Absolute Photon Yield Research Articles

Absolute photon yields in the sputter-induced optical emission process

The absolute photon yield expressed as the number of photons emitted at a particular wavelength per sputtered atom has been measured for 34 elements in NBS standard glass targets bombarded by 20-keV Ar+ ions. The yield values lie in the range 10−2–10−6. The detection limits for these elements in parts per million by weight have also been estimated for given bombardment conditions.

Visible chemiluminescence from the reaction of metastable Sr with N20: Absolute cross section and photon yield

Visible chemiluminescence from the reaction of metastable Sr with N20: Absolute cross section and photon yield

Determination of the absolute chemiluminescence cross section and photon yield for the Ca(4s4p 3P) + N 2O reaction

A novel method for determining absolute chemiluminescence cross sections without knowledge of the absolute detection sensitivity is described for reactions involving electronically excited reactants. A study of the Ca(4s4p 3P) + N 2O reaction under single-collision conditions is presented, and photon yield of 10 ± 3% is determined with the help of a measured beam attenuation cross section.

Determination of absolute photon yields under single-collision conditions

The experimental procedure is presented to measure the absolute photon yield, the percentage probability of emitting a visible photon per reactant molecule consumed, for chemiluminescent reactions under single-collision conditions. Using a well-defined metal beam directed into a scattering gas at submillitorr pressures, this procedure is applied to the reactions Sm+N2O, Sm+F2, Ba+N2O, and Ba+NO2 to obtain the photon yields in the 350–800 nm range of 0.39%, 11.8%, 2.4%, and 0.18%, respectively, where the estimated uncertainty is about 50%. The absolute photon yields for each of these reactions initially increases with scattering gas pressure, demonstrating that secondary collisions ’’feed’’ radiating states from dark, reservoir states. It is suggested that other relative photon yields can be put on an absolute basis by comparison with the Sm+N2O chemiluminescent reaction.

Crossed-beam chemiluminescence: The alkaline earth rearrangement reaction M + S 2Cl 2 → S 2* + MCl 2

The reaction of the heavier Group IIA metal atoms Ba, Sr, and Ca with S 2 Cl 2 under crossed-beam conditions produces a chemiluminescent spectrum in which the S 2 B 3ϵ u −-X 3ϵ g − emission bands are the most prominent features. In addition, chemiluminescence is also observed from excited states of the alkaline earth monochlorides and from a “pseudo-continuum” which is believed to originate from an excited polyatomic metal halide, probably the alkaline earth dichloride. More than 100 bands are assigned to the S 2 B-X band system; the vibrational levels of the S 2 B state are found to be populated by the reaction in a non-thermal manner. The reaction forming S 2 * is determined to be the first order with respect to both the ground state metal atom and the disulfur dichloride molecule. Phenomenological cross sections for removal of metal atoms from the metal beam by S 2 Cl 2 are 162 ± 20 A 2 for Ba, 105 ± 20 A 2 for Sr, and 92 ± 20 A 2 for Ca, which is consistent with an electron jump model if the vertical electron affinity of S 2C l 2 is 3.25 eV. Absolute photon yield measurement in- dicate that less than 1% of the reacton products are S 2 * molecules that emit in the 2800–6000 A region. From the single-collision chemiluminescence spectra the following dissociation energies are estimated: D o o( Cl-S 2 Cl)=44.2 ± 5 kcal/mole and D o o(S 2- Cl=54 ± 5 kcal/mole. A speculation is offered that most of the reaction exoergicity appears in the S 2 * product when the S 2 Cl 2M collision complex passes through a nearly-planar, five-member ring structure in separating to form products.

Multiple Element Soft X-Ray Source

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Absolute Calibration and Use of a Soft X-Ray Source of Characteristic Carbon Radiation

A measurement of the absolute photon yield of characteristic carbon K radiation produced by direct electron bombardment of a target is described. The carbon K emission is produced by a simple and economical source. The radiation is detected with a commercially available proportional counter, using both commercially available windows and thinner windows made at this laboratory. A method for determining gas absorption efficiency of the counter and transmission of the counter window is described. The pulse height distributions obtained from the proportional counter vary in shape with target voltage indicating the presence of a bremsstrahlung continuum as well as characteristic K radiation. The continuum contribution is subtracted from the total counting rate to obtain the photon yield of characteristic K radiation as a function of target voltage. The results of this determination are used in the calibration of a grazing incidence grating spectrometer at 44 Å.

The angular and spectral distribution of transition radiation from thin silver foils

The angular and spectral distributions of photons emitted by Ag foils 660 A and 1 980 A in thickness when bombarded by 40 keV electrons have been determined experimentally in the wavelength region from 2 500 A to 5 600 A. The spectral distribution of light polarized in the plane of incidence showed a sharp peak at 3 300 A ± 12 A, this value being an average over photon directions from 10° to 50° from the foil normal. In addition to the peak, the spectrum showed a continuum which decreased slowly with increasing wavelength and a deep minimum at 3 200 A with a rise in the shorter wavelengths. The angular distribution of photons emitted at the peak wavelength showed a maximum at 30° from the foil normal with zero intensity at 0° and near 90°, whereas the photons emitted at other wavelengths in the continuum, e.g., 2 700 A and 4 500 A, were most intense at 50° from the foil normal. The absolute photon yield was determined by calibrating the spectrometer, analyzer, and photomultiplier with a tungsten filament lamp obtained from the U. S. National Bureau of Standards. For photon directions from 10° to 40° the photon yield was found to agree with the theoretical predictions in all respects at all wavelengths except at that of the sharp peak where the experimental values were about 30 % lower.

Transition Radiation and Optical Bremsstrahlung from Electron-Bombarded Thin Gold Foils

Unbacked gold foils 340- and 530-\AA{} thick were bombarded by a 1.5-\ensuremath{\mu}A electron beam of energy 25 to 100 keV. The light emitted at 30\ifmmode^\circ\else\textdegree\fi{} from the foil normal on the beam exit side was analyzed between 2500 and 5500 \AA{} with a vacuum ultraviolet spectrometer and a glan prism polarizer. The spectral distribution and absolute photon yield in the plane containing the electron beam and photon direction agree well with the transition radiation theory if the bremsstrahlung contribution is assumed to be unpolarized. The photon intensity perpendicular to the plane (of incidence) has the dependence on primary energy indicated by the bremsstrahlung theory of Gluckstern, Hull, and Breit, but only 40% of the expected absolute value. The discrepancy is thought to be due to neglect of optical absorption and refraction in the calculation.