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Using the high resolution vacuum ultraviolet (vuv) photon source provided by the monochromatized undulator synchrotron radiation of the Chemical Dynamics Beamline at the Advanced Light Source, we have measured the photoionization efficiency (PIE) spectrum for Ne in the energy range of 21.56–21.67 eV at a wavelength resolution of 0.3 meV [full width at half‐maximum (FWHM)]. The PIE spectra for Ne obtained using 0.76 and 2.4 V/cm electric fields reveal autoionizing features attributable to the Rydberg states Ne[2p5ns′(1/2)1; n=14–29] and Ne[2p5nd′(3/2)1; n=12–35] converging to the spin–orbit excited Ne+(2P1/2) state. The positions of these Rydberg states are compared to previous experimental results and those calculated using the quantum defects and IE for Ne+(2P1/2) given in Moore [Natl. Stand Ref. Data Ser. Natl. Bur. Stand. 35 (1971)]. We have also observed mass analyzed threshold ions (MATI) for Ne formed in the Ne+(2P3/2,1/2) states. For Ar, only the MATI peak for Ar+(2P3/2) is observed. The failure to...

We investigated the dissociation dynamics of positive-ion and negative-ion fragments of gaseous and condensed HCCl(3) following photoexcitation of Cl 2p electrons to various resonances. Based on ab initio calculations at levels HF/cc-pVTZ and QCISD/6-311G∗, the first doublet structures in Cl L-edge x-ray absorption spectrum of HCCl(3) are assigned to transitions from the Cl (2P(3/2),(1/2)) initial states to the 10a(1)(∗) orbitals. The Cl 2p → 10a(1)(∗) excitation of HCCl(3) induces a significant enhancement of the Cl(+) desorption yield in the condensed phase and a small increase in the HCCl(+) yield in the gaseous phase. Based on the resonant photoemission of condensed HCCl(3), excitations of Cl 2p electrons to valence orbitals decay predominantly via spectator Auger transitions. The kinetic energy distributions of Cl(+) ion via the Cl 2p → 10a(1)(∗) excitation are shifted to higher energy ∼0.2 eV and ∼0.1 eV relative to those via the Cl 2p → 10e(∗) excitation and Cl 2p → shape resonance excitation, respectively. The enhancement of the yields of ionic fragments at specific core-excited resonance states is assisted by a strongly repulsive surface that is directly related to the spectator electrons localized in the antibonding orbitals. The Cl(-) anion is significantly reinforced in the vicinity of Cl 2p ionization threshold of gaseous HCCl(3), mediated by photoelectron recapture through post-collision interaction.

With x-ray spectroscopy and first-principles calculations, we expose the electronic structure, near the Fermi level, of Co-doped PbPdO2 composed of O 2p-Pd 4d hybridized states with an additional contribution of a spin-polarized Co 3d state at either a greater or smaller energy. The spin-polarized Co 3d states interacting with O 2p-Pd 4d hybridized states cause spin splitting at the band edge. Fascinating physical properties such as high-temperature ferromagnetism thus arise in Co-doped PbPdO2. Results will help in the design of materials with desired electronic structures and the control of spin polarization with chemical doping.

We present experimental evidence of the gapless band structure of PbPdO2 by combined x-ray photoemission and x-ray absorption spectra complemented with first principles band structure calculations. The electronic structure near the Fermi level of PbPdO2 is mainly composed of O 2p and Pd 4d bands, constructing the conduction path along the Pd-O layer in PbPdO2. Pd deficiency in PbPdO2 causes decreased O 2p-Pd 4d and increased O 2p-Pb 6p hybridizations, thereby inducing a small band gap and hence reducing conductivity. Hall measurements indicate that PbPdO2 is a p-type gapless semiconductor with intrinsic hole carriers transporting in the Pd-O layers.

The state-selective dissociation dynamics for anionic and excited neutral fragments of gaseous SiCl4 following Cl 2p and Si 2p core-level excitations were characterized by combining measurements of the photon-induced anionic dissociation, x-ray absorption and UV/visible dispersed fluorescence. The transitions of core electrons to high Rydberg states/doubly excited states in the vicinity of both Si 2p and Cl 2p ionization thresholds of gaseous SiCl4 lead to a remarkably enhanced production of anionic, Si− and Cl−, fragments and excited neutral atomic, Si*, fragments. This enhancement via core-level excitation near the ionization threshold of gaseous SiCl4 is explained in terms of the contributions from the Auger decay of doubly excited states, shake-modified resonant Auger decay, or/and post-collision interaction. These complementary results provide insight into the state-selective anionic and excited neutral fragmentation of gaseous molecules via core-level excitation.

The state-selective dissociation pathways for ionic and excited neutral fragments of gaseous and condensed CCl4 following Cl?2p core-level excitation have been characterized by combining photon-induced ionic dissociation, x-ray absorption, resonant photoemission and UV/visible dispersed fluorescence measurements. The excitation of CCl4 induces significant enhancement of the Cl+ desorption yield in the condensed phase and the CCl+ yields in the gaseous phase. Based on the resonant photoemission studies, excitations of Cl?2p electrons to valence orbitals and Rydberg states decay predominantly via the spectator Auger transitions. The transitions of Cl?2p electrons to the Rydberg state of gaseous CCl4 lead to a noteworthy production of excited atomic neutral fragments (C*) and excited diatomic neutral fragments (CCl*). These results provide insight into the state-selective ionic and neutral fragmentation processes of molecules via core-level excitation.

