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  • 1
    Electronic Resource
    Electronic Resource
    Valence ionization of HCl. An investigation of many-body effects (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 4331-4341 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Many-body Green's-function calculations which employ large configurational expansions and which are accurate to third and fourth order of perturbation theory are reported for the valence ionic states of HCl. Large polarized basis sets including Rydberg functions s-, p-, and d-type symmetries have been used. Third-order calculations are sufficient in the outer valence region, but in the inner valence region where the complete breakdown of the one-particle picture of ionization is observed a fourth-order theory is necessary in conjunction with large basis sets. The synchrotron and x-ray excited spectra can be assigned nearly up to the double-ionization threshold in a very satisfactory way. No indication of strong outer valence satellite lines is found.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457740
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  • 2
    Electronic Resource
    Electronic Resource
    Doubly ionized states of ethylene: Auger spectrum, potential energy surfaces and nuclear dynamics (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1734-1753 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Theoretical investigations of the outer valence doubly ionized states of ethylene are presented using ab initio Green's function and configuration interaction methods. The vertical double ionization potentials computed by the Green's function method using the ADC(2) scheme are discussed in connection with the Auger spectrum of ethylene and found to reproduce the experimental spectrum quite accurately. Another main purpose of the present work is the investigation of the potential energy surfaces of selected states via ADC(2) and the study of the nuclear dynamics on them. It is shown that only the ground state of the dication is nonplanar with a torsional angle of 90°. The investigation of the topology of the surfaces reveals a low energy conical intersection of the dicationic ground and first excited states. The associated vibronic coupling problem turns out to include, additionally, the second excited state and thus, we encounter here a three-state problem. In the approximation of linear vibrational and vibronic coupling the static and dynamic aspects of the vibronic interaction are investigated by a model Hamiltonian comprising three nonseparable nondegenerate vibrational modes. The input data for the model are extracted from the ADC(2) results. With the aid of this Hamiltonian we have computed a hypothetical "sudden'' double ionization spectrum of ethylene whose complex structure is analyzed stepwise and by cuts through the pertinent adiabatic and diabatic surfaces. The analysis exhibits that the conical intersection of the two lower lying states dominates the vibronic dynamics and leads to strong vibronic mixing between them. This mixing is indirectly reinforced by the interaction between the intermediate and the upper states. As a consequence, strong nonadiabatic effects occur in the ethylene dication. Furthermore, two of the three surfaces involved in the conical intersection are also involved in a multidimensional avoided crossing. A more detailed analysis presented earlier has exhibited that the states belong to an interesting class where two potential energy surfaces are likely not to coincide although plenty of nuclear degrees of freedom exist, in principle, to allow for an intersection to occur.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457080
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  • 3
    Electronic Resource
    Electronic Resource
    Valence ionization and the electron affinities of the open and cyclic forms of Se3 and Te3 (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3582-3588 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Se3 and Te3 exist as the S3 molecule in an open C2v and a closed D3h form with different electronic configurations but nearly the same total energy. The ionization spectra of Se3 and Te3 are calculated in both the C2v and D3h forms by many-body Green's function methods. The D3h forms behave quite regularly, but very strong correlation effects are found in the C2v forms. If a theory accurate to third order is used, satellite lines appear at the ionization threshold. If higher order terms are included the satellite lines at the ionization threshold experience a slight change in energy, but a considerable reduction of intensity. Se3 has a positive electron affinity in both the C2v and D3h forms (2B1, 2A'2, states), and Te3 appears to have two positive electron affinities (2B1, 2A1 for the C2v form and 2A2 and 2E' for the D3h form).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456890
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  • 4
    Electronic Resource
    Electronic Resource
    Symmetry breaking and symmetry restoring in ions of loosely bound systems (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7039-7047 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a theoretical analysis of the effects that lead to distortion or symmetry restoration of the nuclear configuration in ions of weakly bound molecular systems and clusters. The vibronic coupling mechanisms involved are reviewed and discussed in terms of simple and general models. Guided by a qualitative characterization of the wave function of a loosely bound system, we study the symmetry determining effects for such ions in terms of first order (Koopmans'), relaxation and correlation distortion energies associated with electron removal from the parent system. Our discussion suggests that the Koopmans' approximation can reasonably be expected to provide a qualitatively correct dependence of the ion energy on nuclear motion. Exceeding these expectations, it is found in numerical applications to the He+3 and He+4 ions that the relaxation and correlation energy changes accompanying symmetry breaking tend to cancel exactly. This implies, and we show in practice, that while insufficiently accurate correlation energy estimates may easily lead to incorrect geometries, the inexpensive Koopmans' approximation can often suffice to obtain the correct energy curvature along symmetry breaking coordinates. Further preliminary results for N3 and CO+2 suggest that these findings have a much wider applicability than anticipated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457321
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  • 5
    Electronic Resource
    Electronic Resource
