Normal Incidence Monochromator Research Articles

High flux and high resolution VUV beam line for synchrotron radiation

A beam line has been optimized for high flux and high resolution in the wavelength range from 30 to 300 nm. Sample chambers for luminescence spectroscopy on gaseous, liquid and solid samples and for photoelectron spectroscopy have been integrated. The synchrotron radiation from the storage ring DORIS (at DESY, Hamburg) emitted into 50 mrad in horizontal and into 2.2 mrad in vertical direction is focused by a cylindrical and a plane elliptical mirror into the entrance slit of a 2 m normal incidence monochromator. The light flux from the exit slit is focused by a rotational elliptic mirror onto the sample yielding a size of the light spot of 4×0.15 mm 2. The light flux at the sample reaches 7×10 12 photons nm −1 s −1 at 8 eV photon energy for a current of 100 mA in DORIS. A resolution of 0.007 nm has been obtained.

Measurements and calculations of the circular polarization and of the absolute intensity of synchrotron radiation in the wavelength range from 40 to 100 nm

The circular polarization and the absolute intensity of the elliptically polarized synchrotron radiation were measured in the VUV range as functions of the wavelength and of the vertical emission angle with respect to the orbit plane of the 2.5 GeV synchrotron in Bonn. In the wavelength range from 100 to 40 nm the absolute intensity and the circular polarization of the radiation with a bandwidth of 0.05 nm has been experimentally determined to be between 10 9 and 10 10 photons s −1 and (83±3)%, respectively. The experimental data are compared with calculated results obtained on the basis of Schwinger's theory taking into account the properties of the synchrotron as well as of the components in the 10 m normal incidence monochromator.

Current Research at NBS Using Synchrotron Radiation at SURF-II

The National Bureau of Standards (NBS) Synchrotron Ultraviolet Radiation Facility (SURF-II) is used in conjunction with a high flux normal incidence monochromator for angle resolved wavelength dependent photoelectron studies. The recent work has concentrated on studies of the effect of shape resonances on molecular vibrational intensity distributions as well as the effects of autoionization upon the vibrational intensity distributions over narrow wavelength regions. Results for CO, N2, Ar and Xe will be discussed.

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

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 Å 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·Å at 1200 Å is observed with a 10 mA circulating current. A flux of 5×10 10 photons/sdÅ at 600 Å 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.

An angle resolved photoelectron spectrometer for atoms and molecules

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 Å −1 permit vibrationally resolved photoelectron spectra of N 2 and CO to be obtained in about 15 min.

A VUV 4 m normal incidence monochromator

A 4 m normal incidence monochromator for use at the Tantalus storage ring is described. A general description of the instrument is given, together with a brief discussion of the optics required to match the angular acceptance of the monochromator to the angular spread of the source. With a 3600 lines mm −1 grating the estimated flux at the blaze wavelength of 304 Å with a bandwidth of 0.01 eV and 100 mA of circulating current, is 6 × 10 9 photons s −1.

High resolution VUV matrix isolation spectroscopy using synchrotron radiation; N2 IN Ne

We have investigated the VUV absorption spectrum of nitrogen in a neon matrix exploiting the intense synchrotron radiation continuum of the storage ring DORIS and the high resolving power of a 3 m normal incidence monochromator. With an improved sample preparation technique we were able to observe both the allowed transitions b 1Πu and b 1σu+ between 12.4 and 14.0 eV and even the for- bidden transitions w 1Δu and a 1Πg between 8.0 and 11.0 eV. All four transitions consist of long progressions of sharp bands (Γ ~ 10 meV) which are deperturbed in the matrix due to the suppression of nearby Rydberg states. Using symmetry arguments, our analysis of the spectra leads us to the conclusion that the N2 molecule is oriented along the (1,1,1) direction in the host lattice. A detailed fine structure is observed for most bands for the first time. This fine structure is caused by dynamical interactions of the excited molecules with the matrix and is interpreted as excitation of librational modes of the N2 molecule and a selective coupling to phonon modes of the neon lattice.