The MQDT of hyperfine structure is greatly simplified in extremely high Rydberg states. A four (or three)-channel quantum defect model is derived for the hyperfine structure of extremely high Rydberg states of msnd configuration in odd alkaline-earth isotopes, in which the transformation matrix Ui(F)i' is given analytically except for one parameter which measures the mixing of singlet 1D2 and triple 3D2 induced by spin-orbit interaction in even isotopes, and in which the four eigen quantum defects mu i' (i'=1, 2, 3, 4) are related to the quantum defects of the fine structure in even isotopes by mu i'=mod( delta i'). This model has been used to interpret the observed sublevel structure of 5snd (n>100) configurations in 87Sr. An excellent agreement between the theoretical and experimental results was achieved.

Mechanisms of the excitation, transformation, and decay of atomic and molecular Rydberg states are described and unsettled questions are discussed.

Rydberg spectra of the chlorine atom 3s/sup 2/3p/sup 4/( /sup 3/P)ns, J = (1/2), (3/2), and (5/2), are calculated by use of an eigenchannel quantum-defect method. The results indicate that the channel interactions due to electrostatic and spin-orbit potentials are weak. The effect due to channel interaction of the 3s3p/sup 6/ configuration is completely negligible. Agreement with emission data is excellent (< or approx. =0.2% accuracy except for the lowest n = 4 levels). Autoionization resonances are predicted.

The collision-induced two-photon forbidden 3s2 1S-3snp 1P transitions (n>11) and collisional enhancement of 3s2 1S-3sns 1S transitions (n>8) of Mg I have been observed and studied as functions of Mg vapour pressure and perturbing gas (Ar) pressure by using two-photon resonant ionisation and thermionic diode techniques. The occurrence of forbidden transitions and collisional enhancement of 3s2 1S-3sns 1S transitions is qualitatively attributed to the formation of a Rydberg Mg2 excimer and a MgxAry Fermi complex. Quantitatively, the above phenomena are interpreted as collisional mixing of high Rydberg states of Mg. The mixing coefficients are estimated by using perturbation theory and the semiclassical (JWKB) approximation. The theoretical estimation is in good agreement with the experimental result.

We report a calculation of the basic structure parameters of 3${p}^{4}$${(}^{3}$P)ns Rydberg spectra of the chlorine isoelectronic ions from Ar ii to Zn xiv as functions of effective nuclear charge by using the eigenchannel quantum-defect theory. The results indicate that the channel interaction gradually diminishes along the isoelectronic sequence. This behavior can be interpreted as the competition between electrostatic and spin-orbit interactions along the sequence. A great amount of basic atomic data can be evaluated with these parameters as input.

In extremely high Rydberg states of odd alkaline-earth isotopes, the Fermi constant ams is comparable to the energy separation between levels belonging to different principal quantum numbers n and hyperfine interactions dominate the structure of the levels. The multichannel quantum defect theory of fine structure has been developed and applied to hyperfine structure and thus the HFS can be analysed directly by MQDT. A two-channel quantum defect model for extremely high Rydberg msns 1S0 and msns 3S1 series in odd alkaline-earth isotopes for F=I is described. The Ui(F)i matrix is given analytically and the relationship between the eigenquantum defects of HFS and the quantum defects of fine structure is derived. The HFS of two extremely high Rydberg 5sns 1S0 and 3S1 series in 87Sr has been calculated by the model and an excellent agreement between the theoretical and experimental results has been achieved. The HFS of extremely high Rydberg msns 1S0 and 3S1 series in 43Ca, 135Ba and 137Ba are also analysed by the model.

Calculation of the motional Stark effect for $B\ensuremath{\sim}50$ kG shows it to become dominant along Rydberg series of light elements at $T\ensuremath{\sim}1000\ifmmode^\circ\else\textdegree\fi{}$C before the diamagnetic shift changes the spectrum from Rydberg to Landau type, thus accounting for the $\frac{1}{2}\ensuremath{\hbar}{\ensuremath{\omega}}_{c}$ spacing observed in Li. This spacing is preserved when both of the competing effects---motional Stark and Landau modulation---are strong. The spectrum appears to depend critically on the relative position of the thresholds for the onset of the two effects.

The quadrupole and exchange interactions representing the short-range electron-ion scattering process have been observed and determined via the measurements of p5s and p5d, J=0 and 1 highly excited Rydberg states in Ne. The quantitative determinations were made possible by the combination of high precision (better than 100 MHz) perturbation-free measurements and multichannel quantum defect theory. A new precise value for the fine-structure splitting of the 20Ne+(2p5) ion is determined to be 780.4269(36) cm-1.

