Calculations of rotational and vibrational relaxation times for gases composed of homonuclear diatomic molecules have been carried out. The model used for the molecular interaction potential consists of an attractive component, which acts between geometrical centers of the molecules, and a repulsive component which is assumed to originate from two centers of force in each of the molecules. For large intermolecular separations, the attractive forces prevail while at close distances the repulsive forces control. Using this model, the number of collisions to establish rotational equilibrium ZR and also the number for vibraton ZV are calculated. Both ZR and ZV contain some of the same molecular parameters and are therefore dependent on each other. From the analysis it turns out that ZR is a gradually increasing function of increasing temperature and ZV is a rapidly decreasing function of increasing temperature. Comparison with experiment for the gases chlorine, nitrogen, and oxygen indicates that the calculations are for the most part reliable.

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A new method is described for determining the energy distribution among the product vibrational energy levels in an exothermic reaction. By careful control of the rotational temperature in the UF6–H2(D2) and UF6–CH4(CD4) chemical laser systems, one can obtain conditions for which two hydrogen fluoride vibration–rotation transitions initiate simultaneously and with the same gain. From the relationship between relative gain, rotational temperature, and the vibrational populations, an accurate value can be determined for the ratio of the populations Nυ / Nυ−1 of the two states involved in the first transition to reach laser threshold. This ratio equals kυ/kυ−1 after vibrational deactivation has been taken into account. This technique has yielded the following rate-constant ratios:F + H2→limυHFυ† + H, k2 / k1∼5.5 F + D2→limυDFυ† + D, k3 / k2∼1.6. The significant observations are that the rate-constant ratios are all above unity, the hydrogen and methane values are very close to each other, and the DF ratios are much closer to unity than their HF counterparts.

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The characteristics of laser-induced breakdown in N2 and O2 at a wavelength of 1.064 μ were investigated using 10-nsec pulses for pressures ranging from 1 to 50 atm. The pressure dependence of the experimentally determined threshold field intensities for these two gases is essentially the same except at the lowest pressures where the O2 values are slightly lower than those for N2. Comparison of this data with existing microwave data can be accomplished most directly by means of a ’’universal plot’’ of the type used effectively in the latter case. The general trends of the microwave and optical data are essentially the same; however, actual numerical values for the optical data tend to be lower than the corresponding microwave values by a constant factor of roughly 0.5. The fact that the two sets of data correspond so closely in spite of the large difference in frequencies has to be regarded as providing substantial evidence supporting the validity at optical frequencies of the cascade theory of breakdown used to interpret microwave breakdown measurements. Time-resolved and time-integrated emission spectra were analyzed, from which the electron density and temperature were obtained for the initial stage of spark development. The spectral characteristics, the electron densities, and temperatures are similar to those determined previously by other investigators for air.