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Dive into the research topics where Geoffrey K. James is active.

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Featured researches published by Geoffrey K. James.


Applied Optics | 1988

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

Joseph M. Ajello; Donald E. Shemansky; Brian Franklin; J. Watkins; S. Srivastava; Geoffrey K. James; W. T. Simms; C. W. Hord; Wayne R. Pryor; William E. McClintock; V. Argabright; D. Hall

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.


Astrophysical Journal Supplement Series | 1995

High-Resolution Electron-Impact Study of the Far-Ultraviolet Emission Spectrum of Molecular Hydrogen

Xianming Liu; Syed Ahmed; Rosalie Multari; Geoffrey K. James; Joseph M. Ajello

The emission spectrum of molecular hydrogen produced by electron-impact excitation at 100 eV has been measured in the wavelength range 1140{endash}1690 A. High-resolution, optically thin spectra ({Delta}{lambda}=0.136 A) of the far-ultraviolet (FUV) Lyman and Werner band systems have been obtained with a newly constructed 3 m spectrometer. Synthetic spectral intensities based on the transition probabilities calculated by Abgrall {ital et} {ital al}. are in very good agreement with experimentally observed intensities. Previous modeling that utilized Allison & Dalgarno band transition probabilities with H{umlt o}nl-London factors breaks down when the transition moment has significant {ital J} dependence or when ro-vibrational coupling is significant. Ro-vibrational perturbation between {ital v}=14 of the {ital B}{sup 1}{summation}{sup +}{sub {ital u}} state and {ital v}=3 of the {ital C}{sup 1}{Pi}{sub {ital u}} state and the rotational dependence of the transition moment in the (6,12) and (7,13) bands of the Lyman system are examined. Complete high-resolution experimental reference FUV spectra, together with the model synthetic spectra based on the Abgrall transition probabilities, are presented. An improved calibration standard is obtained, and an accurate calibration of the 3 m spectrometer has been achieved. {copyright} {ital 1995 The American Astronomical Society.}


Journal of Geophysical Research | 1998

Electron‐impact excitation and emission cross sections of the H2 Lyman and Werner Systems

Xianming Liu; D. E. Shemansky; Syed Ahmed; Geoffrey K. James; Joseph M. Ajello

Excitation functions of the H2 Lyman (Ly) ( B1∑∑u - X1∑∑g) and Werner (W) (C1∏u - X1∑∑g ) band systems have been reanalyzed using a combination of measurements and theoretical considerations. These systems are prominent emitters in outer planet atmospheric dayglow and auroral activity and can be used to infer energy deposition and heating rates. Earlier measurements of the cross sections reported by Shemansky et al. [1985a] have been found to be inaccurate in the threshold energy region. A combination of high-resolution spectral and shape function measurements obtained at the Jet Propulsion Laboratory (JPL) in low- and medium-energy regions have been used to obtain relative excitation shape functions of the two systems. At energies above 250 eV, we have used the measurements obtained earlier by De Heer and Carriere [1971] in order to define the first two terms of the Born electric dipole collision strength. The complete set of collision strength terms is obtained by combining the JPL and De Heer and Carriere [1971] data. We have found that, contrary to results of Shemansky et al. [1985a], the shape functions of the H2 Ly and W systems, expressed in units of threshold energy, are the same within experimental error. Absolute cross sections of the H2 Ly and W systems are established using the theoretical oscillator strengths of Abgrall et al. [1987, 1993a, b, c] and Abgrall and Roueff [1989]. The atomic hydrogen H Ly α emission cross section resulting from dissociative excitation of H2 at 100 eV obtained from relative intensities of H2 W emission lines is also derived and compared with other experimental measurements. At a gas temperature of 300 K and electron energy of 100 eV, the cross sections for the H Ly α, H2 Ly, and W emissions are (0.716 ± 0.095) × 10−17, (2.62 ± 0.34) × 10−17, and (2.41 ± 0.31) × 10−17 cm2, respectively. The accuracy of the absolute values and shape functions is limited primarily by electron gun performance, as was the case for Shemansky et al. The rotational dependence of the transition dipole matrix elements and perturbations between the B 1∑+u and C1∏+u states have a significant effect on the cross sections.


Journal of Geophysical Research | 1992

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

Joseph M. Ajello; Geoffrey K. James; Isik Kanik; Brian Franklin

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.


Journal of Physics B | 1998

High resolution EUV emission spectroscopy of the and 4 levels by electron impact

Joseph M. Ajello; Geoffrey K. James; Marco Ciocca

A high resolution 36 mA (FWHM), optically thin emission study of the and (3, 2) Rydberg bands excited by electron impact at 100 eV has been completed in the extreme ultraviolet (EUV). A model of the perturbed rotational line intensity distribution of the bands shows the effects of electronic state mixing between the Rydberg state and the valence state. By normalizing the model to the published predissociation yield for the laboratory spectrum can be used to determine the predissociation yields for each rotational level of and 4. Based on a 25% accuracy of the model fit to the measured signal intensities it is found that the predissociation yields of the rotational levels increase as the percentage of character increases. On the other hand, the predissociation yields of the rotational levels reach a maximum for . The mean predissociation yields for and 4 levels are a function of temperature and are found to be 0.41 and 0.58, respectively at 300 K. The -dependent predissociation yields indicate that the emission cross section is a function of temperature. The remainder of the bands forming the progressions -5) from and -6) from were studied at 64 mA (FWHM) resolution. Using this composite spectrum of the two progressions the electron impact emission cross sections of the and 4 levels at 300 K were determined and compared with previous results.


