G. L. Weissler

Photoionization Analysis by Mass Spectroscopy

A vacuum ultraviolet monochromator has been combined with a mass spectrometer to study photoionization processes between 1570 and 430 A. Monochromatic photons were used to ionize, A, He, Ne, O2, N2, CO, NO, CO2, N2O, and NO2; and preliminary measurements of ion intensity as a function of photon energy have been obtained for the parent ions together with their fragments. The appearance of ionization and the structure of curves, in which ions per unit photon flux versus λ are plotted, will be related to previously reported energy levels, to photoionization cross sections, and to dissociative photoionization mechanisms.

Optical and Photoelectric Properties of Thin Metallic Films in the Vacuum Ultraviolet

Measurements have been made on evaporated thin films of Al, Sn, In, Bi, Au, Ag, and Cd in order to correlate optical transmission, reflection, and photoemission in the far ultraviolet. Thin unbacked films prepared outside the spectrograph and glass-backed films prepared inside the spectrograph were used. The frequency at which the films change from a reflecting medium to a transmitting medium has been compared with the plasma frequency predicted by Bohm and Pines and also with electron energy eigenlosses in metals observed by Marton et al. Some new absorption transitions have been observed and related to x-ray absorption edges. A qualitative correlation between the photoelectric yields and corresponding optical properties has been attempted.

Absolute Absorption Coefficients of Nitrogen in the Vacuum Ultraviolet

Absorption in nitrogen at wavelengths longer than 796A, the Worley-Jenkins Rydberg series limit 2∑g+, was found to be principally the result of band spectra, whereas below this limit continuous absorption set in, very rapidly reaching a first maximum of k=680 cm−1 near 760A and a second one near the limit of the Hopfield Rydberg series 2∑u+. From the appearance of the contour of the absorption continuum and from the intensity distribution of the preceding bands, one is led to believe that it was a composite one consisting of two ionization continua adjoining the two Rydberg series limits. The integrated absorption coefficient gave an f-value of 3.3 which was in agreement with the predicted value of 3.27 from classical electron theory. Between 796A and 661A some very large absorption coefficients were found with k=2.76×103 cm−1 because of the strong, diffuse bands in this region. Experimental results were internally consistent to about ten percent and Beer’s law was shown to be obeyed.

Ultraviolet Absorption of Atomic Nitrogen in its Ionization Continuum

A method has been developed to measure absolute absorption coefficients of atomic nitrogen in the region of its ionization continuum below 800 A. This was achieved by passing radiation from an appropriate light source through the plasma of a Philips Ionization Gauge (P.I.G.) type discharge into a normal incidence vacuum spectrograph. Light intensities at various wavelengths between 400 A and 800 A were recorded on film and measured photometrically for the cases of the absorbing plasma either on or off. By taking into account information on the concentration of ions in such discharges, and by considering the relative probabilities of formation of other plasma constituents from their appearance potentials by electron impact methods, it was possible to estimate the concentration of atomic nitrogen in the plasma and to show that aside from neutral N2 atomic nitrogen was the sole cause of the attenuation of light. The experimental absorption cross sections were found to agree well with the theoretical values of Bates and Seaton.In order to support some of the assumptions used, separate experiments were performed in which the light source pulse was passed through the plasma region after the P.I.G. discharge had been extinguished. It could be shown that the ions had been swept out and that excited and metastable states had also disappeared. Thus, the only remaining plasma component was atomic nitrogen, and its absorption cross sections were within the error limits the same as those of the first investigation.

A Constant Intensity Vacuum Spectroscopic Light Source

A vacuum spectroscopic light source was developed which furnished a constant intensity emission line spectrum. Reproducibility was achieved by controlling with an oscilloscope the spark breakdown potential and the number of breakdowns per cycle of the applied high voltage. The gas composition in the source was found to be critically important. The constancy was checked photometrically and found to be about 2 percent.

