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The visible-infrared self-amplified spontaneous emission amplifier (VISA) free electron laser (FEL) is an experimental device designed to show self-amplified spontaneous emission (SASE) to saturation in the near infrared to visible light energy range. It generates a resonant wavelength output from 800–600 nm, so that silicon detectors may be used to characterize the optical properties of the FEL radiation. VISA is designed to show how SASE FEL theory corresponds with experiment in this wavelength range, using an electron beam with emittance close to that planned for the future Linear Coherent Light Source at SLAC. VISA comprises a 4 m pure permanent magnet undulator with four 99 cm segments, each of 55 periods, 18 mm long. The undulator has distributed focusing built into it, to reduce the average beta function of the 70–85 MeV electron beam to about 30 cm. There are four FODO cells per segment. The permanent magnet focusing lattice consists of blocks mounted on either side of the electron beam, in the undulator gap. The most important undulator error parameter for a free electron laser is the trajectory walk-off, or lack of overlap of the photon and electron beams. Using pulsed wire magnet measurements and magnet shimming, we were able to control trajectory walk-off to less than ±50 μm per field gain length.

One of the principal advantages of electron momentum spectroscopy (EMS) is that peaks in the binding-energy spectrum can be assigned with greater certainty than in photoelectron spectroscopy, through a comparison of the EMS triple-differential cross sections with the theoretically calculated spherically averaged momentum distributions (MD’s) of Dyson orbitals. While the target Hartree-Fock approximation is commonly used to calculate the Dyson orbital MD’s for this purpose, a computationally less demanding method would allow the advantages of EMS to be extended to larger molecules. This paper considers the use of Kohn-Sham density-functional theory for this purpose. Although Dyson orbitals are not among the quantities that can be calculated exactly (in the limit of the exact exchange-correlation functional) within the framework of Kohn-Sham density-functional theory, the Kohn-Sham equation can be regarded as an approximate form of Dyson’s quasiparticle equation, with a local self-energy. The well known shortcomings of this approach for estimating ionization potentials and band gaps do not a priori imply a corresponding problem with the orbitals. After discussing these formal considerations, we introduce the ‘‘target Kohn-Sham approximation’’ as a means of approximating Dyson orbitals by Kohn-Sham orbitals. The quality of this approximation for the calculation of MD’s is assessed by comparison with high-quality configuration-interaction calculations, the target Hartree-Fock approximation, and experiment, for several small molecules. The quality of the target Kohn-Sham approximation MD’s is found to be comparable to that of the MD’s from the target Hartree-Fock approximation, with evident practical implications for EMS.

A Monte Carlo technique for modeling spectra with many resolved lines of ions in intermediate coupling was presented in the preceding paper [Phys. Rev. A 44, 5707 (1991)]. We use the same concept here with a modification for the near-LS-coupling case. Rosseland and Planck means of transition arrays of Fe v and Fe vi are evaluated. Comparison with ‘‘exact’’ computations shows much better agreement than is found using the simple unresolved-transition-array approximation.

A method is proposed for simulating resolved transition arrays of ionized atom spectra in full intermediate coupling. The wave number and intensity of each line in an array are picked at random from separated but correlated distributions. Even though each line is not exactly reproduced, this procedure yields the correct following characteristics of the supposedly symmetric array: total intensity; second and fourth moments of the distributions of unweighted wave numbers, of intensity-weighted wave numbers, and of transition amplitudes; numbers of lines and sums of intensities in consecutive narrow energy ranges. All the parameters of the distribution are obtained by means of compact formulas, or tabulated. Applications to the arrays 4d⁴-4d³5p of Pd⁶⁺ and 4d⁷5s-4d⁷5p of Cd⁴⁺ are presented. Comparison with the explicit results of the Slater-Condon method shows good agreement. It is proposed to use this method for fast and reliable computation of Rosseland means and other opacity properties.