Intensity Calibration of Normal Incidence Monochromator by the Double Monochromator Method

The double monochromator method is applied to the calibration of the sensitivity of a monochromator in the vacuum ultraviolet region. A pre-monochromator for delivering monochromatic light and a main monochromator to be calibrated are connected in tandem as a double monochromator. The detector attached to the main monochromator is a photomultiplier coated with sodium salicylate. The monochromatic light flux from the pre-monochromator is measured by absolute detectors such as a photodiode calibrated in the region 1200 to 2300 Å. The pre-monochromator is rotated about the exit principal ray through 90 deg. so as to compensate for the polarization effect of the monochromatic light. The molecular branching ratio method is also applied to the calibration and the results are compared with the double monochromator method.

Normal incidence monochromators for synchrotron radiation sources

General considerations for the design of normal incidence monochromators for synchrotron radiation sources are described. The small divergence and the polarization of the incoming beam can be exploited in order to have a high proton flux and polarized light behind the exit slit. The size of the particle beam in the accelerator determines the wavelength resolution or the ratio between intensity and resolution. In particle one has to take into account in addition the distance from the source, the acceptance of the ootics and the aperture of the monochromator. The use of normal incidence monochromators is discussed in view of experiments requiring either high intensity, like the study of secondary processes (luminescence, photoemession, photofragmentation, …) or high wavelength resolution (spectroscopy on atoms and molecules).

Recent instrumentation developments at the university of Wisconsin synchrotron radiation center

Two new monochromators have been installed at the Wisconsin Synchrotron Radiation Center and four more are being designed and constructed for installation later this year. The new instruments installed are an uhv version of a vertically dispersing Seya-Namioka monochromator and an adjustable slit version of the “Grasshopper” grazing incidence monochromator built at the University of Wisconsin Physical Sciences Laboratory for use at the Stanford Synchrotron Radiation Project The new instruments to be installed later are a 4 normal incidence monochromator designed around a high efficiency Bausch and Lomb grating ruled for the Sky Lab Program and three grazing incidence toroidal grating monochromators designed for use with special Jobin-Yvon holographic gratings. The work on the toroidal grating monochromator is proceeding in collaboration with research groups from the IBM Watson Laboratories and the University of Pennsylvania. Operating characteristics achieved or to be expected with these instruments will be discussed.

Normal incidence monochromator for synchrotron radiation

The design of a normal incidence monochromator is described, taking into account the small divergence and the polarization of the synchrotron radiation in order to raise at a maximum the photon flux and the polarization of the light on the exit slit. It is quite common to see a succession of very different experiments at the extremity of a given beam line of a synchrotron radiation laboratory. The required wavelength range and band pass of the exciting light may vary a lot from one experiment to another. We will describe a method we have used to modify completely the characteristics of the monochromator by using the properties of holographic concave gratings, without any mechanical modification.

EUV impurity study of the Alcator tokamak

The intensity of resonance line radiation from oxygen, nitrogen, carbon and molybdenum impurities has been measured in the high-field (80 kG), high-density (6 × 1014 cm−3) discharges of the Alcator Tokamak, using a 0.4-m normal-incidence monochromator (300–1300 Å) with its line of sight fixed along a major radius. Total light-impurity concentrations of a few tenths of a percent have been estimated by using both a simple model and a computer code which included Pfirsch-Schlüter impurity diffusion. The resulting values of Zeff, including the contributions due to both the light impurities and molybdenum, were close to one. The power lost through the impurity line radiation from the lower ionization states accounted for ∼10% of the total Ohmic input power at high densities.

High flux medium resolution normal incidence monochromator for sunchrotron radiation

Based on the peculiar spatio-optical properties of synchrotron radiation (SR) we have designed a wavelength dispersing system in the normal incidence spectral range, the entrance slit being the tranverse section of the electron bunch itself. The system can work only in conjunction with an SR source and at a given (optical) distance to the electron orbit tangent point. Our design employs very asymmetrical grating conjugates (r∼95 m; r′0.5 m). Tuning is accomplished by simple rotation of the grating. The angle between the monochromator arms is of the order of 4°; the astigmatic length of the image is very short (0.3 mm). In our design, light from the exit slit is refocused by a toroidal mirror to a spot 60 μm in diameter. The optical elements of the system are placed on a vertical plane with respect to the electron orbital plane, so that the already high degree of polarization of SR is increased to 95%. With a 600-grove/mm grating the measured resolving power in the 1500-Å region is of the order of 1000. The flux measured in the same spectral region is of the order of 1010 photons sec−1 Å−1, which gives a transmission factor T∼3×10−2. Some examples of utilization are given.

