W. R. Hunter

Errors in using the Reflectance vs Angle of Incidence Method for Measuring Optical Constants

The sensitivity of the reflectance vs angle of incidence method for measuring the optical constants n and k has been investigated using computer methods. Reflectance values for the perpendicular and parallel components Rs and Rp, and their average Ra, corresponding to unpolarized radiation, were computed. With these values, isoreflectance curves were plotted in the n–k plane using a computer–plotter combination. The angles of intersection of these isoreflectance curves provide a measure of the sensitivity of the method and are a function of n and k. For a given n and k, the sensitivity of the method is greatest using Rp and least using Rs Error studies showing the effects of nonparallelism and polarization of the incident radiation and error in reflectance measurement were made for Rp and Ra, but not for Rs because the lack of sensitivity of the method for this component renders it practically useless. The effect of errors was simulated by altering the computed reflectance values by a given amount and plotting the corresponding isoreflectance curves. It was found that the method is least sensitive to errors when using Rp. A scheme for plotting isoreflectance curves using a computer–plotter combination is given as well as an alternative scheme for digital computation of n and k without recourse to isoreflectance curves.

On the use of classical and conical diffraction mountings for xuv gratings

A description is given of the properties of plane diffraction gratings used in conical diffraction. Formulas are given for computing the direction of the diffracted orders. Experiments were performed to investigate the behavior of gratings used on conical diffraction mountings. Comparisons made with classical diffraction mountings show a significant increase in the efficiency of the −1 order. An empirical formula to predict the efficiencies of gratings used in conical diffraction mountings has been verified by the measurements.

Reflectance of Evaporated Aluminum in the Vacuum Ultraviolet

The effect of speed of evaporation, pressure, thickness, and age on the reflectance of evaporated Al films was investigated in the extreme ultraviolet from 900 A to 2200 A. High speed of evaporation was found to be the most important factor in producing Al films of highest ultraviolet reflectance. By changing the evaporation time required to produce an 800 A thick coating from 2 sec to 130 sec at a pressure of 1×10−5 mm Hg, the reflectance is reduced from 90% to 67% at 2000 A, from 74% to 50% at 1600 A, and from 38% to 30% at 1200 A. For films produced at higher pressures such as 1×10−4 mm Hg the effect of evaporation speed on the reflectance is even more pronounced. Film thickness has very little effect on the reflectance as long as films are not thinner than 400–500 A and not thicker than 2000 A. The effect of aging on the reflectance increases strongly with decreasing wavelength. Between 2 hr and 1 month exposure to air the reflectance of good Al films decreases from 91.5% to 90.0% at 2200 A, from 75% to 69% at 1600 A, and from 44% to 27% at 1200 A.

A grating/crystal monochromator for the spectral range 5 eV to 5 keV

Abstract An ultrahigh vacuum scanning monochromator using gratings and crystals for the unusually large spectral range of roughly 5 eV to 5 keV has been designed for installation at the X-ray storage ring at Brookhaven National Laboratory. The electron beam at the tangent point of the storage ring is imaged at unit magnification on an exit slit by means of fixed paraboloidal collimating and focusing mirrors illuminated at a small glancing angle. Two dispersing elements, located in the approximately parallel radiation beam between the mirrors, are simultaneously translated and rotated such that the direction of the beam emerging from the monochromator is fixed. When blazed gratings are used, the scanning motion maintains them always at the blaze condition, thus ensuring maximum efficiency. In the double-grating mode, a resolving power of 2000 over the entire range is indicated by ray tracing. When using a pair of crystals, one has a conventional double crystal monochromator. In the resulting instrument design, all motions are fed into the ultrahigh vacuum system via linear translation through opposing bellows, using computer-driven stepper motors and air bearings for the main supports. The rotational motion is coupled from a translational feedthrough via a metal tape drive. Translations and rotations are independent and encoded interferometrically.

