Brian Franklin

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

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 complete UV spectrum of SO2 by electron impact, 1, The vacuum ultraviolet Spectrum

We have measured the middle ultraviolet (MUV) electron impact induced fluorescence spectrum of SO2 from 200 to 430 nm in a crossed beam experiment. The spectrum is dominated by two features at the experimental resolution of 0.5 nm. These two features are referred to as MUV1 and MUV2. MUV1 is the SO(A3∏ → X3 ∑-) band system extending from 240 to 265 nm produced by dissociative excitation. MUV2 is a blend of the SO2(ā(1A2),B~(1B1)→X~(1A1))andSO2+(C~(2B2)→X~(2A1)) molecular band systems in the range 264-430 nm. In addition, the excitation function measurements of MUV2 indicate that low-energy electrons effectively prepare SO2 in one or more electronically excited triplet states that involve the SO2 (ā(3B1)) state by direct excitation and/or cascading. A candidate triplet band system for this emission process is the E~-ā electronic transition. This emission process is the largest contributor to the MUV2 emission cross section at low electron impact energy. The peak cross section for MUV1 occurs at 20 eV with a value of 25.0 ± 5.5 × 10-19 cm2. The peak cross section for MUV2 arises at 9 eV with a value of 368 ± 81 × 10-19 cm2. The laboratory measurements of the excitation functions of both MUV features were made from 0 to 1 keV. The emission cross sections are an important part of the total inelastic cross section of SO2 needed in modeling the Io atmosphere. The laboratory results give a plausible explanation of the Io auroral hot spots observed by Voyager 2.

Electron impact excitation cross section studies of methane and acetylene

We have measured the electron impact emission cross sections for CH4 and C2H2 at 200 eV in a crossed beam laboratory system. Included in the study are all vacuum ultraviolet (VUV) emission features from 40 to 200 nm. The features are entirely from the atomic dissociation fragments (C i, C ii, and H). The Lyman series of H is observed to truncate near principal quantum number n=10 due to the long lifetime and to the high kinetic energy of the excited H fragments. The threshold region of the excitation functions has been measured at an energy resolution of 0.2–1.0 eV for the Lyman‐α and Lyman‐β transitions of H and C i (165.7, 193.1 nm) multiplets; and several distinct appearance potentials (AP) have been detected. For example, appearance potentials of Lyman‐α from dissociation of CH4 and C2H2 are noted at several energies, including the first observations of a Lyman‐α AP from C2H2 at 16.3 eV.

Study of electron impact excitation of argon in the extreme ultraviolet: emission cross section of resonance lines of Ar I, Ar II

The authors have studied in a crossed-beam experiment under optically thin conditions the extreme ultraviolet (EUV) spectrum of argon produced by electron impact excitation. The most prominent features of the EUV spectrum between 40 and 110 nm are the resonance lines of Ar I at 104.8 nm and 106.7 nm and of Ar II at 91.96 nm and 93.21 nm. Absolute cross sections of these lines at 200 eV are measured by the relative-flow technique and compared with previous estimates. The measured emission cross section values at 200 eV for the Ar I lines at 104.8 nm and 106.7 nm are 23.1*10-18 cm2 and 9.32*10-18 cm2, respectively, with an uncertainty of 14%. When compared with electron energy loss estimates of the direct excitation cross section these values establish that cascading is larger for the Ar I resonance lines than previous emission experiments have indicated. The far ultraviolet (FUV) spectrum of Ar is also surveyed for the first time and is found to consist of Ar II multiplets from simultaneous ionization-excitation.

The extreme ultraviolet emission spectrum of CO produced by electron impact at 20 and 200 eV

In this first part of a laboratory study of predissociation in CO the authors have measured in the electron-impact-induced fluorescence spectrum of CO in the wavelength range 40-125 nm at a spectral resolution of 0.5 nm. The experiment was performed in a crossed-beam configuration under optically thin conditions. Their spectral measurements at 200 eV electron impact, energy provide the emission cross sections of atomic dissociation fragments (C I, C II, O I, O II) and the vibrational transitions of the B 1 Sigma +, C 1 Sigma +, E 1 Pi -X 1 Sigma + band systems of CO. The measured emission cross sections of these molecular transitions will be needed for comparison with excitation cross sections from electron energy-loss spectra to determine predissociation yields.

Architecture and design of the aquarius instrument for RF and thermal stability

In this paper, we present the architecture and design of the Aquarius instrument: a spaceborne combination radiometer-scatterometer in L-band, for measuring ocean surface salinity. In order to achieve the unprecedented measurement stability of 0.1 Kelvin for the radiometer, the Scatterometer (for correction of the sea surface roughness) is required to have a calibrated stability of 0.1 dB. Active and passive thermal control was utilized as well as RF self calibration. Novel test techniques were also developed to verify the stability requirement was met.