High resolution absorption spectrum of nitrogen in the vacuum ultraviolet

The photoabsorption cross section of molecular N2 has been determined in the range from 10- 35 eV utilizing the continuum of synchrotron radiation from the DORIS storage ring, a 3m normal incidence monochromator of 0.03Åresolution and photoelectrical recording. New detailed features are observed which make possible a refined analysis of the valence-Rydberg interaction of the b1Πu–c1Πu, b′ 1Σ+u–c′1Σ+u states as well as an improved analysis of Rydberg series leading to the X2Σ+g, the A 2Πu, the B 2Σ+u and the C2Σ+u states of the N+2 ion.

Synchrotron spectroscopy: a normal incidence monochromator and an approach to order sorting problems

A normal incidence Wadsworth-type monochromator is described which is suitable for use with a UHV electron spectrometer and for employing a synchrotron radiation source. The optical and practical issues to be resolved when designing an instrument to work with a synchrotron source are discussed in a general way. One of the principal problems associated with using a continuum source is the presence of second- and higher-order radiation in the output of monochromators. A method is presented by which the amount of second-order radiation can be determined by means of a simple photo-yield measurement. A second grating monochromator is thus unnecessary.

High resolution monochromator for the VUV radiation from the DORIS storage ring

The unique properties of the DORIS storage ring at DESY as a synchrotron radiation source are exploited for high resolution spectroscopy in the vacuum ultraviolet. We describe a new experimental set up with a 3-m normal incidence monochromator for wavelengths between 3000 A and 300 A (4 </= ?w </= 40 eV) using a vertical dispersion plane. The storage ring provides a light flux intense and stable enough for rapid photoelectrical scanning of the spectra with a resolution of 0.03 A in first order.

Holographic diffraction gratings used in normal incidence monochromators for synchrotron radiation

Holographic diffraction gratings used in normal incidence monochromators for synchrotron radiation

Measurement of the Polarization of Extreme Ultraviolet Synchrotron Radiation with a Reflecting Polarimeter

The polarization of extreme uv light provided by a normal incidence monochromator used with synchrotron radiation from the 6-GeV electron synchrotron DESY has been measured. The measured polarization increases from 82% at 500 A to 87% at 1000 A, and is in excellent agreement with the calculated polarization of the light incident upon the monochromator grating. A reflecting polarimeter consisting of a polarizer and an analyzer each made of four gold mirrors has been used in the measurement.

Investigations of aluminum films with synchrotron radiation of wavelengths 500 to 1000 Å

Measurements of the photocurrent from thin Al cathodes in a windowless electron multiplier as a function of wavelength, polarization, angle of incidence, and film thickness have been carried out in the extreme vacuum ultraviolet. A normal incidence monochromator utilizing synchrotron radiation provided highly polarized light. A marked difference is found between the photocurrent measured during irradiation of thin films with “s” and “p” polarized light at wavelengths near the Al plasma wavelength (835 A). For films thicker than about 500 A pronounced interference effects are found in both “p” and “s” light at wavelengths less than the plasma wavelength. The observations can be explained by assuming the photocurrent is related to the photon density (electromagnetic energy density) in the photocathode. Calculations of the energy density in films irradiated with light give the structure found in the measured photocurrent. The measurements indicate our monochromator yields light with a degree of polarization consistent with the calculated polarization of the synchrotron radiation incident upon our grating (about 85%).

Argon, Krypton, and Xenon Continuum Light Sources for the Vacuum Ultraviolet

Argon, krypton, and xenon rare gas continuum light sources have been developed which are suitable for photoelectric scanning in the vacuum ultraviolet. Intense continuum light sources are required for many applications, including study of discrete absorption spectra. The argon, krypton and xenon continua have maximum intensities at about 1270, 1500, and 1650 Å respectively. Together with the intense Hop field helium continuum which we have earlier produced by similar techniques, these continua provide overlapping coverage of the 580–2000 Å wavelength region at bandwidths of at least 0.25 Å. The continua are excited with a mild condensed discharge which is controlled by a hydrogen thyratron modulator developed for this purpose. A 2.2m normal incidence monochromator equipped with a sodium salicylate-photomultiplier detector was used. Optimum conditions for producing these continua, details of the instrumentation, and typical recorder scans will be discussed.