Monochromator.us

Let there be light! Fiat lux!

XUV is a thing!


This post is taken from ISBN 0-918626-15-3 "Techniques of Vacuum Ultraviolet Spectroscopy" by James A.R. Samson. Copyright 1967, Pied Publications, Lincoln, Nebraska USA. Find and buy this book!

Various names have been given to the region below 2000 Angstroms. However, because of the opacity of air between about 2 and 2000 Angstroms and the consequent need to evacuate spectrographs, this region will be called, following Boyce [5], the vacuum ultraviolet. The major divisions of optical radiation are as follows: infrared, wavelengths longer than 7000 Å; visible, 7000 to 4000 Å; ultraviolet, 4000 to 2000 Å; and the vacuum ultraviolet 2000 to 2 Å. The vacuum region covers a very large energy range, 6 eV to 6 keV; however, it can be conveniently subdivided owing to the nature of the optical instruments used. For example, below 1040 Å no window materials transmit radiation, and below about 300 Å the low reflectance of gratings necessitates the use of grazing incidence spectrographs. Too, the use of different light sources tends to establish separate regions for the experimenter. The H2 glow discharge, which provides useful radiation from 2000 to 900 Å is frequently used in the Schumann region. At shorter wavelengths, different light sources must be used. Generally these are of the high voltage condensed spark type. This is the region of extreme ultraviolet. Since this region includes the soft x-ray region, it has been suggested by Tousey [6] that it be contracted to XUV. The figure shows a chart of subdivisions and the nomenclature introduced above. The limits should not be taken as precise boundaries.


Techniques of Vacuum Ultraviolet Spectroscopy


This post is taken verbatim from ISBN 0-918626-15-3 "Techniques of Vacuum Ultraviolet Spectroscopy" by James A.R. Samson
Copyright 1967, Pied Publications, Lincoln, Nebraska USA. Find and buy this book!

Although the energy range covered by the vacuum ultraviolet region of the spectrum, 6 to 6000 electron-Volts, greatly exceeds that of the combined infrared, visible and ultraviolet regions, no text in English since that written by Lyman in 1928 has been devoted to vacuum ultraviolet radiation.
James A. R. Samson Waltham, Mass. April 1967

Introduction
Owing to the high absorptance of air, early spectroscopic studies in the ultraviolet region of the spectrum were limited to wavelengths longer than about 2000 Angstroms. In 1893 Viktor Schumann [1] built the first vacuum spectrograph and made the first investigations of vacuum ultraviolet radiation. He employed a fluorite prism as the dispersing element, but because the dispersion of fluorite was unknown, he was unable to determine the wavelength of dispersed radiation. Nevertheless, he was able to show that air, and in particular, oxygen was responsible for absorption below 2000 Angstroms. Theodor Lyman [2], using a vacuum spectrograph equipped with a concave diffraction grating, was first to measure wavelengths in this region. He found that Schumann’s spectrum had a short wavelength limit around 1250 Angstroms, the limit due to transmission characteristics of fluorite. The region from 2000 to 1250 Angstroms is known as the Schumann region.
The use of the concave diffraction grating as the dispersing element in a vacuum spectrograph together with the absence of windows enables the entire vacuum ultraviolet region to be investigated down to about 5 Angstroms. Thus, the vacuum uv region overlaps the soft x-ray region. The difference between the two regions is simply that ultraviolet or optical radiation corresponds to energy changes in the outer electrons of an atom or ion, while x-radiation corresponds to energy changes in inner electrons.”

figure showing the division of vacuum ultraviolet, extreme ultraviolet and soft x-ray regions

Various names have been given to the region below 2000 Angstroms. However, because of the opacity of air between about 2 and 2000 Angstroms and the consequent need to evacuate spectrographs, this region will be called, following Boyce [5], the vacuum ultraviolet. The major divisions of optical radiation are as follows: infrared, wavelengths longer than 7000 Å; visible, 7000 to 4000 Å; ultraviolet, 4000 to 2000 Å; and the vacuum ultraviolet 2000 to 2 Å. The vacuum region covers a very large energy range, 6 eV to 6 keV; however, it can be conveniently subdivided owing to the nature of the optical instruments used. For example, below 1040 Å no window materials transmit radiation, and below about 300 Å the low reflectance of gratings necessitates the use of grazing incidence spectrographs. Too, the use of different light sources tends to establish separate regions for the experimenter. The H2 glow discharge, which provides useful radiation from 2000 to 900 Å is frequently used in the Schumann region. At shorter wavelengths, different light sources must be used. Generally these are of the high voltage condensed spark type. This is the region of extreme ultraviolet. Since this region includes the soft x-ray region, it has been suggested by Tousey [6] that it be contracted to XUV. The figure shows a chart of subdivisions and the nomenclature introduced above. The limits should not be taken as precise boundaries.

Å, Angstroms are currently replaced either by nanometers (nm) or electronVolts (eV) units - Sal F.

