R. D. Birkhoff

Optical Properties of Some Silicone Diffusion‐Pump Oils in the Vacuum Ultraviolet—Using an Open‐Dish Technique

The optical properties of Dow Corning‐704 and ‐705 diffusion‐pump oils have been measured from 4→24.8 eV using an open‐dish technique. These are the first liquids for which optical constants have been obtained above 11.8 eV. In the region above 10.6 eV, the real part of the dielectric constant is structureless. A collective oscillation involving π and σ electrons is seen in the region between 21→25 eV. Sum rule calculations indicate approximately 130 as the effective number of electrons per molecule participating in absorption processes up to 25 eV.

A Reflectometer for Studying Liquids in the Vacuum Ultraviolet

A reflectometer has been built for measuring the reflectance of liquids in the vacuum ultraviolet. A monochromator is rotated about a liquid sample in equilibrium with its vapor in a windowless dish. A continuous range of angles of incidence is provided between 15 and 70°. The light traverses a path of approximately 1 cm in length in the vapor adjacent to the sample. The reflected beam is collected by a light pipe. A description of the mechanisms for positioning the light pipe, adjusting the sample dish, and rotating the monochromator with respect to the reflecting surface is given, as well as the system used to maintain a pressure differential between the monochromator and chamber containing the liquid sample.

Vacuum ultraviolet optical properties of squalane and squalene

The reflectance of liquids squalane and squalene between 2 and 25 eV have been determined by closed cell double ionization chamber methods. Kramers–Kronig analysis of the data gives optical functions from which are calculated the dielectric functions, energy loss functions, and mean free path for photons and electrons. Values of yield predicted from a theoretical model of photoemission are found to be about 11% at 16 eV declining to 4% at 25 eV. However, no photoemission was observed over this energy range.

Photoemission and electron mean free paths in liquid formamide in the vacuum UV

Our apparatus has been modified to permit measurements of absolute photoemission as well as reflectance of liquids in the vacuum UV. Photon fluxes are determined by filling the tandem ion chambers with argon. An electrometer is connected to the sample cup, which must be properly biased, to read electron currents leaving the cup. After argon ion drift into the cup is taken into account, the yields of photoelectrons per incident photon and per absorbed photon are obtained. These yield curves for liquid formamide (HCONH2) are similar to each other over the energy range studied (16–25 eV). They are nearly constant between 16 and 18 eV at 13.5% and decline toward higher energies. The surface and volume plasmon energies for formamide are near 19 and approximately 27 eV, respectively. We suggest that photoelectron excitation of surface plasmons may enhance the photoemission yield, while excitation of volume plasmons may reduce it. Values of electron mean free path derived from the yields and a simple three‐step ...

Optical and dielectric functions of liquid hexamethylphosphoric triamide between 2 and 25 eV

The reflectance of liquid hexamethylphosphoric triamide has been determined between 2 and 9 eV by the semicylinder method and between 10 and 25 eV by the double ionization chamber method. Kramers‐Kronig analysis of the data gave the optical and dielectric functions, the energy loss functions, and the effective number of electrons, all as a function of photon energy. Structure in the absorption spectrum and the imaginary part of the complex dielectric function at 7.5 and 11.5 eV are associated with excitation of π and σ electrons, respectively. The σ→σ* excitation is shifted in the energy loss function −Im(1/e) to 21 eV, indicating an appreciable degree of collective behavior. Of the total 68 valence electrons, the number of electrons taking part in optical processes and energy loss processes increases to 38 and 25, respectively, at 25 eV.

Optical studies of liquid formamide in the vacuum ultraviolet

Optical and dielectric functions of liquid formamide have been measured between 2 and 24.5 eV. Below 7.8 eV a semicylinder technique was used, whereas above 10.4 eV a new ionization method was employed. Peaks in the absorption spectrum at approximately 6.8 and 16.0 eV are attributed to excitation of π and σ electrons in the formamide molecule. The latter electrons participate in a collective oscillation as evidenced by a shift in the peak in the energy loss function from the corresponding peak in the absorption spectrum. Sum rule calculations give 11.8 and 4.3 of the eighteen formamide valence electrons participating in optical absorption and energy loss processes, respectively, out to 24.5 eV.

Yields and mean free paths of photoelectrons from liquid hexamethyl phosphoric triamide

Monochromatic photons between 775 and 500 A were incident almost normally (15°) on the surface of liquid hexamethyl phosphoric triamide (HMPT). Incident and reflected beam strengths were measured with a double ion chamber filled with argon and the current leaving the cup of liquid was determined. After subtracting a portion of the current due to Ar+ drift into the cup, we obtained the photocurrent and, hence, the absolute photoemission yield. The yield falls from 9% at 16 eV to 5% at 25 eV. Such a falloff with increasing photon energy is expected from theory. The values are about double those calculated from theory using a three‐step model for the photoemission and using the Born approximation to calculate electron mean free paths. Conversely, if the experimental yields are used in the three‐step model, the electron mean free paths calculated from the data range from about 935 A at 16 eV to about 80 A at 24 eV and are about two to three times the theoretically predicted values.

Optical properties of polycrystalline anthracene in the 3. 2--9. 3-eV spectral region

The optical functions of polycrystalline anthracene films have been determined in the spectral region between 3.2 and 9.3 eV. Conventional reflectance techniques using the semicylinder method were used to obtain the optical and dielectric functions for our sublimed films. In general the maxima in our data occur at energies comparable to those where maxima occur in single‐crystal data. However, the magnitudes are frequently quite different. Our data indicate that the a and b axes of the crystallites tend to lie in a plane parallel to the surface of the substrate.

Scattering of low‐energy electrons on micron‐size pinholes

In exploring forces between charged particles and surfaces, we have sent a beam of low‐energy (0.5–40 eV) electrons through two (laser drilled) micron‐size pinholes in thin films of gold spaced 10 cm apart. Image forces associated with the solid around the second pinhole cause the beam to diverge into an angular distribution which is scanned with a movable slit and Channeltron detector. Distributions have widths which depend inversely on electron energy as expected from transit time considerations. The paraxial portion of the distribution appears to diverge as though from a weak negative electron lens. The impulse approximation for the radial component of the electron motion is used to derive a radial potential averaged along a line parallel to the pinhole axis. This potential approximately equals the expected value e/4x at x=10 A from the pinhole edge but deviates from this function at greater distances. This deviation may be due to geometric effects, to the impulse collision model employed, or to geomet...

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.