Recent papers have presented data on the high Rydberg states of rubidium in external electric fields. We present an analysis which accounts for the energy positions and symmetry aspects of the resonances seen in these experiments. Our analysis places these new data in the framework of previous studies of such resonances.

The diamagnetic and Stark shifts of strongly perturbed Rydberg spectra such as those of the alkaline earths are expressed in terms of multichannel\char22{}quantum-defect parameters. The theory is applicable when the shifts produced by the external fields are smaller than the neighboring Rydberg separations. It is applied to evaluate the diamagnetic shifts in Ba, an atom in which Rydberg and doubly excited channels are strongly mixed. In such complex spectra, the diamagnetic effect can serve a useful purpose as a tool to disentangle the interacting series. The radial sizes and, therefore, the diamagnetic shifts are quite different for singly and doubly excited states even when they are at the same energy. In fact, for the field strengths considered ($\ensuremath{\simeq}40$ kG), the diamagnetic contribution of the doubly excited configuration is quite negligible. As a result, the observed shift is a direct measure of the amount of singly excited configuration in the state of interest. Exactly similar conclusions apply to Stark shifts. The last part of this paper considers states of very high excitation further up in the spectrum which are no longer in a perturbative regime. Spectra of atoms in a magnetic field have shown equally spaced resonances near the ionization threshold with characteristic spacings. By the consideration of the effects of electric fields which are also often simultaneously present, a criterion is established for the nature of these spacings depending on the relative strengths of the crossed electric and magnetic fields. The spacing seen in any particular experiment is shown to depend on the mass and quantum defect of the atom, the strengths of the external fields, and experimental conditions such as the temperature of the atomic vapor whose absorption spectrum is being studied.

A method of analysis of all level positions in multiple strongly perturbed series of levels is presented and illustrated by examples from the rare gas and Ba spectra. The method is suggested by Seaton's multichannel quantum defect theory but is presented here as an empirical approach. It emphasizes the dependence of a perturbation on the periodicity of the perturbing series and aims at extracting significant information from experimental data in compact form; this goal will be pursued in later works.

This paper comments on the recent observation of Rydberg "progressions" in atomic uranium. A multichannel quantum-defect analysis shows that the observed ion peaks are unresolved groups of levels with different total angular momenta $J$ and approximately the same effective quantum number ${\ensuremath{\nu}}_{\mathrm{ln}}$, and configuration interactions are apparent. The possible number of channels and their assignment are outlined.

The long and glittering history of the study of atoms in external electric and magnetic fields dates from the late 19th century work of Zeeman(1) and the early 20th century works of Stark(2) and Paschen and Back.(3) The birth of quantum mechanics was followed by the pioneering studies of diamagnetic effects of Van Vleck(4) and Jenkins and Segre,(5) and by the prediction of Landau resonances(6) in free-electron spectra of solids. More recently, observations of highly excited atoms in external fields, (7–10) made possible by the advent of high-resolution spectrometers, superconducting magnets, and lasers, have led to a revitalization of atomic spectroscopy. In particular, the observation of field-induced resonances in an otherwise smooth continuum(11–13) has compelled theory(14–16) to deal with a class of phenomena involving competing forces of different symmetries and comparable magnitudes. Fragmentary advances of theoretical understanding have in turn pointed to possibilities of using external fields as probes of atomic structure.

The purpose of this paper is to present a quantum defect theory (QDT) basis for a unified approach to Rydberg and auto-ionizing states in atoms and molecules. A scattering-theoretic outline of QDT was given by A. R. P. Rau in the previous paper in this volume; the present paper focuses on atomic aspects of QDT and provides comment on the transition from atoms to molecules; finally, the MQDT of diatomic molecules will be discussed in detail by C. H. Greene in the following paper in this volume.

A multichannel quantum-defect theory focused on the time-delay matrix is applied to the analysis of Rydberg levels and autoionization resonances of the chlorine atom. The analysis shows that recently observed photoionization cross sections above the 3${p}^{4}$${(\mathrm{}}^{3}$P) threshold of the chlorine ion may be interpreted in terms of unresolved groups of resonances with three different total angular momenta J and approximately the same energy. Experimental data of Rydberg levels were used to determine the phase shift of the collisional eigenstate (quantum defect) and the time-delay matrix for the electron-ion scattering process. The LS coupling scheme is assumed for the electron + ion compound state. The channel interaction strengths and dipole matrix elements of even-parity states relevant to electron-ion collisional and recombination processes are obtained. The branching ratio of photoelectrons is calculated.

The effects of the np2 configurations on interacting Rydberg series are studied by a simple graphic method that exploits the variation of the quantum defect of energy levels along the ioselectronic sequences of alkaline earths. Regularities and relationships between different isoelectronic sequences can thus be observed and perturbations can be identified; extrapolations and interpolations can then be done to check and to locate energy levels and their assignments and to understand term inversions. Extensive data are available for this purpose for the sequences of Be i and Mg i; Ca, Sr, and Ba will be treated summarily. Interpolated energy values of 3p2 1S0 for Al ii, S v, and Ar vii were obtained.