Journal of Physics B | 1999

HIGH-RESOLUTION FAR ULTRAVIOLET EMISSION SPECTRA OF ELECTRON-EXCITED MOLECULAR DEUTERIUM

H Abgrall; E Roueff; Xianming Liu; Donald E. Shemansky; Geoffrey K. James

The high-resolution (full width at half maximum = 0.11 A) emission spectrum of the deuterium molecule produced by electron-impact excitation at 100 eV has been measured in the wavelength range 1200 - 1660 A. In conjunction with the experimental measurement, transition probabilities of and D band systems are calculated. Synthetic spectra based on the calculated transition probabilities are in very good agreement with the optically thin experimental spectra. While centrifugal effects on the overall Lyman band emission intensity in D2 and H2 are similar, the effect of rotation-vibrational perturbations between the B 1u+ and C 1u+ states on spectral intensities is found to be less significant in D2. Excitations via isotropic ( J = 0) and anisotropic ( J = ±2, and 0) interactions for the dipole forbidden E, F 1g+-X 1g+ transition are inferred from the measured spectra. Excitation cross sections of the B 1u+-X 1g+ and C 1u-X 1g+ bands, along with the estimated E, F 1g+-B 1u+ cascade cross section, for D2 are obtained.


Journal of Physics B | 1990

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

Joseph M. Ajello; Geoffrey K. James; Brian Franklin; Simon Howell

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.


Journal of Geophysical Research | 1998

Galileo ultraviolet spectrometer observations of Jupiter's auroral spectrum from 1600–3200 Å

Wayne R. Pryor; Joseph M. Ajello; W. Kent Tobiska; Donald E. Shemansky; Geoffrey K. James; C. W. Hord; S. K. Stephens; Robert A. West; A. Ian F. Stewart; William E. McClintock; Karen E. Simmons; Amanda R. Hendrix; Deborah A. Miller

In 1996 and 1997 the Galileo Ultraviolet Spectrometer (UVS) obtained the first measurements of Jupiters nightside midultraviolet (MUV) polar auroral spectrum from 1620 to 3231 A at 13 A resolution. The reduced polar spectra, after removal of off-axis scattered radiation from the sunlit dayside of Jupiter, contain a spectrum that matches laboratory spectra of the H 2 continuum in the a-b dissociative emission transition. This is the first direct identification of the H 2 a-b transition in astronomy. The a-b emission is excited by electron impact exchange reactions with H 2 that peak in cross section near 15 eV. The emission threshold is at 1216 A, and the continuum peaks in intensity in the 2000-2500 A range. Jupiters observed wavelength-integrated MUV H 2 a-b emissions (1620-3231 A) have a photon flux ∼8 times smaller than simultaneously observed wavelength-integrated far-ultraviolet (FUV) H 2 band emissions (1230-1650 A). Because the FUV H 2 emissions have an emission cross section that peaks at higher energies near 50 eV, this FUV/MUV brightness ratio is diagnostic of the secondary electron energy distribution and is consistent with a warm distribution of electrons.


Journal of Geophysical Research | 2002

Middle ultraviolet and visible spectrum of SO2 by electron impact

Joseph M. Ajello; D. L. Hansen; Luther W. Beegle; C. A. Terrell; Isik Kanik; Geoffrey K. James; O. P. Makarov

[i] Electron-impact-induced fluorescence spectra of SO 2 in the middle ultraviolet and visible wavelength regions (200-600 nm) have been measured in the laboratory using a crossed beam experiment at three electron impact energies. The emission spectra at 8, 18, and 98 eV exhibit a broad and continuous emission region extending from 225 to near 600 nm with a peak emission close to 330 nm. The quasicontinuous SO 2 bands arise primarily from direct excitation of SO 2 . At 18 and 98 eV, simultaneous excitation and dissociation of SO 2 produces distinct vibrational bands from SO and from atomic emission lines from S I, S II, O I, and O II that are superimposed on the SO 2 electronic transitions. The laboratory spectra were compared to green/violet color ratios obtained at lo by the Galileo Orbiter Solid State Imaging experiment. The laboratory spectra were also applied to the Cassini Imaging Subsystem to determine which filter combinations are particularly sensitive to electron energy, if the atmospheric gas present in the auroral atmosphere is solely or primarily SO 2 .


Journal of Geophysical Research | 1998

The middle ultraviolet‐visible spectrum of H2 excited by electron impact

Geoffrey K. James; Joseph M. Ajello; Wayne R. Pryor

The electron-impact-induced emission spectrum of H 2 has been measured in the extended wavelength region 175-530 nm at a spectral resolution of 1.7 nm (full width at half maximum). The laboratory spectra observed in the middle ultraviolet (MUV) and visible spectral region are characterized by underlying H 2 (a 3 Σ g + → b 3 Σ b + ) continuum emission, together with many strong lines assigned to the radiative decay of the gerade singlet states of H 2 , and to members of the H Balmer series resulting from dissociative excitation of H 2 . Our calibrated MUV spectral data, obtained at 14, 19, and 100 eV electron-impact energies, provide absolute emission cross sections of these H 2 lines and will assist in the interpretation of planned Galileo ultraviolet spectrometer observations of Jupiters aurora in this wavelength region.

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Joseph M. Ajello

California Institute of Technology

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Isik Kanik

California Institute of Technology

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Brian Franklin

California Institute of Technology

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Xianming Liu

California Institute of Technology

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Dariusz Dziczek

Nicolaus Copernicus University in Toruń

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C. W. Hord

University of Colorado Boulder

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William E. McClintock

University of Colorado Boulder

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Luther W. Beegle

California Institute of Technology

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