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The Lylman–Birge–Hopfield system of N2(a1Πg–X1Σg+) and the CO fourth-positive system (A1Π–X1Σ+) are readily excited by low-energy electrons (100 eV). Together, these molecular systems have numerous emission bands spread uniformly over the wavelength region 1150–2500 Å. Because the absolute transition probabilities for both these systems have been accurately measured, relative measurement of the intensities of these bands excited by electron impact can be used as a convenient technique for calibrating vacuum-ultraviolet (vuv) monochromators. This is the basis of the molecular branching-ratio technique described in this paper. These relative measurements can be placed on an absolute basis by comparing the band intensities to the intensity of the hydrogen Ly-α line (1215 Å) and the O i resonance triplet (1302, 4, 6 Å) produced by the dissociative excitation of H2 and O2, respectively, and to a standard lamp at wavelengths greater than 1500 Å. The results obtained by this method are shown to be in excellent agreement with relative spectral-response measurements obtained by use of the atomic branching-ratio technique.
© 1971 Optical Society of America
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