R. N. Hamm

Optical Constants of Vacuum-Evaporated Silver Films*

The optical constants of vacuum-evaporated silver films were determined in the wavelength region 2500–3700 A from reflectance measurements made at various angles of incidence. These data were obtained to investigate the discrepancy between the experimental optical emission studies from electron-bombarded thin silver foils and transition radiation theory when the optical constants of bulk silver were used as input data. The discrepancy of about 30% in the photon intensity at the peak of emission is removed when the optical constants of evaporated films are used. These optical constants also predict accurately the resonance absorption found in the optical transmission of thin silver foils.

Polarization Studies in the Vacuum Ultraviolet

A method is presented, well suited to the ultraviolet region, for determining the degree of polarization produced by a polarizer at a given wavelength and for determining the polarization introduced by a grating monochromator. An analysis is made of the degree of polarization required by a polarizer for use in optical studies to determine, for example, the reflectance of a surface for light of parallel or perpendicular polarization. Data are given in the spectral region 500 to 1300 A for gold and silver reflection-type polarizers and for a grating used in the Seya geometry.

Optical Properties of Sodium in the Vacuum Ultraviolet

The index of refraction, n, of sodium films evaporated onto quartz substrates has been determined in the spectral region from 420 to 2000 A from interference patterns observed in the reflected beam and from the critical angle marking the onset of “total reflection.” The growth of an oxide layer on the surface of a film is shown not to alter appreciably the position of interference maxima and minima. The real part of the dielectric constant was consistent with a nearly-free-electron model. The effective volume–plasmon energy was found to be 5.69±0.06 eV, in agreement with the values determined from electron-energy-loss experiments.

Collective electron effects in molecular liquids

Abstract A knowledge of the interactions of radiation with molecular liquids such as glycerol, water and benzene has direct biological application in terms of understanding radiation damage to living material. Not only the amount of energy but the modes of energy deposition can be extracted from the dielectric functions if these are known over a sufficiently wide energy range. The experimental techniques and methods of data analysis which yield the dielectric functions are reviewed. The conditions necessary for the existence of collective electron effects in molecular liquids are considered and the collective behavior associated with a single oscillator and with a collection of oscillators is compared with the behavior of a free-electron gas. Experimental spectra for some molecular liquids in the energy region from 0 to 26 eV are analyzed for the degree of collective behavior exhibited.

Extinction Coefficient and Imaginary Part of the Dielectric Constant for Sodium and Potassium above the Plasma Energy

The extinction coefficients, k, of sodium and potassium have been found by measuring the transmittance of vacuum-evaporated films 400–900 nm thick deposited on quartz or lithium fluoride substrates. Corrections were made for losses due to surface reflections, substrate absorption, and contamination of the exposed surface of the metal. Previously determined values of the refractive indices, n, of Na and K were used to compute the imaginary part, ∊2 = 2nk, of the dielectric constant. Above the interband energies, at 2–3 eV, ∊2 for both Na and K decreases rapidly with increasing photon energy up to the plasma energy. Beyond the plasma energy, ∊2 remains nearly constant within the limits of our measurements. The values of ∊2 for K are more than twice those for Na in the region above the plasma energy.

Optical Constants of Vacuum Evaporated Films of Cd and Tl in the Vacuum Ultraviolet

The optical constants, n and k, and energy-loss functions of vacuum evaporated Cd and Tl films were determined in the wavelength region 500 to 2800 A from reflectance measurements made at 20°, 45°, and 70° angles of photon incidence. The energy-loss functions were found to have maxima at 8.55 and 10.25 eV for Cd and Tl, respectively, as compared to the values of 8.1 and 9.6 eV found from characteristic electron energy-loss experiments.

Slope method for determining extinction coefficients

A method is presented for obtaining the extinction coefficient, k, of an absorbing medium from critical-angle measurements. The maximum value of the slope of the reflectance vs angle of incidence curve is very sensitive to k and relatively insensitive to the refractive index, n, the polarization, P, and to surface imperfections. Theoretical curves are presented relating the magnitude of this maximum slope to k for constant values of n and P. In practice, the accuracy obtainable is limited by problems of normalization of the measured reflectance.

Optical properties and electron-attenuation lengths from photoelectric-yield measurements*

The photoelectric yield of an infinitely thick medium is examined as a function of the polarization and angle of incidence of the incident light, the refractive index, n, and extinction coefficient, k, of the photoemitter, and the attenuation length, L, of the photoexcited electrons in the photoemitter. It is found that the photoelectric yield is a maximum at an angle very close to θc given by n = sinθc and that the magnitude of the yield at this angle, relative to that for normal incidence, is dependent on k and L. In practice, if we know the polarization of the incident light, a least-squares fit of the experimental photoelectric yield as a function of angle of incidence can be made to the theory using n, k, and L as the adjustable parameters. This method is most sensitive for the determination of optical constants when n is close to, but less than, unity and k and L are small. These are just the conditions generally found in the soft x-ray region where it is difficult to obtain accurate optical constants from reflectance measurements. It is suggested that, in the energy region where it is applicable, this method for obtaining optical constants may be easier and more accurate than reflectance methods. When the magnitude of the yield near θc is dependent on both k and L, this quantity may be used to determine L if k is measured independently.

A laser reflection technique for studying contamination buildup on a fusion reactor liner

Abstract Calculations and measurements of optical reflectance of a gold substrate for various carbon overlayers suggested monitoring a Tokamak liner optically for contamination. A He-Ne laser beam directed at the inner surface of the ORM AK liner reflected back several centimeters in diameter due to the optically rough and irregular surface. Gentle scouring of the liner greatly increased the reflected intensity. A beam splitter in the laser beam directed a portion through a chopper to a photo-multiplier and oscilloscope, this acting as a reference signal. The remainder struck the liner and photomultiplier without being chopped. The reflectance decreased during the first ~200 shots and returned to its original value at about shot 500. Separate reflectance curves were found after clockwise and counterclockwise discharge. Strong temperature effects were seen from liner expansion of gas condensation. Reflectances became erratic after neutral particle injection. We conclude the method is feasible and many have use in plasma diagnostics.

Optical Properties of Liquids in the Vacuum UV

One of the most rapidly developing fields of physics in the last decade has been the study of solids in the vacuum ultraviolet spectral region, that is, the wavelength region between 400 angstroms and 2,000 angstroms or between 30 eV and 6 eV photon energies. Reflection and transmission studies have revealed the properties of many metals, semi-conductors, and insulators and in some cases rather complete band-structure models have been invoked to explain these properties.