The XUV spectra of free metal atoms obtained by absorption, photoion, photoelectron and two-photon spectroscopy are reviewed. The experimental results are discussed in comparison with the predictions of the different theoretical approaches. Emphasis is put on the basic excitation and de-excitation processes, the importance of electron correlation, on common features of groups and on trends along series.

https://iopscience.iop.org/article/10.1088/0034-4885/55/7/002/pdf

XUV spectroscopy of metal atoms

We describe a detailed study on the properties of alkali atoms in extrasolar giant planets, and specifically focus on their role in generating the atmospheric free electron densities, as well as their impact on the transit depth observations. We focus our study on the case of HD 209458b, and we show that photoionization produces a large electron density in the middle atmosphere that is about two orders of magnitude larger than the density anticipated from thermal ionization. Our purely photochemical calculations, though, result in a much larger transit depth for K than observed for this planet. This result does not change even if the roles of molecular chemistry and excited state chemistry are considered for the alkali atoms. In contrast, the model results for the case of exoplanet XO-2b are in good agreement with the available observations. Given these results we discuss other possible scenarios, such as changes in the elemental abundances, changes in the temperature profiles, and the possible presence of clouds, which could potentially explain the observed HD 209458b alkali properties. We find that most of these scenarios cannot explain the observations, with the exception of a heterogeneous source (i.e., clouds or aerosols) under specific conditions, but wemore » also note the discrepancies among the available observations.« less

https://iopscience.iop.org/article/10.1088/0004-637X/796/1/15/pdf

Electron densities and alkali atoms in exoplanet atmospheres

Chemi-ionization in thermal-energy binary collisions of optically excited atoms

The 'quasi-resonant' energy pooling process in a rubidium vapour (Rb(5p2P32/)+Rb(5p2P32/)+68 cm-1 to Rb(5d)+Rb(5s2S12/)) is experimentally investigated. An asymptotic theoretical model is applied to this reaction and gives a cross section (1*10-14 cm2) close to the experimental value (3*10-14 cm2). The results are discussed together with available data for other alkali atoms.

https://iopscience.iop.org/article/10.1088/0022-3700/16/17/014/pdf

Energy pooling process in rubidium vapour

Publisher Summary This chapter presents an account of research into fundamental questions, spanning roughly the past decade, and provides continuity from earlier reviews of collisional ionization. The chapter discusses progress along three principal lines: (1) experimental investigation of the fundamental questions, (2) the calculation of molecular potential curves required for the interpretation of experiments, and (3) a review of theoretical approaches developed to grapple with the physics of electron bound-free coupling in heavy particle associative and dissociative processes. The chapter explains the problem of molecular potentials and dynamics of associative ionization. The treatment of associative ionization necessitates knowledge of accurate potential curves and dynamical couplings characterizing the quasi-molecule formed during the collision. Dynamics of associative ionization reviews and classifies the theoretical approaches. It assumes that the basic molecular data, the potential curves and couplings responsible for the ionization process are either known from first principles or have been parametrically fit to experimental results.

Associative Ionization: Experiments, Potentials, And Dynamics

A sodium atomic beam with density ${10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ was illuminated by cw dye laser radiation (a few watts per square centimeter) tuned to the ${D}_{2}$ resonance line. In the energy spectrum of the emitted electrons, several lines were observed between 4 and 7 eV. Their positions and intensities indicate that seed electrons are produced either via associative ionization or via collisional ionization from excited states populated by energy-pooling collisions. These electrons are then heated through successive superelastic collisions with excited $3p$ atoms.

Direct Observation of Hot-Electron Spectra from Laser-Excited Sodium Vapor

We have obtained oscillator strengths for transitions between a laser-excited initial state and autoionizing final states. In the case of sodium, a laser was used to populate the 3p initial state, and synchrotron radiation was used to excite the autoionizing resonances. These transitions are of the type 2p3p P-+ 2p(P)3s3p(>P)>S,>*P,>*D\\ they decay to 2pS + e. The sum of the oscillator strengths for all the observed transitions between the 2p3p initial-state and the 2p3 s 3p finalstate configurations was found to be equal to 0.22(4).

Oscillator strength measurements of even-parity autoionizing resonances by combined synchrotron-radiation-laser excitation.

Electron spectrometry study of associative and penning ionization in laser excited sodium vapor

Experimental and theoretical determinations of the 5d photoionization cross section in laser-excited barium atoms between 15 and 150 eV photon energy.

In a triple, orthogonal crossed beam experiment, we have studied photoionization of excited Na atoms. A cw ring dye laser (few W/cm2) locked to the D1 or D2 absorption lines excited up to 20% of the 2013 cm−3 ground state atoms to Na(3p 2p3/2). Monochromatized synchrotron radiation from the ACO storge ring provided the photoionizing radiation. A cylindrical mirror electron spectrometer was used to measure photoelectron spectra. First measurements of the 2p63p→2p53pEl photoionization were obtained. Decay of autoionizing resonances to the 2p53p ionic channel were also observed.

J. L. Picqué

Papers

Direct Observation of Hot-Electron Spectra from Laser-Excited Sodium Vapor

Oscillator strength measurements of even-parity autoionizing resonances by combined synchrotron-radiation-laser excitation.

Electron spectrometry study of associative and penning ionization in laser excited sodium vapor

Experimental and theoretical determinations of the 5d photoionization cross section in laser-excited barium atoms between 15 and 150 eV photon energy.

Observation of ionization of laser excited sodium atoms by synchrotron radiation