A collision-type formalism for spectral theory, described by Lu and applied originally to Xe, is improved and is applied to analyze the strongly perturbed absorption spectrum of Ar. The analysis expresses experimental data in terms of three sets of empirical parameters: five eigenquantum defects p, , one 5 X 5 orthogonal transformation matrix tt,-, and five dipole matrix elements D . The energy dependence of the parameters is studied and is found to be approximately linear for the p. and insignificant for 11; and D . With these parameters thus determined by fitting to the experimental data, the mixing coeKcients of the Rydberg levels have been determined and their oscillator strength have been predicted and compared pvith available experimental data.

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A differentially pumped rare gas cell has been developed to suppress undulator harmonics on the Chemical Dynamics Beamline at the Advanced Light Source. Greater than 104 suppression of the harmonics has been demonstrated with no measurable (<5%) attenuation of the fundamental. The overall design is presented, and vacuum and optical performance are reported.

Measurements of the relative photoionization cross sections of Rb in the presence of various strengths of external electric fields are reported. Systematic, field-dependent, resonance structure is observed not only for energies above the classical field-ionization limit, but above the zero-field ionization limit as well. A striking dependence of the cross section upon light polarization is also observed.We report the results of an investigation of the wavelength dependence of the single-photon photoionization cross section of ground-state Rb in the presence of externally applied electric fields, For each field value, a systematic resonance structure versus wavelength was observed, not only for energies above the classical field-ionization threshold (which has been previously reported), but above the zero-field ionization limit as well. The observation of electric-field-dependent resonances in the photoionization cross section above the zero-field limit in a one-electron-like atom is surprising and to our knowledge has not been previously reported, In this Letter we present some relevant results of our experiment, discuss the physical mechanism responsible, develop a model whose predictions are in good agreement with our data, and attempt to place the results in proper perspective relative to recent measurements of field ionization and Stark effects in alkali atoms 0 For detailed study of the cross section, we employed a collison-free, low-density (%> 10 9 /cm 3 ) atomic beanu The Rb atomic beam was irradi-1463