L. R. Painter

Optical and dielectric functions of liquid glycerol from gas photoionization measurements

In the vacuum ultraviolet spectral region, all solids strongly absorb electromagnetic radiation. Thus, optical properties of liquid surfaces must be obtained from reflectance measurements on a sample exposed to near vacuum rather than from a conventional closed‐cell arrangement. In the first application of a new reflectance method which measures photon intensities before and after reflection by gas ionization, we have redetermined the optical functions of glycerol. The new data agree generally with those published before in the region of overlap and extend to a higher energy, 24.4 eV. Although the reflectance between 12 and 18 eV is slightly higher than reported previously, the interpretation of dielectric data calculated from the reflectance is unaltered. The plasmon energy now appears to the at 22 eV instead of the value of 20.4 eV measured earlier.

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.

Measurement of reflectance by gas ionization

In an attempt to improve the accuracy of reflectance determinations, particularly from high vapor pressure samples, we have built and tested a double ionization chamebr which measures the ionization from a photon beam in the vapor or added gas before and after reflection. The photon beam strikes the sample at nearnormal incidence after passing through an oblong ionization chamber containing two collector wires. These are connected to one side of an electrometer-ratiometer of new design. A similar geometry is used for the specularly reflected beam, with the wires here connected to the other side of the ratiometer. A potential of 20 V on the sample is found to be sufficient to prevent the large photoelectric current from the sample from entering the chambers for photon energies to 26 eV. A sitnple calculation shows that the current ratio should decline exponentially with pressure in the operating pressure range from 100 microm to 5000 microm. Extrapolation of the data to zero pressure yields the sample reflectance. In testing, we found the reflectance to be 12.47% at 841 A for an evaporated gold film using a filling gas of air.

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.

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.

Optical and dielectric functions of liquid methylated benzenes in the 2- to 10-eV spectral region

The optical and dielectric functions have been determined in the 2 to 10‐eV spectral region for four polycyclic hydrocarbons: benzene, toluene, xylene, and trimethylbenzene. The effect of substitution on the spectra of these liquid alkyl benzenes is investigated and comparisons are made to the appropriate gas‐phase spectra.