Burton L. Henke

The characterization of x‐ray photocathodes in the 0.1–10‐keV photon energy region

A method and an instrument are described for the measurement of the absolute quantum yield for front‐surface and transmission photocathodes in the 0.1–10‐keV photon energy region. The total and the secondary electron photoemission yields have been measured for the Al, Au, CuI, and CsI photocathodes as required for the absolute calibration of the x‐ray diode detectors and for the x‐ray streak cameras. The relative secondary electron yields have also been measured for the same photocathodes by high resolution electron spectroscopy of the secondary electron energy distributions, which are in good agreement with the absolute yield measurements. The secondary electron yield of CsI is ten to one‐hundred times higher than that for Au in the 0.1–10‐keV region and with a secondary energy distribution that is appreciably sharper. For these reasons, CsI should be an effective photocathode for sensitive, time‐resolved spectroscopy into the picosecond region. It is verified experimentally that the secondary electron quantum yield varies approximately as Em(E), with E as the photon energy and m(E) as the photoionization cross section, and that the primary (fast) electron quantum yield is a small fraction of the total yield and varies approximately as E2m(E). A simple model for x‐ray photoemission is described which leads to semiempirical equations for front‐ and back‐surface secondary electron photoemission as based upon an escape depth parameter that may be obtained from yield‐versus‐photocathode thickness data. The model predictions are in good agreement with experiment.

0.1–10‐keV x‐ray‐induced electron emissions from solids—Models and secondary electron measurements

Analytical models are presented describing the x‐ray‐excited emission of ’’no‐loss’’ photoelectrons and Auger electrons and the energy distribution of emitted secondary electrons. The secondary electron energy distribution is given in terms of the electron kinetic energy EK, work function W, photon energy Eo, and mass photoionization coefficient μ (Eo), as proportional to Eoμ (Eo) EK(EK+W)−4. Techniques of electron spectral measurements utilizing uniform field preacceleration and limited acceptance angle spectrometers are discussed. Secondary electron energy distributions are measured at about 10−8 Torr from thick evaporated films of gold and aluminum at photon energies 277, 1487, and 8050 eV. The shapes of these distributions do not depend significantly upon photon energy. The full width at half‐maximum (FWHM) of these distributions are 3.9, 6.7, and 4.4 eV for Au and ion‐cleaned Au and Al photocathodes, respectively. The data agree well with the model predictions.

Sulfur LII,III emission spectra and molecular orbital studies of sulfur compounds

The fluorescent sulfur LII,III emission spectra have been quantitatively measured and analyzed for the relative strengths (radiative yields) of the allowed transitions, and for the corresponding emission linewidths. These were investigated for the sulfur compounds in the solid, vapor, and gas states—Na2SO4, K2SO4, CdSO4, Na2SO3, K2SO3, C4H4S, H2S, SO2, and SF6. The S LII,III spectra for the solid compounds (all strongly ionic) revealed essentially the same localized molecular orbital character about a central sulfur atom as for the molecular compounds—showing no significant influence of the cations and no evidence of crystal band structure. The measured molecular orbital energies and radiative yields were found to be generally consistent with the eigenvalue and eigenvector calculations based upon current CNDO and ab initio molecular orbital approximation methods. The measured sulfur LII,III spectra for the compounds reported in this paper have been interpreted according to the valence orbital configuratio...

High‐efficiency low‐energy x‐ray spectroscopy in the 100–500‐eV region

The lead myristate multilayer analyzer has provided a basis for a relatively simple and efficient spectroscopy for the low‐energy x‐ray emissions in the 20–80‐A region (where conventional crystal spectroscopy and grazing incidence grating spectroscopy are generally inefficient). The percent reflectivity, the integrated coefficient of reflection, and the Bragg diffraction width of the lead myristate analyzer have been measured and found to be consistent with the predictions of a simple theoretical model for multilayer diffraction. This multilayer spectroscopy at large Bragg angles has a high efficiency (high instrument transmission) as compared to grazing incidence grating spectroscopy in this 20–80‐A region. However, the resolution is limited to that set by the diffraction width of the lead myristate analyzer of about 1 eV. Because the collimator‐crystal broadening function can be precisely defined, a simple and effective deconvolution procedure can be applied with this multilayer spectroscopy to bring th...

X-ray spectroscopy in the 100–1000 eV region

Abstract Some current methods for achieving low energy X-ray spectroscopy in the 10–100 A region are reviewed. Gratings, crystals and multilayers can be used as monochromators or dispersive analyzers. Some of the important characteristics are noted here which can help to determine their applicability to a given spectroscopic analysis situation. The “trade-off” between resolution and spectrographic speed (gratings versus multilayers) may be an important consideration when the number of photons available for measurement is limited, as, for example, by the excitation dosage allowed for a given sample. For pulsed X-ray sources and for time-resolved spectroscopy, special fixed-crystal spectrographs have been developed. These may be applied with X-ray diodes and fast oscilloscopes or with X-ray streak cameras for detection. The optimum design and characterization of the photocathode systems for such detection have been studied in detail and some of the results of this work are briefly reviewed.

