Even-parity autoionizing states in the extreme-ultraviolet photoabsorption spectra of Mg, Al⁺, and Si²⁺

The dual-laser-produced plasma (DLP) photoabsorption technique has been used to study 2p→3s excitations in the isoelectronic species Mg, Al+, and Si2+ prepared in the excited configuration 2p63s3p. The autoionizing upper states belong to the 2p53s23p even-parity configuration. The versatility of the technique is demonstrated through a careful combination of space- and time-resolved photoabsorption scans. Plasma conditions optimized for the observation of the inaccessible parity regime were successfully reproduced along the isoelectronic sequence of interest. All the observed transitions were interpreted with the help of multiconfigurational atomic structure calculations. In the case of magnesium, the photoabsorption data are compared with the ejected-electron spectra excited by low-energy electron impact of Pejcev et al. [J. Phys. B 10, 2389 (1977)]

Critical parameters influencing the EUV-induced damage of Ru-capped multilayer mirrors

Ongoing endurance testing of Ru-capped multilayer mirrors (MLMs) at the NIST synchrotron facility has revealed that the damage resulting from EUV irradiation does not always depend on the exposure conditions in an intuitive way. Previous exposures of Ru-capped MLMs to EUV radiation in the presence of water vapor demonstrated that the mirror damage rate actually decreases with increasing water pressure. We will present results of recent exposures showing that the reduction in damage for partial pressures of water up to 5 x 10{sup -6} Torr is not the result of a spatially uniform decrease in damage across the Gaussian intensity distribution of the incident EUV beam. Instead we observe a drop in the damage rate in the center of the exposure spot where the intensity is greatest, while the reflectivity loss in the wings of the intensity distribution appears to be independent of water partial pressure. (See Fig. 1.) We will discuss how the overall damage rate and spatial profile can be influenced by admixtures of carbon-containing species (e.g., CO, CO{sub 2}, C{sub 6}H{sub 6}) at partial pressures one-to-two orders of magnitude lower than the water vapor partial pressure. An investigation is underway to find the cause of the non-Gaussian damage profile. Preliminary results and hypotheses will be discussed. In addition to high-resolution reflectometry of the EUV-exposure sites, the results of surface analysis such as XPS will be presented. We will also discuss how the bandwidth and time structure of incident EUV radiation may affect the rate of reflectivity degradation. Although the observations presented here are based on exposures of Ru-capped MLMs, unless novel capping layers are similarly characterized, direct application of accelerated testing results could significantly overestimate mirror lifetime in the production environment

EUV testing of multilayer mirrors: critical issues

Recently, while performing extensive EUV irradiation endurance testing on Ru-capped multilayer mirrors in the presence of elevated partial pressures of water and hydrocarbons, NIST has observed that the amount of EUV-induced damage actually decreases with increasing levels of water vapor above {approx} 5 x 10{sup -7} Torr. It is thought that the admitted water vapor may interact with otherwise stable, condensed carbonaceous species in an UHV vacuum system to increase the background levels of simple gaseous carbon-containing molecules. Some support for this hypothesis was demonstrated by observing the mitigating effect of very small levels of simple hydrocarbons with the intentional introduction of methyl alcohol in addition to the water vapor. It was found that the damage rate decreased by at least an order of magnitude when the partial pressure of methyl alcohol was just one percent of the water partial pressure. These observations indicate that the hydrocarbon components of the vacuum environment under actual testing conditions must be characterized and controlled to 10{sup -11} Torr or better in order to quantify the damage caused by high levels of water vapor. The possible effects of exposure beam size and out-of-band radiation on mirror lifetime testing will also be discussed

Anomalous behavior of the near-threshold photoionization cross section of the neon isoelectronic sequence: a combined experimental and theoretical study

We present a combined theoretical and experimental investigation of photoionization along the Ne isoelectronic sequence and show that the near-threshold behavior of the cross section for Si4+ differs radically from the nearby ions in the sequence. We demonstrate that the general nature of the underlying physics implies that dramatic changes in near-threshold behavior may be expected for many other ions

