2,616 research outputs found

    Theoretical studies of photoexcitation and ionization in H_2O

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    Theoretical studies are reported of the complete dipole excitation and ionization spectrum in H_2O employing Franck‚ÄďCondon and static‚Äźexchange approximations. Large Cartesian Gaussian basis sets are used to represent the required discrete and continuum electronic eigenfunctions at the ground‚Äźstate equilibrium geometry, and previously devised moment‚Äźtheory techniques are employed in constructing the continuum oscillator‚Äźstrength densities from the calculated spectra. Detailed comparisons are made of the calculated excitation and ionization profiles with recent experimental photoabsorption studies and corresponding spectral assignments, electron impact‚Äďexcitation cross sections, and dipole (e,‚ÄČ2e)/(e,‚ÄČe+ion) and synchrotron‚Äźradiation studies of partial‚Äźchannel photoionization cross sections. The various calculated excitation series in the outer‚Äźvalence (1b(^‚ąí1)_1, 3a(^‚ąí1)_1, 1b(^‚ąí1)_2) region are found to include contributions from valence‚Äźlike 2b_2‚ÄČ(ŌÉ*) and 4a_1(ő≥*) virtual orbitals, as well as appropriate nsa_1, npa_1, nda_1, npb_1, npb_2, ndb_1, ndb_2, and nda_2 Rydberg states. Transition energies and intensities in the ‚ąľ7 to 19 eV interval obtained from the present studies are seen to be in excellent agreement with the measured photoabsorption cross section, and to provide a basis for detailed spectral assignments. The calculated (1b(^‚ąí1)_1)X(^‚ÄČ2)B_1, (3a_1(^‚ąí1))^2A_1, and (1b_2(^‚ąí1))(^2)B_2 partial‚Äźchannel cross sections are found to be largely atomic‚Äźlike and dominated by 2p‚Üíkd components, although the 2b_2(ŌÉ*) orbital gives rise to resonance‚Äźlike contributions just above threshold in the 3a_1‚Üíkb_2 and 1b_2‚Üíkb_2 channels. It is suggested that the latter transition couples with the underlying 1b_1‚Üíkb_1 channel, accounting for a prominent feature in the recent high‚Äźresolution synchrotron‚Äźradiation measurements. When this feature is taken into account, the calculations of the three outer‚Äźvalence channels are in excellent accord with recent synchrotron‚Äźradiation and dipole (e,‚ÄČ2e) photoionization cross‚Äźsectional measurements. The calculated inner‚Äźvalence (2a_1(^‚ąí1)) cross section is also in excellent agreement with corresponding measured values, although proper account must be taken of the appropriate final‚Äźstate configuration‚Äźmixing effects that give rise to a modest failure of the Koopmans approximation, and to the observed broad PES band, in this case. Finally, the origins of the various spectral features present in the measured 1a_1 oxygen K‚Äźedge electron energy‚Äźloss profile in H_2O are seen to be clarified fully by the present calculations

    Soft gluon resummation for squark and gluino pair-production at hadron colliders

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    We report on the study of soft gluon effects in the production of squarks and gluinos at hadron colliders. Close to production threshold, the emission of soft gluon results in the appearence of large logarithmic corrections in the theoretical expressions. In order to resum these corrections at next-to-leading-logarithmic accuracy appropriate one-loop anomalous dimensions have to be calculated. We present the calculation of the anomalous dimensions for all production channels of squarks and gluinos and provide numerical predictions for the Tevatron and the LHC.Comment: 6 pages, talk given at RADCOR 2009 - 9th International Symposium on Radiative Corrections (Applications of Quantum Field Theory to Phenomenology) October 25-30 2009, Ascona, Switzerlan
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