D. L. Ederer

High‐efficiency soft x‐ray emission spectrometer for use with synchrotron radiation excitation

A new soft x‐ray spectrometer designed for use with photon excitation from synchrotron light sources is described and characterized. Special design features, including a close‐spaced input slit, large toroidal gratings, and a two‐dimensional charge‐coupled‐device array based detector system, provide exceptional measuring efficiency in a 5‐m Rowland circle design. Descriptions are given of the spectrometer’s mechanical and detector design, and of calibration and alignment procedures. The beam line providing photon excitation from a synchrotron light source is described. Typical electron beam and/or photon excited emission spectra of Al, Si, and LiF are presented and compared with those produced by other instruments.

Triply differential photoelectron studies of non-Franck–Condon behavior in the photoionization of acetylene

Vibrational branching ratios and photoelectron angular distributions for alternative vibrational levels of C2H2+X 2Πu have been measured in the range 13 eV⩽hν⩽25 eV using synchrotron radiation. Below hν∼16 eV, these data exhibit strong non‐Franck–Condon effects, namely, wavelength‐dependent vibrational branching ratios and vibrational‐state‐dependent photoelectron asymmetry parameters. Moreover, enhanced excitation of bending modes of the ion is observed below hν∼16 eV, in addition to the C–C stretch mode, which is the only mode readily observed in photoelectron spectra of C2H2 at shorter wavelengths, e.g., at the He I (21.2 eV) resonance line. The non‐Franck–Condon behavior is attributed to resonant photoionization processes, whose identification is discussed in the framework of several recent theoretical and experimental studies on acetylene and related molecules.

The angular distribution parameters of argon, krypton and xenon for use in calibration of electron spectometers

Abstract Measurements are presented of the angular distribution parameter for the photo ejected valence electrons of argon, krypton and xenon, from threshold to hv ≈25 eV. The experimental arrangement at the NBS (SURF II) storage ring is described and the method of data analysis discussed. The results are compared with other experimental data and theoretical predictions.

A high throughput 2 m normal incidence monochromator for surf-II

Abstract The high intrinsic brightness of the circulating electron beam at SURF-II is used as the entrance slit for a two-meter normal incidence monochromator. A typical beam size for the electron beam is 100 μ high by 2 mm wide yielding an observed resolution of 0.4 A with a 200 μm exit slit and a 2400 lines/mm grating. The instrument accepts a beam with a 65 mrad horizontal divergence and a 10 mrad vertical divergence. A plane pre-mirror used near normal incidence reflects the incoming radiation onto the 2 m grating; this combination provides a horizontal exit beam, and enables the experiment to be located three meters from the orbit tangent point. With magnesium fluoride coated aluminum optics a flux of 2×10″ photons/s·A at 1200 A is observed with a 10 mA circulating current. A flux of 5×10 10 photons/sdA at 600 A is observed with an osmium coated grating and a 10 mA circulating current. Sample spectra of the angle-resolved photoelectron spectrum of CO are presented.

Wavelength and vibrational‐state dependence of photoelectron angular distributions. Resonance effects in 5 σ photoionization of CO

Photoelectron angular distribution were studied for the photoionization of CO for v=0−3 in the range 16–26 eV. (AIP)

Absolute Radiometric Calibration of Detectors Between 200–600 Å

Radiometric transfer standards consisting of windowless diodes with cathodes made of anodized aluminum oxide on aluminum are now available from the National Bureau of Standards with calibrations in the 200-600-A wavelength range. This extends the previously existing range of calibration for these diodes (600-1200 A). For wavelengths shorter than 600 A, synchrotron radiation at NBS-SURF is used as the source of radiant energy. A noble gas double ionization chamber is used to calibrate a secondary standard diode that is then intercompared with the transfer standards. Monitors take into account variations in the intensity of synchrotron radiation and in beam position. Methods of accounting for the effects of second-order radiation in the incident flux and secondary ionization in the double ionization chamber are discussed. Calibration uncertainties are about 10%.