Reflectance of Aluminum Overcoated with MgF 2 and LiF in the Wavelength Region from 1600 Å to 300 Å at Various Angles of Incidence

Since the discovery that Al overcoated with MgF(2) or LiF produces high reflectances to wavelengths as short as 1150 A and 1000 A, respectively, these coatings have been used extensively in vacuum ultraviolet instruments in the wavelength region where their reflectance is high. If the instrument is intended to cover wavelengths shorter than the two given above, usually either Pt or Ir is used, with a loss of speed at the longer wavelengths. This paper presents reflectance data showing that fluoride-overcoated Al can be useful to wavelengths as short as 500 A. Measurements were made from 1600 A to about 300 A at normal, 35 degrees , and 85 degrees angles of incidence, angles used in normal, Seya, and grazing incidence spectrometers, respectively. These measurements show that from the boundary of the high reflectance region to 500 A, the reflectance at normal and 35 degrees depends on the thickness of the fluoride coating and can be as high as 24% at 800 A for a MgF(2) thickness of 150 A. For shorter wavelengths, the reflectance shows a decreasing thickness dependence and at 304 A is very low-about 1%. At grazing incidence, the reflectance shows some thickness dependence from 1500 A to about 1000 A, but toward shorter wavelengths the dependence disappears and the reflectance increases slowly to about 80% at 500 A. In addition to the reflectance measurements, polarization effects are discussed.

Optical Properties of Evaporated Iridium in the Vacuum Ultraviolet from 500 Å to 2000 Å

The reflectance and optical constants of evaporated iridium films were measured in the wavelength region from 500 to 2000 A. The films were evaporated by electron bombardment and deposited at various rates onto glass substrates of various temperatures. Their optical constants were determined from reflectance measurements made at different angles of incidence. In contrast to aluminum and other oxide-film-forming metals, iridium showed very little change in reflectance during exposure to air. Films with highest normal incidence reflectance were obtained by evaporation onto substrates heated to 300°C and more. Owing to interference, semitransparent films of 100 to 200 A on glass showed higher reflectance than opaque films at most wavelengths in the vacuum ultraviolet. Iridium films produced under optimum conditions had a reflectance of about 27% at 550 A.

Influence of Purity, Substrate Temperature, and Aging Conditions on the Extreme Ultraviolet Reflectance of Evaporated Aluminum

An investigation was made of the effects of aluminum purity, substrate temperature, aging, and ultraviolet irradiation on the reflectance of coatings of aluminum deposited on glass by evaporation. The wavelength range covered was 900–2200 A. Data are presented supporting the following conclusions. Whenever aluminum mirrors or diffraction gratings of the highest reflectance are required for use in the extreme ultraviolet, it is important that they be produced from the purest grade of aluminum (99.99%) on a nonheated substrate. Growth of the oxide should be retarded, so far as possible, by storing the coatings in a dessicator, rather than leaving them exposed to normal room air. If left in an unevacuated spectrograph some drying agent should be introduced. Intense ultraviolet irradiation should be avoided as much as possible, especially if water vapor is present. For use at wavelengths below 1100 A heavily anodized aluminum may be preferred to fresh aluminum.

Multilayer-coated blazed grating performance in the soft x-ray region.

Useful grating efficiences for a variety of new spectrometer types have been attained. These results are discussed.

Measurement of optical properties of materials in the vacuum ultraviolet spectral region.

Measurement of optical properties in the VUV depends on photometric data rather than a mixture of photometric and relative phase data. Generally the measurement technique is that of measuring the reflected intensity at a number of angles of incidence and fitting these data to a calculated reflectance curve. The actual measured reflectance values need not always be known but are helpful in the final analysis of the data. Other more specialized methods include a critical angle method and the determination of extinction coefficients from transmittance measurements. The accuracy of the reflectance methods depends on the optical properties of the material being measured and the angles of incidence at which reflectance measurements are made. Instrumental errors and the condition of the surface being measured can also cause appreciable errors in the results. Accurate values of the optical constants can be used to characterize the electronic structure of a material and to design special reflecting and transmitting coatings for use in EUV astronomy and laboratory applications and possibly for use with excimer lasers in the VUV.

Optical Constants of Metals in the Extreme Ultraviolet. II. Optical Constants of Aluminum, Magnesium, and Indium at Wavelengths Shorter than Their Critical Wavelengths*

Measurements of the index of refraction of Al, Mg, and In as a function of wavelength have been made from 300 A to their critical wavelengths. The measurements, made on films evaporated onto glass substrates, were obtained by using a “critical-angle” technique. It was found that the index of refraction of Al and Mg in this wavelength range can be described to a good approximation by the free-electron theory of Drude, while that of In cannot. The extinction coefficient was calculated for Al and Mg on the basis of the free-electron theory and, in the case of Al, compared with measured values. The optical properties of Al over a wavelength range from 300 to 6000 A are discussed and compared with results predicted by the free-electron theory.