We have measured the probability of transition of the forbidden Vegard—Kaplan system (A3Σ—X1Σ) in N2, working from the definition of this probability as the ratio of number of photons emitted per second to number of excited molecules, both measured in the same discharge. We measure the absolute intensity of the 0,6 Vegard—Kaplan band as emitted from the discharge. Under identical conditions we find the population of molecules in the A3Σ level by measuring the strength of absorption of the 1,0 first positive band. By calculation we relate the observed loss (which is a sum of absorption by all rotational levels) to the absolute population of the excited level, in terms of the probability of transition of the first positive system. Our measurements of absorption and emission give a value of (1.6±0.4) × 106 for the ratio of the transition probability of the first positive system to that of the Vegard—Kaplan system. From recent shock-tube measurements of the former transition probability we obtain a value of 2.0(±0.9) sec for the lifetime of the A3Σ state of N2.
We have studied emission of the four forbidden lines of atomic oxygen, λλ 6364, 6300, 5577, and 2972 Å from a discharge through flowing tank oxygen at pressures of a few torricelli, in a tube of 8 mm i.d., with 40–120 mA of current flowing. We found that under these discharge conditions none of the lines were perceptibly broadened or shifted (to within 0.2 Å), and that none showed any response to changes in conditions that would suggest collisional stimulation of emission. Hence we conclude that the emission of these lines is governed by their true radiative transition probabilities. Typical intensities were, for the four lines (in units of photons per cubic centimeter·second): 3.3×109, 1.0×1010, 4.0×1010, and 1.8×109, respectively. The ratio of intensities of 5577 and 2972 Å (which have the common upper level 1S) has a constant value of 22±2 within the range of conditions in which we can observe these lines, as compared with the theoretical value of 16 computed by Garstang. We discuss, in terms of molecular selection rules, the collisional stimulation of the line 5577 Å that is observed by others at high pressures.
(1) In order to make the OH absorption bands applicable to a quantitative chemical test for free OH their probability of transition (f value) was measured by the concentration of ``dispersion electrons'' in a known concentration of OH. (2) This concentration of OH is produced by water vapor dissociated at 1473°C, the equilibrium of which was computed. (3) The concentration of the ``dispersion electrons'' was measured by photometry of the absorption lines photographed with high resolving power. (4) The f values of the various absorption lines are given in a table. The most intense lines have f values of the order of magnitude 2×10—4. The accuracy is estimated to be 15 percent, the sensitivity (smallest detectable pressure under the conditions of the present experiment) 0.01 mm. (5) Comparison with other f values leads to the conclusion that the OH absorption band belongs to a half-forbidden transition. The lifetime of the excited state is computed as approximately 4×10—6 sec.
Various origins are considered for the atmospheric sodium observed in the twilight and nightglow above an altitude of 70 km. The transport of sea salt to these altitudes in sufficient quantities seems impossible except in the unlikely case of complete atomization of the sodium at low altitudes (30 km). Other terrestrial sources seem even more unlikely. The sodium has not accreted from the sun, as shown by a consideration of the amount of neon now present in the atmosphere. Meteoric influx does provide sufficient quantities of atomic sodium at altitudes of 70 km and greater, and is probably the source of the airglow sodium. The influx of micrometeoritic material is capable of providing even more sodium than do the meteors, but there is great uncertainty as to the fraction of this material which vaporizes. Other metals which have been observed in the high atmosphere are Ca, Li, and Mg. However, no conclusions with regard to the origin of these constituents can be drawn from the presently available evidence.
The various lines of evidence regarding the energy of dissociation H2O→H+OH are critically discussed. This gives a basis for discussing the energy of activation of the process OH+OH→H2+O2.
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