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Following photoexcitation of Cl 2p electrons to various resonances, the dissociation dynamics of ionic and excited neutral fragments of gaseous CH{sub 2}Cl{sub 2} was investigated by combined measurements of photon-induced ionic dissociation, x-ray absorption, and uv-visible dispersed fluorescence. The Cl 2p core-to-Rydberg excitations near the ionization threshold produce a notable enhancement of excited neutral fragments (C* and CH*), which is attributed to the contribution from a shake-modified resonant Auger decay and postcollision interaction. The excitation Cl 2p {yields}10 a{sub 1}* induces an enhanced yield of CH{sub 2}{sup +}, possibly originating from fast dissociation via a strongly repulsive surface. The experimental results provide insight into the dissociation dynamics of ionic and excited neutral fragment production following core-level excitation.

The dissociation dynamics of anionic and excited neutral fragments of gaseous ${\text{CCl}}_{4}$ and ${\text{CCl}}_{4}$ adsorbed on Si(100) $\ensuremath{\sim}90\text{ }\text{K}$ following $\text{Cl}\text{ }2p$ core-level excitations were investigated on combining measurements of photon-induced anionic dissociation, x-ray absorption, and uv-visible dispersed fluorescence. The transitions of core electrons to high Rydberg states or doubly excited states near $\text{Cl}\text{ }2p$ ionization thresholds of gaseous ${\text{CCl}}_{4}$ remarkably enhance the production of excited neutral fragments (${\text{C}}^{\ensuremath{\ast}}$ and ${\text{CCl}}^{\ensuremath{\ast}}$); this enhancement is attributed to the contribution from the shake-modified resonant Auger decay and/or postcollision interaction (PCI). The ${\text{Cl}}^{\ensuremath{-}}$ anion is significantly reinforced in the vicinity of the $\text{Cl}\text{ }2{p}_{1/2,3/2}$ ionization threshold of gaseous ${\text{CCl}}_{4}$, originating from PCI-mediated photoelectron recapture. The $\text{Cl}\text{ }2p\ensuremath{\rightarrow}7{a}_{1}^{\ensuremath{\ast}}$ excitation for ${\text{CCl}}_{4}/\text{Si}(100)$ at $\ensuremath{\sim}90\text{ }\text{K}$ enhances the ${\text{Cl}}^{\ensuremath{-}}$ desorption yield at a submonolayer level. This resonant enhancement of ${\text{Cl}}^{\ensuremath{-}}$ yield at the $7{a}_{1}^{\ensuremath{\ast}}$ resonance in the $\text{Cl}\text{ }2p$ edge at a submonolayer level occurs through the formation of high-lying molecular-ion states of ${\text{CCl}}_{4}$ adsorbed on a Si surface.

By using two-photon resonant ionisation and a thermionic diode detector, the collision-induced two-photon forbidden 6s2 1S-6snp 1P transitions (n>29) have been observed at P(Ba) approximately 0.13 Torr and P(Ar) approximately 15 Torr. The forbidden transitions are attributed to the dimer excimer Ba2 processes. The minimum quantum number, n approximately 28 is calculated by using the l-mixing model and perturbation theory, which is in good agreement with the authors' observation.

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.

Efficient VUV generation near 160 nm has been demonstrated in Mg vapor at pressures between .1 -1. Torr and an estimated laser intensity of 10 W/cm2 The highest efficiency measured is on the order of .1 % and it is found that the vUv generation scales as P35 j the above pressure range. The data indicate that stimulated emission from molecular Mg plays an important role.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Non-resonant multiphoton ionization of atomic lead at 1064nm laser field has been studied. Both the angular distribution and the intensity dependence of photoelectrons have been measured. The results show that they are very sensitive to the process of multiphoton ionization.

In an odd isotope with nuclear spin I not=O of an atom, the sublevels usually degenerate at certain points in energy ordinate which they call crossings. In alkaline-earth atoms, there is a series of crossings and it is determined only by the hyperfine splitting of the first ionisation limit ms 2S12/. A formula which is used to calculate the crossings has been derived. By using this formula, the framework of the sublevels and the experimental profile can be anticipated without knowing any detailed information about the odd isotope except for the hyperfine splitting of the first ionisation limit ms 2S12/.

A two-channel quantum defect model is applied to analyse the strongly perturbed principal absorption series of Sr near threshold. Experimental data in the discrete and in the autoionization region are expressed in terms of five parameters: two eigenquantum defects μ α , one transformation angle θ , and two dipole matrix elements D α . Values of the parameters, obtained by fitting the experimental data, provide information on the mixing coefficients of the levels and oscillator strength as well as on photoabsorption cross sections in the autoionization region. The two-channel model is sufficient to describe the discrete spectrum. Discrepancies show up between theory and experiment in the autoionization region because of opening up of additional channels above the first ionization threshold.

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 present a systematic densitometer mapping of diamagnetic effects on the principal series of Ba and Sr in absorption, with magnetic fields up to 4.7 T. The n - and l -mixing spectra are resolved in both σ- and π -polarization while the quasi-Landau resonances are observed near and above the ionization limit only in σ-polarization. Configuration interaction produces striking differences in the intensities and spacings of the n - and l -mixed spectra while only the shapes of the quasi-Landau resonances depend on the atomic systems.

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 oly 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.

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 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.