Photoabsorption and photoion spectroscopy of atomic uranium in the region of 6p and 5d excitations

The photoabsorption process in atomic uranium has been investigated experimentally and theoretically in the 15–150-eV region. Using the dual laser plasma technique, the 6p photoabsorption spectrum has been recorded while for the first time the 5d region has been remeasured photoelectrically using both photoabsorption and photoion spectroscopy. Interpretation of the photoabsorption spectra is supported by Hartree-Fock calculations which take into account spin-flip decay and the interaction of many discrete states with many continua. The 6p spectrum is entirely dominated by spin-orbit split 6p⃗6d transitions. The 5d-subshell photoabsorption is shown to consist predominantly of discrete 5d⃗5f excitations; here the electrostatic and spin-orbit interactions are comparable in strength

Atomic-scale visualization of initial growth of homoepitaxial SrTiO3 thin film on an atomically ordered substrate

The initial homoepitaxial growth of SrTiO3 on a (\surd13\times\surd13) - R33.7{\deg}SrTiO3(001) substrate surface, which can be prepared under oxide growth conditions, is atomically resolved by scanning tunneling microscopy. The identical (\surd13\times\surd13) atomic structure is clearly visualized on the deposited SrTiO3 film surface as well as on the substrate. This result indicates the transfer of the topmost Ti-rich (\surd13\times\surd13) structure to the film surface and atomic-scale coherent epitaxy at the film/substrate interface. Such atomically ordered SrTiO3 substrates can be applied to the fabrication of atom-by-atom controlled oxide epitaxial films and heterostructures

Doubly excited autoionization resonances in the absorption spectrum of Li⁺ formed in a laser-produced plasma

In a technique using a continuum generated by focusing a laser beam on a target of high atomic number, the spatially resolved absorption spectrum of Li+ formed in a second laser-produced plasma was studied from 200 to 50 Å. The doubly excited resonances, 1s21S-2snp1P, as well as the principal series and its adjoining photoionization continuum were observed. The Fano parameters, q and Γ, of the 2s2p1P were derived from the absorption profile. Comparison is made with theory

Trends in autoionization of Rydberg states converging to the 4s threshold in the Kr-Rb⁺-Sr²⁺ isoelectonic sequence: theory and experiment

We have measured the photoabsorption spectra of the Kr-like ions Rb+ and Sr2+ at photon energies corresponding to the excitation of 4s-np resonances using, the dual laser plasma photoabsorption technique. Dramatic changes in the line profiles, with increasing ionization and also proceeding along the Rydberg series of each ion, are observed and explained by the trends in 4s-transition amplitudes computed within a framework of configuration-interaction Pauli-Fock calculations. Total photoionization cross sections show very good agreement with relative absorption data extracted from the measured spectra

4 f(P1) giant dipole resonance in La³⁺

Photoabsorption of free La3+ ions in the 4d excitation region has been measured using the dual-laser plasma technique. A dramatic strong and broad 4d94f(1P) giant dipole resonance was observed. The interpretation of the results is provided using theoretical techniques which go beyond the independent particle approximation. In particular the strong term dependence for 4f(1P) gives evidence of strong polarization effects for the description of the giant dipole resonance

ANALYSIS OF Ba++Ba^{++} PHOTOABSORPTION SPECTRA: COMPARISON WITH XeI, BaI AND BaXI

Author Institution:Photoabsorption in a controlled doubly ionized plasma (fully ionized and in the ground state) of $Ba^{++}$ revealed several long series arising from the 4d and 5p closed shells. The following series have been observed: (1) $4d^{9}$ np (to n=14), (2) $5p^{5}$ ns (to n=20), and (3) $5p^{5}$ nd (to n=19). These spectra can be directly compared with existing published and unpublished isoelectronic spectra of XeI. Series (2) and (3) are strongly mixed and are treated by multi-channel quantum defect theory. Transitions to $4d^{9}$ nf $^{3}D_{1}, ^{3}P_{1}$, and $^{1}P_{1}$, not seen in XeI, were identified. They are bracketed by the previously identified, corresponding transitions in BaI and Ba XI