Characterization of some autoionization resonances in CO2 using triply differential photoelectron spectroscopy

We report vibrationally resolved branching ratios and asymmetry parameters for two sets of autoionizing resonances in CO2 near 680 and 750 A. These resonances were excited with monochromatized synchrotron radiation from the National Bureau of Standards storage ring and the energy and angle of ejection of the photoelectrons were analyzed. The results show striking non‐Franck–Condon behavior.

An angle resolved photoelectron spectrometer for atoms and molecules

Abstract An angle resolved photoelectron spectrometer has been constructed and coupled to the high throughout 2 m normal incidence monochromator now in operation at SURF-II. The electron analyzer whose design has been previously reported [2] scans the angular distribution of the ejected photoelectrons in a plane perpendicular to the partially polarized exit beam from the monochromator. The intensity of the photon beam from the monochromator is monitored by measuring the photocurrent from a screen in front of a three mirror polarization analyzer which is used to determine the degree of polarization of the photon beam. The function of the spectrometer is under computer control which is also acquires and processes the data output from the apparatus. Two novel features are employed to make the system highly effective: one is a capillary which channels the monochromatized output photon beam to the gas-photon-interaction zone and isolates the 10 −4 vacuum int the spectrometer chamber from the 10 −9 vacuum in the monochromator; the second feature is a high speed cryopump which permits a high density gas jet to interact with the photon beam while maintaining a high enough vacuum to minimize electron scattering and permit operation of the channeltron electron detector. These features combined with output fluxes of 5 × 10 10 photons/s −1 A −1 permit vibrationally resolved photoelectron spectra of N 2 and CO to be obtained in about 15 min.

Instrumental aspects of synchrotron XUV SPECTROSCOPY.

Electron synchrotrons are becoming increasingly important as sources of extreme uv radiation for physical experiments. The NBS 180-MeV machine has been utilized for gas absorption studies over a four-year period, during which a 3-m grazing incidence spectrograph and monochromator have been designed, constructed and put into operation. The instruments are extremely stable to vibration and temperature variation, and are operating with a slit limited resolution of the order of 0.06 A. The design principles and features of these instruments are described, and a highly successful prealignment procedure for grazing incidence spectrometers is outlined. The effect of the unusual characteristics of this light source on the illumination and performance of the spectroscopic instruments is discussed.

The development of far UV spectrometers for synchrotron radiation facilities

Abstract In the early days of synchrotron radiation research conventional instruments were adapted for use with the radiation. While these instruments yielded fluxes that were adequate for simple experiments, such as absorption spectroscopy, and produced exciting results, stringent demands imposed by the higher flux requirements of photoelectron spectroscopy and fluorescence spectroscopy created a need for instruments that were designed and optimized for synchrotron radiation as a source. Furthermore, the fundamental challenge of developing designs that maintain a fixed exit slit position and exit beam direction has yielded over the years an evolving array of instrumentation that deliver higher and higher fluxes to the specimen with resolutions adequate for solid state and gas phase experiments of greater and greater sophistication. Since the last conference on Synchrotron Radiation Instrumentation instruments have been proposed, designed, and constructed for the synchrotron radiation facilities throughout the world, and in particular, in the United States at existing facilities and at the new laboratories under construction. Monochromators that yield 10 11 photons/s - 100 mA - 0.1 eV at a photon energy of 100 eV with 0.03 eV resolution are in operation. Monochromators have also been constructed that yield 10 12 photons/s - 100 mA - 0.01 eV at photon energy of 10 eV. Designs that exploit the wavelength region below 100 A with planned resolution of 0.02 A and with expected throughputs of about 10 11 photons/s - 100 mA - 0.1 eV are under construction. These are exciting developments which provide dramatic evidence of the continuing progress in monochromator design for the exploitation of synchrotron radiation as an important tool for scientific research.