Optical Properties of Sb, Te, and Ti Films in the Vacuum Ultraviolet*

The reflectivity and transmissivity of Sb, Te, and Ti films have been measured in the range from 1600 to 450 A in order to compare the frequency at which the films change from a reflecting to a transmitting medium with the plasma frequency predicted by Bohm and Pines and also with electron energy characteristic losses in metals observed by Marton et al. and by Powell.

Properties of a Plane Grating Predisperser Used with a Grazing Incidence Vacuum Spectrograph

The performance of a plane, unblazed grating (600 lines/mm) as a predisperser has been investigated. Placed 7 cm in front of the primary slit of a 2-m grazing incidence vacuum spectrograph, the foregrating allowed passage of uv (100 to 500 A) into the spectrograph in 50 to 150 A bands. The spurious background observed when radiation passed directly through the primary slit was found to be absent from the predispersed spectra, which have been observed by both photographic and photoelectric methods. These spectra, free from scattered background and second-order images, exhibited measurable line displacement, up to 0.3 mm from the line image positions of the direct spectra. Such displacement, occuring naturally with the plane foregrating and shown to be in agreement with theoretical predictions, are sometimes found as a result of source misalignment with grazing incidence instruments. Use was made of the foregrating for isolation of true continuum radiation for the Vodar vacuum sliding spark and linear pinch sources.

The Characteristics of Ultraviolet-Sensitized Photographic Plates in the Vacuum Ultraviolet*

The D vs log t curves of Eastman 103a–O uv-sensitized plates have been studied in the near ultraviolet and vacuum ultraviolet, respectively, together with the absorption and fluorescence spectrum of the sensitization lacquer. It was found that the contrast γt, obtained from the D vs log t curves calibrated in the vacuum spectrograph, was a constant within an experimental deviation of 4 percent throughout all observed wavelengths from 1320A to 360A and was equal to the contrast of the emulsion at the wavelength of maximum intensity of fluorescence of this lacquer. Numerous methods for the calibration of plates in air have been suggested and one of them was tested in detail. The reciprocity law of these uv-sensitized plates was studied and suggestions are made on the procedures leading to correct calibration curves of commercial plates for the purpose of photographic photometry in the vacuum ultraviolet.

Optical Constants of Evaporated Barium in the Vacuum Ultraviolet

The optical properties of evaporated barium films have been investigated in the wavelength range from 1500 to 3000 A. Reflectance measurements were made at angles of incidence of 17.5° and 72.5° and the complex index of refraction obtained from graphical solutions of the Fresnel reflection equations. The films were prepared in an ultrahigh-vacuum reflectometer having a base pressure of about 5 × 10−10 torr. Radiation from a hydrogen glow-discharge source was dispersed by a normal-incidence vacuum monochromator which was optically connected to the ultrahigh-vacuum system by means of a sapphire window. The energy-loss function Im(1/∊), as computed from the optical constants, is compared with published characteristic electron-loss (CEL) data. The complex index of refraction was fitted to a Drude model for which the plasma energy was fixed at 7.4 eV and the damping at 1.6 × 10−16 sec.

Electrodeless Discharges at High Frequencies and Low Pressures

In the last ten years electrodeless discharges have become increasingly important to the spectroscopist as light sources that possess distinct advantages over d.c. discharge tubes. An attempt was made to gather information on the characteristics of such discharges at frequencies of 10, 15, 20, and 25 megacycles and pressures ranging from 10−2 to 10−4 mm of Hg using dry air as the filling gas. It is hoped that the data presented here in form of onset and offset potentials versus pressure will help the spectroscopist in the design of his light sources and of the associated electrical equipment, thus taking this aspect of his research at least partly out of the realm of trial and error.—With this as our primary objective the experimental conditions were not appropriate for yielding data that contribute to the understanding of the fundamental mechanisms of such discharges. Therefore a detailed discussion of the shapes of the characteristic curves Vsversusp is omitted.