Intensity Decay Curves Research Articles

Systematic problems with the beam-foil technique

Three problems which can seriously affect the accuracy of and confidence placed in beam-foil data are discussed: (1) mean lives obtained from exponential decays are a surprisingly strong function of the statistics, number of data points, effect of cascades and decay length observed; (2) mass contaminations occur in “isotopically pure” beams — oxygen being the major source of impurity in most cases; (3) the slope of the intensity decay curve from which mean lives are extracted appears to be 3 function of the beam current.

Radiative mean lives of some electronic states in beam-foil excited atomic and ionic carbon

A monoenergetic (≈ 350 keV) beam of magnetically analyzed C + ions from the University of New Hampshire 400 kV Van de Graaff electrostatic accelerator was passed through a 10 μg/cm 2 carbon foil in order to produce a beam-foil spectroscopic source of atomic and ionic carbon in various stages of ionization and various states of electronic excitation. The radiative mean lives of the upper excited terms of the strongest emission-multiplets were measured by photometrically monitoring the intensity of the multiples as a function of distance from the point of initial excitation; i.e., the foil. A 1 2 - m (Model 235) McPherson Vacuum Monochromator was used for this purpose. The photon detector was an EMI 6256S photomultiplier (with a sodium salicylate wavelength conversion window for use in the UV spectral region). Some of the semi-logarithmic intensity decay curves were non-linear due to cascading and/or multiplet blending but many appear free of these effects. Where a transition is singular or dominant, it has been possible to extract multiplet transition probabilities directly. The results are compared with other recent beam-foil measurements and with theoretical values.

Studies of the Nitrogen Afterglow. I. Surface Catalytic Efficiency and Diffusion-Controlled Decay of Atomic Nitrogen

The temporal behavior of the afterglow light intensity in decaying nitrogen plasmas, as well as that of the number density of N+, N2+, N3+, and N4+ ions, was studied in the pressure range of about 0.1 to 1.7 torr. The analysis of the light intensity decay curves resulted in a method of measuring the surface catalytic efficiency γN for the recombination of nitrogen atoms at the plasma boundary. The simultaneous identification of the ions present in the plasma by means of a quadrupole mass spectrometer ensures that the data relate to pure nitrogen as well as to extremely clean surfaces. The values obtained for γN of molybdenum sputtered on a glass surface decreased with pressure from a value of 0.22 at 0.09 torr to 0.04 at 1.35 torr. The studies also showed that part of the afterglow light emission is due to collisions between metastable nitrogen molecules which are produced in the afterglow by the atomic volume recombination process.