CLOUDY is a large‐scale spectral synthesis code designed to simulate fully physical conditions within an astronomical plasma and then predict the emitted spectrum. Here we describe version 90 (C90) of the code, paying particular attention to changes in the atomic database and numerical methods that have affected predictions since the last publicly available version, C84. The computational methods and uncertainties are outlined together with the direction future development will take. The code is freely available and is widely used in the analysis and interpretation of emission‐line spectra. Web access to the Fortran source for CLOUDY, its documentation Hazy, and an independent electronic form of the atomic database is also described.

https://iopscience.iop.org/article/10.1086/316190/pdf

CLOUDY 90: Numerical Simulation of Plasmas and Their Spectra

Since the appearance of the title paper, a number of new developments have occurred which need to be included in that body of material. We present additional remarks and clarifications which supplement and update numerous aspects of the Bethe theory discussed in the earlier paper. We also bring the bibliography up to date. Plasma stopping power, the ${z}^{3}$ effect, and stopping power for particles at extreme relativistic energies are among the new topics included. We make several comments on Fano's earlier review article, Ann. Rev. Nucl. Sci. 13, 1 (1963).

Inelastic Collisions of Fast Charged Particles with Atoms and Molecules-The Bethe Theory Revisited

Inelastic collisions of fast charged particles with atoms and molecules the Bethe theory revisited

A model for the photodissociation of ICN in its lowest continuum is developed and is used to predict the partitioning of available energy between translational, rotational, and vibrational energies of the recoiling fragments. The model is based on three major assumptions: (i) that only one upper electronic state is involved, (ii) that light absorption affects only the breaking C–I bond, thus allowing the upper‐state potential surface to be calculated quasidiatomically from spectral and thermodynamic data, and (iii) that the mechanics of the “half‐collision” of the recoiling fragments on this potential surface may be treated as classical to predict the average partitioning between translational, rotational, and vibrational energies, and by a classical‐energy forced quantum oscillator approximation to predict the partitioning between vibrational states. The model predicts that most of the available energy will go into translation, which is consistent with flash photolysis studies and with crude measurements of the photofragment spectrum.

Molecular Dynamics of Photodissociation: Quasidiatomic Model for ICN

Publisher Summary This chapter discusses collision experiments using laser excited atoms in crossed beams. With the advent of tunable narrow-band lasers, especially cw dye lasers, the situation has changed and it has become clear that it should be possible to excite atoms optically within the scattering region of an otherwise conventional crossed-beam experiment. In this way a steady-state upper-state population could be reached that may be comparable to the ground-state population. When an atomic beam is excited, it is free of internal collisions and, for right angle intersection with the laser, free of Doppler broadening. Then the laser properties allow selecting the state into which the atom is excited. Specific fine- and hyperfine-structure states may be chosen as well as a particular combination of sub-states. The novel techniques allow preparing states with an angular momentum different from zero and to vary systematically the alignment and orientation of the resulting non-spherical interaction potentials. Frequency doubling of dye lasers can also widen the scale of possible applications. Very high powers are needed and one probably would have to use a pulsed laser. An alternative to frequency doubling is the direct two-photon excitation of atoms.

Collision Experiments with Laser Excited Atoms in Crossed Beams

Publisher Summary This chapter describes the classical theory of atomic scattering. When classical methods are applied to such collisions, they complement close coupling and variational methods, which are more appropriate when the number of quantum states involved in a collision is relatively small. A classical theory of atomic scattering requires the choice and interpretation of a classical model, and the solution of the classical model collision problem. No distinction is made between different states of excitation in defining the channel, as the classical energy has a continuous range of values. In practice, the initial states of atomic systems cannot be controlled sufficiently to study individual collisions precisely. Observed physical phenomena depend on statistical properties of ensembles of collisions, that is to say, on very large numbers of collisions, with variable initial states whose probability distribution is determined by the macroscopic conditions. For direct excitation of low-lying states, the method is usually less satisfactory because the energy transfer is small, and there is a large amount of ambiguity in choosing the final state energy band. It is also difficult to see how to split up this classical energy band, so as to represent the various possible quantized angular momentum final states separately.

A. Burgess

Papers

Classical Theory of Atomic Scattering