Potential-Barrier Effects in Photoabsorption. III. Application to 4\u3ci\u3ed\u3c/i\u3e-Shell Photoabsorption in Lanthanum

An exploratory calculation of the photoabsorption cross section near the 4d ionization threshold of lanthanum metal is carried out. The level structure of the configuration 4d94f is found to be altered by interaction with the continuum configuration 4d9εf. The Fano profile of the resonance above threshold is calculated, as are the oscillator strengths of the discrete states below threshold. Our results provide the ground work for more detailed calculations

ANGULAR DISTRIBUTIONS OF PHOTOELECTRONS AND NON-THERMAL PHOTOIONS FROM ATOMS AND MOLECULES

Durant ces dix dernières années, les distributions angulaires des photo-électrons ont été exclusivement utilisées pour étudier la dynamique des processus de photoionisation dans les atomes et les molécules. Nous faisons le point, ici, sur les résultats les plus marquants survenus depuis la dernière Conférence Internationale d'il y a trois ans. De loin, les plus grands progrès ont eu lieu pour les atomes, l'intérêt s'étant plus particulièrement porté sur les points suivants : amélioration de l'ordre zéro (Hartree-Slater, Cooper-Zare), compréhension du paramètre d'asymétrie β(ε), en considérant les corrélations électroniques, les effets relativistes, les interactions de l'état final anisotropique. L'étude des gaz rares a bénéficié de l'étroite coordination entre l'expérience et la théorie, tandis que l'étude des atomes non sphériques est restée principalement théorique, mis à part les mesures récentes sur l'oxygène atomique. Les études de distribution angulaire sur les molécules sont encore à un stade très préliminaire de développement. Ceci résulte du manque de mesures en fonction de la longueur d'onde, mises à part les expériences très récentes, et l'absence de méthodes théoriques pratiques et réalistes. Le point sera donc fait sur ce travail récent, ainsi que sur une sélection de travaux plus anciens concernant les molécules et obtenus avec des raies de résonance. De plus, une nouvelle série d'études angulaires sur les molécules — la distribution angulaire des ions non thermiques formés par photoionisation dissociative — fournissant des informations complémentaires sur les mesures correspondantes avec des photoélectrons, sera présentée.During the last ten years, photoelectron angular distributions have been used extensively to study the dynamics of the photoionization process in atoms and molecules. Here we review some major advances in this body of work, with special emphasis on results emerging since the last Conference on VUV Radiation Physics three years ago. By far the greatest progress has occurred for atoms, where interest is focussed on improving our zero-order (Hartree-Slater, Cooper-Zare) understanding of the asymmetry parameter β(ε), by considering electron correlations, relativistic effects, and anisotropic final-state interactions. The study of the rare gases has benefitted from extensive coordination between experiment and theory, whereas work on non-spherical atoms has been mainly theoretical, with the only measurements being performed very recently on atomic oxygen. Angular distribution studies on molecules are in a much earlier stage of development. Progress has been impeded by the lack of practical, realistic theoretical methods and wavelength-dependent measurements, both of which are becoming available only now. This recent work, together with selected topics from earlier resonance-line work on molecules will be reviewed. In addition, a new class of angle-dependent studies of molecules will be discussed — the angular distribution of non-thermal ions formed by dissociative photoionization — which provides information complementary to the related measurements on photoelectrons

Photoionization of excited molecular states using multiphoton excitation techniques

Photoelectron spectra are reported for three photon resonant, four photon ionization of H/sub 2/ via the B /sup 1/..sigma../sub u//sup +/, v = 7 (J = 2,4) and C /sup 1/..pi../sub u'/, v = 0-4 (J = 1) levels and of N/sub 2/ via the o/sub 3//sup 1/..pi../sub u'/, v = 1,2, b /sup 1/..pi../sub u'/, v = 3-5, and c /sup 1/..pi../sub u'/, v = 0 levels. The results reflect both the spectroscopy and the dynamics of photoionization of excited molecular states and are discussed in terms of the selection rules for photoionization and the relative probabilities of photoionization from Rydberg and valence states. In some cases, in accordance with the Franck-Condon principle, the results demonstrate that resonant multiphoton ionization through Rydberg states may be a powerful technique for the production of electronic, vibrational, and rotational state selected ions. However, in other cases, systematic departures from Franck-Condon factors are observed, which reflect the more subtle dynamics of excited state photoionization. 23 references, 6 figures, 2 tables

Raising of Discrete Levels into the Far Continuum

Broad peaks observed in the photoabsorption spectra of rare earths ∼10-20 eV above the 4d edge are attributed to transitions 4d104fN→4d94fN+1. Exchange interaction splits the 4d94fN+1 configuration and raises some multiplets by ∼20 eV. Autoionization to 4d9-4fNεf broadens the high levels. The interpretation extends to p→d processes observed in the absorption of synchrotron light by transition elements

Calibration of a monochromator/spectrometer system for the measurement of photoelectron angular distributions and branching ratios

We describe the techniques used in calibrating a monochromator/spectrometer system for gas-phase photoelectron angular distribution and branching ratio measurements. We report a self-consistent set of values for the Ne 2p, Ar 3p, Kr 4p/sub 3/2/ and 4p/sub 1/2/, and Xe 5p/sub 3/2/ and 5p/sub 1/2/ photoelectron asymmetry parameters and for the Kr 4p/sub 3/2/:4p/sub 1/2/ and Xe 5p/sub 3/2/:5p/sub 1/2/ branching ratios for the energy regions from threshold to approximately 15 iV. 22 refs., 7 figs

A STUDY OF VIBRONIC COUPLING IN THE [MATH] STATE OF CO+2

Les effets du couplage vibronique dans l'état [MATH](2∑g+) du CO2+ ont été étudiés pour des photons d'énergies comprises entre 20 et 28,5 eV. Deux analyseurs d'électrons hémisphériques suivis de détecteurs sensibles à la position combinés au rayonnement synchrotron de SURF-II ont permis un analyse vibrationnellement résolve de l'état [MATH](2∑g+). La distribution angulaire caractérisée par le paramètre β de l'état vibrationnel interdit [MATH](1,0,1) se révèle très différent de celle de l'état permis [MATH](0,0,0). Les similarités entre la distribution angulaire (β vs. hν) de l'état interdit [MATH](1,0,1) et celle de l'état [MATH](2∑u+) supportent l'hypothèse de couplage vibronique entre ces deux états comme origine pour la bande interdit.We have studied vibronic coupling in vibrationally resolved photoionization to the [MATH] (2∑g+) state of CO2+ in the photon-energy range hν=20-28.5 eV. The measurements utilize high-resolution hemispherical electron analyzers, equipped with area detectors, and the SURF-II synchrotron radiation source at the National Bureau of Standards. The angular distribution asymmetry-parameters (β) for the allowed [MATH](0,0,0) and forbidden [MATH](1,0,1) (19.747 eV binding energy) peaks are found to be quite different. However, similarities between the [MATH](1,0,1) β curve and that for the [MATH] state suggest that vibronic coupling to the [MATH] (2∑u+) state of CO2+ is the explanation for the intensity of the [MATH] state forbidden band in the first 8 eV above threshold