References
[1] V. Schumann, Akad. Weiss. Wien. 102, 2A, 625 (1983)
[2] T. Lyman, Astrophys. J. 5, 349 (1906)
[3] B.C. Fawcett, A.H, Gabriel, B.B. Jones, and N. J. Peacock, Proc. Phys. Soc. 84, 257 (1964)
[4] A.H. Gabriel, J.R. Swain, and W.A. Waller, J. Sci. Instr. 42, 94 (1965)
[5] J.C. Boyce, “Spectroscopy in the Vacuum Ultraviolet,” Revs. Mod. Phys. 13, 1-57 (1941)
[6] R. Tousey, J. Opt. Soc. Am. 52, 1186 (1962)
[7] V. Schumann, Ann. D. Phy. 5, 349 (1901)
[8] H.A. Rowland, Phil. Mag. 13, 469 (1882)
[9] H.A. Rowland, Phil. Mag. 16, 197 and 210 (1883)
[10] G.L. Weissler, “Handbuch der Physik” (Springer-Verlag, Berlin, 1956), Vol. XXI
[11] K. Watanabe, Advances in Geophysics 5, 153-223 (1958)
[12] “Proc. First Intl. Conf. Vacuum UV Radiation Physics,” ed. G.L. Weissler, J. Quant. Spectrosc. Radia. Transfer 2, 313 (1962)
[13] R. Tousey, Applied Optics 1, 679 (1962)
[14] H. Bomke, “Vakuumspektroskopie” Johann Ambrosius Barth, Leipzig (1937)
[15] T. Lyman, “The Spectroscopy of the Extreme Ultraviolet” (Longmans, Green, New York, 1928) 2nd ed


Merry Thanksgiving


just a postcard we sent to a few people during November 2015

Just a postcard we sent to a few people during November 2015



Fusion and Hot Plasma Measurements


McPherson delivers multi chordal visible light and extreme vacuum ultraviolet spectrometers to experimental fusion facilities around the world. Over the past few months there has been a lot of chatter about fusion. This idea has legs. Bountiful clean energy, who doesn’t like that idea? Princeton Plasma Physics Laboratory even just produced a comic book about fusion research “A Star for Us”.

Internet and other trolls point out that we do not actually have any energy produced by fusion. Nonetheless, a small business like McPherson in Chelmsford Massachusetts is excited about the prospect. McPherson makes spectrometers ideally suited to studying the fusion process. McPherson spectrometers are used in the USA and all over the world at experimental fusion experiments. The spectrometers diffract the light emitted from the fusion plasma and identify component wavelengths. Measuring what wavelength the light is helps scientists learn about plasma temperature, ion rotation, constituents (e.g. contamination, are the containment walls burning); basically to learn more about what works and what doesn’t.

image from Lockheed skunkworks article, McPherson grazing spectrometer fron and center

Traditionally, customers for spectrometers that measure light coming from the hot fusion plasma are at really large laboratories. For example, McPherson has customers at Oak-Ridge, Livermore, Sandia, Max-Planck (Germany) and NIFS (Japan). And also at large university labs like Princeton and MIT. Now there is new excitement from smaller groups, like Lockheed’s skunk works or companies like Tri Alpha Energy and General Fusion. In the last few months Lockheed announced truck-sized (portable) fusion reactors. Tri Alpha Energy announced that their experiment worked. They are ready to scale it and demonstrate its capabilities for real.

Even though these experiments are relatively small they cost tens or hundreds of millions of dollars to set up and run. They also still use McPherson spectrometers to learn more about what works. Ultimately, if this green, clean power comes from the federally funded national labs or the smaller private sector labs it doesn’t matter. We will all benefit. Until then McPherson’s visible light and extreme vacuum ultraviolet light spectrometers will continue helping discover what makes the process tick.


Czerny-Turner Monochromator Design


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M. Czerny and his graduate student A.F. Turner invented one of the most useful methods for using plane diffraction gratings for spectroscopy in 1930. The “Czerny-Turner” monochrometer consists of two concave mirrors and one plane diffraction grating. Generally, two identical concave spherical mirrors are used off-axis and for collimating and focusing. The mirrors are arranged such that coma is cancelled. Occasionally flat (plane) turning mirrors are added for steering or adding capability (paths). With careful implementation, these simple elements constitute a very high performance optical system. The Czerny-Turner monochromator design is popular and regularly used to this day.

For you German readers, here is the cover page of the paper by Czerny and Turner. Within the following thirty years or so, the grating moved from the same line as the slits (as shown in their paper) to a position roughly 0.8 * focal length in order to flatten the focal plane. They tried several optical layouts for monochrometers. English readers, just look at the pictures.

the criss-cross Czerny-Turner design

See also the crossed Czerny-Turner (US Patent 3,409,374, Paul McPherson) configuration. This monochromator development specifically increased workspace around the slits and maintained a small angle of incidence at the grating. After this patent expired, the criss cross design concept found way into many contemporary compact instruments used with fiber optic input and (linear) array detectors.

the criss-cross Czerny-Turner design

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We intend to provide general information about monochromators, spectrometers, spectrographs, their uses and applications. We hope it is useful.

A monochromator is an optical instrument that transmits a mechanically selectable discrete band of wavelengths of light, or other electromagnetic radiation, chosen from a wider range of wavelengths available at the input. The name is from the Greek roots mono-, "single,” and chroma, "color", and the Latin suffix -ator, denoting an agent.