Experimental Verification of the Theory of the Continuous Spectra ofH2andD2

Abstract

The continuous spectra of molecular hydrogen and deuterium have been produced and studied under conditions permitting control of the excitation processes involved, the electron beam being subjected to definite accelerating potentials. An immediate comparison of the results with those predicted on purely theoretical grounds in previous papers by the author and others was not possible because of the unavoidable thermal spread in the kinetic energies of the electrons, an effect which had not been considered in those papers. The necessary modification in the theoretical predictions has been carried out, the essential result being that no sharp separation of even the lowest vibrational level of the radiating electronic state can be expected at any accelerating potential; consequently, even at the lowest potential at which any radiation is perceptible, that coming from the lowest vibrational level will be appreciably contaminated by the contribution from the next higher. The form of functional relationship between the rate of excitation of a given level and the accelerating potential is investigated and found to be linear if the latter is higher than the critical excitation potential of the given level (but not too much higher), and exponential if it is lower. Advantage is taken of this relationship to determine the corrections due to contact potentials and possible other causes which prevent the true accelerating potential in a thermionic device from being read directly from a voltmeter connected between the source of electrons and the collector. It has been shown that at certain wave-lengths the observed radiation can come only from a single vibrational level; when the corresponding intensity is plotted as a function of the voltmeter reading, a curve of the form described is obtained, and the point at which the linear and exponential parts merge is taken as that at which the true accelerating potential is equal to the (known) critical potential. When the true potential scale has been thus established, it is found that there is good agreement between calculated and observed spectral energy distribution. Thus the original calculations, which were in painful disagreement with the only quantitative experimental data then extant, are now satisfactorily confirmed.