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 H(2) and N(2) and the n(1)P(0) Itydberg series of He. The model spectrum from H(2) 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 H(2) and N(2) 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 n(1)P(0) 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, H(2), and N(2), 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.

THE HIGH-RESOLUTION EXTREME-ULTRAVIOLET SPECTRUM OF N 2 BY ELECTRON IMPACT

“…The assignment of the latter and weaker bands in the spectrum of Ajello et al (1989) likely neglected the entanglement of these with other emission features, thereby overestimating their cross sections. The new cross sections for c 4 (1-6, v ) are also an improvement on the previous measurements of Ajello et al (1988Ajello et al ( , 1998, and we attribute the scattered differences to the partition of cross sections between bands in the previous experiment. There is apparently an imperfect absolute agreement between the two experiments, with the present cross sections lying primarily below those of Ajello et al (1989) despite their nominally identical calibration procedures.…”

“…The polarization of the radiation is assumed to be negligible for N 2 transitions when employing 20 and 100 eV excitation energies and averaging over many rotational levels. The magnetically collimated electron beam source is described in detail by Ajello et al (1988). In brief, the thermionic electrons are created by heating a tungsten filament.…”

Detection of Rapidly Varying H2 Emissions in Jupiter's Aurora from the Galileo Orbiter

“…In a laboratory spectrum obtained with the Galileo UVS flight instrument of H 2 gas excited by impact of 100 eV electrons (Ajello et al 1988), the two wavelengths selected for the C3 observations are both bright, with a color ratio c (defined here as the total counts z(t) at 1611Å minus the background b, divided by the total counts z(t) at 1240Å minus the background b) of…”

High‐Resolution Electron‐Impact Emission Spectrum of H 2 . II. 760–900 A

“…Low-resolution (*j \ 5 electron-impactÈinduced A ) emission studies of molecular hydrogen in the EUV region have been reported by Ajello et al (1984Ajello et al ( , 1988. Because of the low spectral resolution, no emissions from the n [ 4 states were clearly observed.…”