The self-consistent-field function for atoms with 2 to 36 electrons are computed with a minimal basis set of Slater-type orbitals. The orbital exponent of the atomic orbitals are optimized as to ensure the energy minimum. The analysis of the optimized orbital exponents allows us to obtain simple and accurate rules for the 1s, 2s, 3s, 4s, 2p, 3p, 4p, and 3d electronic screening constants. These rules are compared with those proposed by Slater and reveal the need of accounting for the screening due to the outside electrons. The analysis of the screening constants (and orbital exponents) is extended to the excited states of the groundstate configuration and to the positive ions.

Minimal basis-set atomic functions for the ground-state atoms from Rb(Z=37) to Rn(Z=86) are presented. These functions are analyzed in order to obtain systematic data for the screening constants and atomic radii following the work initiated by Slater.

Results of Hartree-Fock calculations on the ions Na~, Al~, Si", P", and CI" in states of the lowest electronic configuration, along with correlation and relativistic corrections, are given. The data are used to compute electron affinities for the atoms of the second row of the periodic table, and the stability of the excited states of the ions relative to the neutral ground-state atoms. The computed electron affinities for atoms on which experimental determinations are not available are 0.78 eV for Na, 0.52 eV for Al, 1.39 eV for Si, and 0.78 eV for P. The computed electron affinities for S and CI are 2.12 and 3.56 eV in fair agreement with the experimental values of 2.07±0.07 and 3.613dz0.003 eV, respectively. For the second-row negative ions, several excited states are lower in energy than the corresponding ground-state neutral atoms; the computed stabilities for the species Al-QD), Si"( 2 D) and Si"( 2 P) are 0.23, 0.58, and 0.08 eV, respectively. While the l D state of P" is estimated to be very close to that of P( 4 S), our estimates are not accurate enough to say definitely whether it is above or below.

The automation of a quadrupole mass spectrometer -residual gas analyzer (RGA) on a time-shared IBM 1800 Data Acquisition and Control Computer is described. The RGA. which is used to determine the partial pressures of various gases in a vacuum system. may be operated up to a maximum data rate of about 20.000 points/sec (pps) in an interleaved manner with multiple slow-scanning (:::20 pps) instruments. We review in detail the hardware and software considerations regarding the design and subsequent interfacing of the instrument to the computer. Methods for initiating data collection. entering experimental parameters. and analyzing the experimental data. such as spectrum plotting. smoothing. peak location. mass identification. and calculation of the partial pressures by using a least-squares approximation to fit mass peaks. are discussed with examples.

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