Distribution Of Line Strengths Research Articles

Electron Shake-Up in Two-Photon Excitation of Core Electrons to Rydberg Autoionizing States

The results of a theoretical and experimental investigation of satellite line strengths in two-photon transitions to Rydberg autoionizing states in multielectron atoms are presented. In studies of $6snd\ensuremath{-}7s{n}^{\ensuremath{'}}d$ two-photon transitions in Ba I, the distribution of line strengths over various final $7s{n}^{\ensuremath{'}}d$ states for a given $6snd$ initial state is found to vary dramatically as a function of detuning from intermediate $6p{n}^{\ensuremath{'}\ensuremath{'}}d$ autoionizing resonances. These results indicate the influence of electron rearrangement during the multiphoton transition.

Real line strength distributions for random band models

An improved random band-model method, which takes into account the real line-strength distribution, is presented. This model is useful for low resolution, infrared observational data of the outer solar system.

Winds in hot stars

The line transfer equation in the fluid frame is solved to evaluate radiation pressure force, and a hydrodynamic structure consistent with this force is constructed for stellar wind models driven by radiation pressure in spectral lines. The models use either a number of driving lines of equal length or several lines with different opacities, which are weighed to simulate a distribution of line strengths. An acceleration near the base of the wind that is more gradual than that of the Castor, Abbott, Klein (1975) model was found even well above the sonic point, though mass-loss rates and wind structure at large radii are in substantial agreement with earlier results.

Curve-of-growth function for a random array of voigt lines

The accuracy of two approximations to the curve-of-growth function V( a, x) for a random array of Voigt lines with exponential line strength distribution is examined. A heuristic extension of the Rodgers and Williams approximation for isolated Voigt lines is accurate to better than 9% for all x and a. A modified version of the approximation used in the NASA plume radiation code is accurate to better than 4% for all x and a.

Nonisothermal band model theory

Theoretical formulations of radiation band models for general nonuniform optical paths are presented. The models are framed within the statistical band model for an array of Lorentz lines and an inverse line strength distribution. Radiative transfer for an isolated line in a general nonuniform medium and the statistical model for a uniform optical path are reviewed in order to provide the foundation required for the band model formulations. Two approaches to the development of these models are taken. The first is based on making approximations to such radiative transfer functions as transmittance or equivalent width and yields models equivalent or similar to the traditional Curtis-Godson approximation. The second treats approximations to the spatial derivatives of these functions. From the standpoint of computing line or band radiance, the spatial derivatives are more fundamental quantities than the transfer functions themselves; consequently, these latter “derivative approximations” are instrinsically more accurate than the Curtis-Godson type approximations. All of the models are formulated to give the spatial derivative of the mean equivalent width function W ̄ (s) δ in the form 1 δ d W ̄ (s) ds =c(s)p(s) k ̄ (s)y(s) , where c( s), p( s) and k̄( s) are the concentration, total pressure and absorption band model parameter, respectively, at the local path position s. The functional form of the derivative function y( s) is derived for each of the models. In general, y( s) is a function of both local and path-averaged (subscript e) band model parameters. These local and averaged parameters are used to define a dimensionless optical depth parameter x e = u k ̄ e β e and three nonuniformity indexes ρ= γ ̄ γ ̄ e , r= β β e and θ = δ ̄ e δ ̄ (γ̄ is line width, ḡd is the mean line spacing band model parameter, and β = 2μ γ ̄ γ ̄ ). The explicit manner in which these parameters enter into the various functional forms for y( s) is derived and discussed. The introduction of the parameter q is an important aspect of the new models and is an explicit measure of nonisothermality along the optical path. In addition to the traditional Curtis-Godson definitions for the effective parameters k̄ e and β e , new definitions for the path averages γ̄ e and δ̄ e are derived. These new definitions are obtained from the fundamental properties of the assumed inverse line strength distribution. A summary of all the models is given as a table of relevant equations for use in practical calculations.

Rotation and Chemical Abundances in the Peculiar A Stars--II

New measurements of the distribution of silicon line strengths amongst early type stars having similar atmospheric structures do not allow a clear-cut separation into normal and anomalous types. It is shown that those stars having line strengths much larger than the mean tend to have rotational velocities less than 200 km s –1 . It is likely that in many, if not most, strong-line stars no single temperature and pressure parameters will suffice to describe the atmosphere, and the assumption of a mean temperature can lead to large errors in the derived abundances of all but a few elements. The production of stars described by the oblique rotator model is discussed briefly.

An experimental determination of the oscillator strengths for some transitions in the Lyman bands of molecular hydrogen

The ultraviolet absorption cross-section for molecular hydrogen at room temperature has been measured absolutely at a resolution of 0·3 Å, between 1060 and 1130 Å. This range of wavelengths covers the (0-0) through (3-0) vibrational bands of the B 1 Σ + u ↔ X 1 Σ + g Lyman system. The nature of the curve of growth of an individual line has been investigated in detail, and shown to agree closely with the expected behaviour of a Doppler-broadened, Lorentz line for hydrogen at room temperature. A curve-of-growth analysis has been used to determine absolute line strengths for all observed transitions within each band. For each band, the distribution of line strengths is in accordance with the expected weighted Boltzmann distribution of initial states at room temperature, with most of the observed absorption concentrated in the R 0, R 1 and P 1 transitions. Oscillator strengths for these bands have been derived from the integrated band intensities, and confirm, within the limits of the experimental errors, the theoretical Franck-Condon factors.

Transmission Function for Bands with Doppler-Lorentz Line Shapes

The mean transmission is calculated for spectral regions which have a random frequency distribution of absorption resonances with Doppler-Lorentz profiles. An effective constant strength band is defined which closely approximates the self-shielding of an arbitrary distribution of line strengths. By appropriately defining the half-width of the lines, the values of the shielding or self-absorption factor when plotted against the ratio of line strength to line width are identical for all line shapes in the region of small optical depth.The transmission calculations may be used in the interpretation of experiments to measure oscillator strengths and in the determination of the emissivity of high-temperature gases.