    Band shape and vibrational structure in Auger spectra: Theory and application to carbon monoxide (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 6634-6644 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A time-dependent approach to Auger spectra is presented and used to derive simple working equations for computing the vibrational broadening and the vibrationally induced shift of the peaks in the spectrum. The formulas give the explicit dependence of the vibrational envelope on the local details of the electronic potential energy surfaces of the intermediate and final states, providing interesting general insights which we discuss in detail. It is shown that, in polyatomic molecules, relevant interaction terms among different nuclear modes arise. The theory applies as well to other processes which involve a core ionized or core excited intermediate state like, for instance, x-ray emission or resonant Auger decay. As a test application, the double ionization spectrum of CO is computed by the Green's function method, and the new equations, together with a two-hole population analysis of the pole strengths, are used to obtain theoretical Auger spectra. The experimental spectral profiles, characteristically shaped by the varying vibrational broadening and substantial energy shifts, are accurately reproduced, giving most peak positions to within a few tenths of eV. The results present very different vibrational effects for the carbon and oxygen spectra, showing the general inadequacy of interpretations based on vertical transition energies only. Hole localization in the dicationic states is discussed in the light of the population analysis results and used to estimate the electronic Auger transition rates.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461533
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  • 6
    Electronic Resource
    Electronic Resource
    Many dicationic states and two-hole population analysis as a bridge to Auger spectra: Strong localization phenomena in BF3 (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 523-532 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The outer valence double ionization spectrum of BF3 is investigated by computing 100 double ionization transitions with the Green's function method. The results show that one-site and two-site hole localization on the fluorine atoms takes place to full extent and the energy split between clusters of states with similar hole distribution exceeds 10 eV. Double hole localization as a symmetry breaking phenomenon is discussed, pointing out the inherent inability of uncorrelated methods to describe it. A two-hole population analysis of the dicationic states for the quantitative measure of their localized character is presented. The strong effects of hole localization on Auger transition rates are discussed, showing that the two-hole population analysis can be used to develop a new statistical approach to the computation of Auger line shapes. Application of this method to the fluorine and boron KLL Auger spectra of BF3 permits, for the first time, an accurate reproduction of the spectral profiles and a complete interpretation of their features.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460369
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  • 7
    Electronic Resource
    Electronic Resource
    Dicationic states of hydrocarbons and a statistical approach to their Auger spectra (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 2984-2999 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The valence doubly ionized states of acetylene and ethane are computed and discussed, complementing previous investigations on other hydrocarbons via the Green's function method. The vertical double ionization potentials are used for the interpretation of the Auger spectra of the molecules. The analysis is performed by employing a simple statistical approach to the Auger rates where the states are weighted by their two-hole components. For all hydrocarbons studied the resulting theoretical spectra exhibit maxima in good agreement with the experimental Auger peaks and reproduce the essential features of the experimental spectra. Strong final-state correlation effects are found for acetylene, ethylene, and benzene. They lead to a breakdown of the molecular orbital picture of double ionization which seems to be a typical phenomenon for unsaturated molecules. This phenomenon manifests itself in the Auger spectrum by the appearance of a high density of dicationic states which cannot be described in the independent-particle approximation. By contrast, the Auger spectrum of ethane is found to be dominated by the appearance of main states accompanied by weak satellite states at high energy. There is a correspondence between many-body effects in the Auger spectrum and the chemical bond. General aspects of correlation effects in dicationic states are discussed and related to the case of singly ionized states.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457894
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  • 8
    Electronic Resource
    Electronic Resource
    On the choice of orbital bases for configuration interaction (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 4170-4179 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study the CI energy dependence on one-particle orbital bases obtained as SCF solutions of a general parametrized Fock operator which reduces to the Hartree–Fock mean field as a special case. In the case of single excitation CI, exact analytical expressions are derived for the curvature of the CI energy surface at the Hartree–Fock point. Numerical applications to N2 and H2O show that the convergence rate of the CI expansion can be drastically increased by an appropriate and largely system independent choice of the mean field parameters. In all configuration spaces, except single plus double excitation CI, the Hartree–Fock basis is found to be the worst orbital basis for use in CI, suggesting in general that the common-practice procedure of optimizing within a given physical model as the first step to go beyond the model itself can be a very inefficient strategy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454852
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  • 9
    Electronic Resource
    Electronic Resource
    Nuclear dynamics of decaying states: A time-dependent formulation (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 9691-9706 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The wave packet dynamics accompanying the excitation to a decaying electronic state and the subsequent decay to final electronic states are discussed. The cross sections for the excitation and for the production of final states are related to the corresponding wave packets. The time-dependent formulation adds insight into the process and is amenable to semiclassical approximations and interpretations. It can also be used to compute the gross features of the observed spectra via a spectral moment expansion. An illustrative application demonstrates the usefulness of the expansion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464348
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  • 10
    Electronic Resource
    Electronic Resource
    Nuclear dynamics of several decaying overlapping electronic states: A time-dependent formulation (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 5871-5884 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The nuclear dynamics accompanying the excitation of a polyatomic target to a manifold of decaying overlapping electronic states and the subsequent decay to final electronic states are discussed. Nonadiabatic interactions within the manifold of decaying states as well as of final states are included. The cross sections for the excitation and for the production of final states are derived. The wave packet dynamics are investigated for both the excitation and the decay and related to the corresponding cross sections. Particular attention is paid to core levels of systems with equivalent atoms. The time-dependent formulation is also used to compute the gross features of the observed spectra via a spectral moment expansion. An interesting example is investigated where the decaying as well as the final states have extremely localized electron vacancies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465940
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