capacities, we have also calculated some thermochemical data for three condensed phases of HMX: β-HMX, δ-HMX and liquid HMX. We present the thermochemical parameters in both the JANAF and CHEMKIN format. We also compare the current calculations with previous estimates.

The effect of reagent vibrational energy on measured reaction rate constants

Abstract The effect of reagent vibrational energy E vib on experimentally measured rate constants for reactions between neutral species at thermal kinetic energies is reviewed. The vibrational acceleration of chemical reactions has recently been characterized by a parameter α describing the efficiency of the usage of the internal energy in overcoming the Arrhenius activation energy, E a . For reactions of vibrationally excited H 2 , HCl, OH, and O 3 studied to date, α is generally less than 0.6, and is uncorrelated with E a , E vib , the excess vibrational energy, or the reaction exothermicity. It is concluded that the utility of this conversion efficiency parameter is limited to the phenomenological description of the vibrational acceleration of rate processes or to the role of an adjustable parameter in calculations involving reactions of excited species.

The ultraviolet absorption spectrum of hot carbon dioxide

Abstract The vacuum ultraviolet absorption spectrum of CO 2 shifts significantly to longer wavelengths with increasing temperature. We have measured this temperature dependence and present absorption cross sections for 230 to 355 nm at temperatures from 1523 to 2273 K. At 2273 K this red shift extends the absorption above 300 nm and well into the blue end of the solar spectrum. In CO 2 the thermal shift is enhanced because of the relative position and shapes of the excited and ground state potential curves.

Effect of CO2 laser-pulse mode quality on multiple photon absorption in SF6

Abstract Unimolecular multiple-photon absorption cross sections are measured for SF 6 for both single and multiple longitudinal mode CO 2 laser pulses at three different frequencies over a four-decade range of energy fluence. The results indicate only a weak dependence on pulse mode structure for fluence levels corresponding to as little as 0.02 photons absorbed per molecule.

The frequency, fluence, and pressure dependence of the absorption of pulsed CO2-laser radiation by SF6 at 140 K

Abstract We report measurements of absorption of pulsed CO 2 laser radiation by SF 6 at 140 K. These measurements cover a broad range of fluence (10 −6 to 0.8 J/cm 2 ), SF 6 gas density (2 × 10 15 to 6 × 10 16 cm −3 ), and frequency (six CO 2 -laser frequencies within the SF 6 v 3 band). We employ two methods of data reduction including one that gives a simple phenomenological function of the three principal independent variables. We conclude that at low fluence a small fraction of the SF 6 molecules absorb the laser radiation and that collisions and higher fluence both increase that fraction. At higher fluence absorption by vibrationally excited molecules becomes increasingly important.