Quantitative low‐energy x‐ray spectroscopy (50–100‐Å region)

The quantitative analyis of emission spectra in the 10–100‐A region has become of considerable importance for high‐temperature plasma diagnostics (106–107 °K region) and for molecular orbital and solid‐state‐band analysis. Because measurement intensities are typically low in these applications, achieving an optimum spectrographic measurement is essential. In order to present specific procedures and methods for optimizing and calibrating a low‐energy spectrographic measurement, a molecular orbital analysis in the 70–90‐A region (S‐LII,III emission spectra) has been carried out quantitatively in energy and intensity using a recently described single‐crystal (lead stearate) spectrographic approach with about 1 eV resolution. Radiative yields, Y, for the radiation process being investigated are determined by the relation, Y=Z/(X0RSTQ), where Z is the area (intensity x angle) under the spectrographic line, X0 the excitation function, R (λ) the coefficient of reflection of the analyzer, S (λ) the effective sour...

Cl‐LII,III fluorescent x‐ray spectra measurement and analysis for the molecular orbital structure of ClO4−, ClO3−, and ClO2−

The chlorine LII, III low energy x‐ray spectra from sodium perchlorate, chlorate and chlorite have been obtained using carbon Kα (277 eV) photon excitation and a lead myristate analyzing ’’crystal’’ (2d=80 A). X‐ray induced decomposition was observed for each of these compounds. By taking repeated spectral scans, systematically distributed over six samples, it was possible to extrapolate to ’’zero‐dose’’ Cl‐LII, III spectra. A specially developed least‐squares fitting program was applied to precisely determine the energy and strength of each spectral component which utilized the known collimation and crystal broadening functions and yielded energy resolutions of less than 1 eV. Broad low‐energy satellite structures were observed for all the oxy‐anions and for chloride (NaCl) and have been compared to similar satellites as measured in the Ar‐LII, III spectrum. These structures were thus identified as resulting from multielectron processes. The other peaks in the Cl‐LII, III spectra of the oxy‐anions could ...

Secondary electron energy distributions for gold as excited by C Kα (277 eV) and Al Kα (1487 eV) x rays

The secondary electron energy distributions for a gold photocathode as excited by C Kα (277 eV) and Al Kα (1487 eV) x rays have been measured. The shapes of the energy distributions are essentially the same for these two x‐ray photon excitation energies. For thick evaporated gold samples on glass substrates (at 150 °C and 3×10−8 Torr), the secondary electron energy distributions peak at about 1 eV and have a FWHM of about 4 eV. As measured immediately after ion cleaning, the distributions peak at about 2 eV and have a FWHM of about 6.6 eV. Approximately 5 h after ion cleaning, the measured distributions appear as those obtained before ion cleaning. The work function of the evaporated gold photocathode temporarily increases by 1 eV upon ion cleaning.

Low energy x‐ray emission spectra and molecular orbital analysis of CH4, CCl4, and CHCl3

The C–K and Cl–LII,III low‐energy x‐ray spectra from solid CCl4, CHCl3, and the C–K x‐ray spectrum from solid CH4 have been obtained using monoenergetic x‐ray excitation and a lead myristate multilayer analyzing crystal. The C–K spectrum of methane and Cl–LII,III spectra of the chloromethanes were also measured in the gas/vapor phase and compared with those measured in the solid phase. The deconvolved spectral components are aligned on a common energy scale with the complementary x‐ray emission and photoelectron spectra by identifying the same molecular orbital in all spectra. Such an alignment procedure yields a C‐ls ionization energy of gaseous CH4, and solid CCl4 and CHCl3 as 290.0 293.5 and 293.1 eV, respectively; and the Cl‐2p3/2 ionization energy of solid CCl4 and CHCl3 as 206.5 and 204.8 eV. Results of the CNDO/2 and MINDO/3 MO calculations have been presented and compared with the available results of the extended Huckel MO method and with the deconvolved spectral components. From the geometry pro...

Soft X-Ray Spectroscopy: Excitation and Dispersion

Soft X-ray emission from a wide variety of chemical compounds, complexes and minerals has been initiated using electron or photon bombardment. A comparison of the spectra showed that in most cases electron excitation caused sample decomposition. In the case of AlF6- - -, SiF6- - and PF6- it was possible to observe decomposition to the element as a function of time. In general fluorescence excitation caused much less sample decomposition.