Simple ultraviolet calibration source with reference spectra and its use with the Galileo orbiter ultraviolet spectrometer

Abstract

We have developed a simple compact electron impact laboratory source of UV radiation whose relative intensity as a function of wavelength has an accuracy traceable to the fundamental physical constants (transitions probabilities and excitation cross sections) for an atomic or molecular system. Using this laboratory source, calibrated optically thin vacuum ultraviolet (VUV) spectra have been obtained and synthetic spectral models developed for important molecular band systems of H₂ and N₂ and the n¹P⁰ Rydberg series of He. The model spectrum from H₂ represents an extension of the molecular branching ratio technique to include spectral line intensities from more than one electronic upper state. The accuracy of the model fit to the VUV spectra of H₂ and N₂ is sufficient to predict the relative spectral intensity of the electron impact source and to serve as a primary calibration standard for VUV instrumentation in the 80–230-nm wavelength range. The model is applicable to VUV instrumentation with full width at half-maximum ≥0.4 nm. The present accuracy is 10% in the far ultraviolet (120–230 nm), 10% in the extreme ultraviolet (EUV) (90–120 nm), and 20% in the EUV (80–90 nm). The n¹P⁰ Rydberg series of He has been modeled to 10% accuracy and can be considered a primary calibration standard in the EUV (52.2–58.4 nm). A calibrated optically thin spectrum of Ar has been obtained at 0.5-nm resolution and 200-eV electron impact energy to 35% accuracy without benefit of models over the EUV spectral range of 50–95 nm. The Ar spectrum expands the ultimate range of the VUV relative calibration using this source with the four gases, He, Ar, H₂, and N₂, to 50–230 nm. The calibration of the Galileo orbiter ultraviolet spectrometer for the upcoming Jupiter mission has been demonstrated and compared to results from other methods.