John R. Holmes

Isotope Shift In Neutral Oxygen

An O18-enriched oxygen sample (donated by A. O. Nier) has allowed examination of isotope structure in 20 atomic lines between 2000 and 11 000A. Strong sharp lines were obtained in a liquid nitrogen-cooled electrodeless quartz discharge tube containing up to 0.5 mm of oxygen in 5 mm of helium excited by a 10 mc oscillator. The discharge was photographed through an external Fabry-Perot interferometer in series with a large prism spectrograph. The shifts found ranged from zero to as high as 0.5 cm−1 for λ4233 (4p3P–3d′3P) and λ2884 (3p3P–3d′3P). Enough data was available to allow a study of the internal consistency of the measured shifts as a test of the mass-effect theory. With the measured shifts in two lines, two theoretical parameters for the specific effect were empirically determined and used to predict shifts in several other lines. It is concluded that the mass-effect theory is generally applicable to oxygen and that the abnormally large shifts found for λ4233 and λ2884 are the result of interactions affecting the common upper level. A comparison is made with earlier nitrogen data. Use is made of the formal relationship of specific effect parameters to certain radiation theory integrals in calculating a transition probability and an f value for the λ1306 resonance triplet. The reasonable values obtained are a further indication of the validity of the theory.

Isotope Shift in the First Spectrum of Silicon (Si i)

Isotope shifts of 30Si relative to 28Si have been observed in eleven ultraviolet lines of the neutral silicon spectrum. The spectra of two highly enriched samples of elemental silicon were excited separately in two hollow-cathode discharge tubes cooled in liquid nitrogen. An externally mounted Fabry-Perot interferometer, used in connection with a medium Hilger, quartz-prism spectrograph, provided the high resolution necessary for these observations. The magnitudes of the observed shifts vary from Δν~0.011±0.001 cm−1 for the (3p2 3P–4s3P0) transition to Δν~0.115±0.002 cm−1 for the (3p2 3P–3a3D0) transition.

Certain Forbidden Spectral Lines of Cadmium

An experimental investigation was made to establish conclusively the source and nature of the highly forbidden lines 3320A×(5 1S0–5 3P00), 3141A(5 1S0–5 3P20), and 3403.1A(5 3P10–5 3D3) in cadmium. All such transitions are strictly forbidden for electric dipole radiation and by most of the selection rules for higher pole and magnetic dipole radiation. Bowen (see reference 1) and others, have suggested that the lines are emitted as the result of the interaction of the optical electron with the magnetic moment of the nucleus and are, therefore, produced by only the odd weighted isotopes.The intensities of the above mentioned lines as emitted by a cadmium sample which had been enriched in one of the odd isotopes were compared with the intensities of the same lines emitted by a sample of natural cadmium. Within the experimental error it was found that the forbidden lines are emitted with an intensity proportional to the odd isotope abundance and, therefore, it seems conclusively established that they arise only from the odd isotopes. Comparison of the intensities of some of the forbidden lines with the intensity of an allowed transition from the same multiplet was made in order to give a measure of the interaction of the optical electron with the nuclear magnetic moment.

Isotope Shift in the Spectrum of Mercury

By using separated isotopes of mercury 198 and mercury 202, in sealed discharge tubes, the isotope shift in several lines from 16 000 A to 4000 A has been measured with direct recording techniques. A grating in crossed with a pressure tuned Fabry-Perot interferometer was coupled to electronic detectors to make a linear recording system. The detectors were a photomultiplier tube in the visible and a lead sulfide cell in the infrared. The output of the detector is amplified and then recorded on a strip chart recorder. By optical switching between the two discharge tubes during the recording both isotopes appear on the same record without overlapping of the intensity pattern. Because of the inherently high accuracy of this type of system the probable errors are small.

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.