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In sharp contrast to molecules, electronic states of clusters with an excited intermediate-shell electron can efficiently decay via an intermolecular Coulombic mechanism. Explicit examples are presented using large scale ab initio propagator calculations. The mechanism is illustrated and its generality is stressed. [S0031-9007(97) PACS numbers: 36.40.Cg, 31.50. + w, 34.50.Gb Quantum states of electronic systems typically decay by photon and/or electron emission. Energetically low lying states decay radiatively while highly excited levels involving the excitation of inner-shell electrons decay more efficiently by emitting an electron (Auger decay). It is only for very deep inner-shell electrons of heavy elements that x-ray emission constitutes the prominent decay channel [1]. States which can decay only radiatively, i.e., states with excited outer-shell electrons, exhibit an exceedingly long lifetime (ഠ nanoseconds) compared to Auger-decaying states. For instance, a vacancy in the 1s shell of a fluorine atom has a lifetime of about 3.3 fs (0.20 eV linewidth) [2]. This lifetime depends only weakly on whether the fluorine atom is free or integrated in a molecular system. The total decay rate is essentially determined by the neighboring atomic electrons. Deeper electron vacancies typically decay faster. An excited or ionized state of the sulfur atom with a vacancy in the 1s shell, for example, lives for about 1.6 fs [3], and if the vacancy is in the 2p shell the lifetime extends to 66 fs [4].As mentioned above, the environment of the atom influences only moderately the lifetime of the deep vacancy (e.g., of a F1s vacancy). If at all, we can only expect interesting environmental effects on the total Auger decay rate to take place for vacancies in intermediate shells.However, a closer look at the energetics of the decay in typical molecules brings a problem to light. The ionization potential (IP) of a F2s electron in, say, the HF molecule, is about 40 eV [5], but the release of these 40 eV is insufficient to ionize a second electron: the lowest double ionization potential (DIP) of HF is about 45 eV [6]. Similarly, the IP of an O2s electron in H 2 O is approximately 35 eV [7], again too small an energy to allow a second electron to leave the system. The lowest DIP of water is about 38 eV [8]. The situation changes substantially as we move to clusters. While the IP of intermediate shells differs only slightly from that of the monomer unit, some DIPs are lowered considerably in the cluster and the autoionization channel opens. More importantly, we shall also show in this Letter that the accessible decay becomes dramatically efficient in clusters.Atomic and molecular clusters have been subject to continuous interest over many years [9][10][11]. Most of the interest has been devoted to the possible geometrical structures and properties of the clusters in their electronic ground state. Much less attention has been paid to excited electronic states and no attention to highly excited states involving intermediate shell ele...

The energies needed to create different types of double core vacancies as well as the resulting redistribution of the valence electrons are analyzed in comparison with single core vacancies. Numerical results are presented for CH 4 and in particular for the molecules C 2 H 2 , C 2 H 4 , and C 2 H 6 • A detailed perturbation theory analysis of the relaxation energies in terms of localized and delocalized molecular orbital pictures is presented. It is shown that the binding energies associated with double core vacancies where each of the two core holes is at a different atomic site sensitively probe the chemical environment of the atoms.

The absorption spectrum of pyrazine to the S2 electronic state can be usefully described by a 4-mode system interacting with a 20-mode bath. In this paper wave packet propagation techniques, using the multiconfiguration time-dependent Hartree approach, are used to study this problem. The investigation was made in stages so as to study the nature of the wave function needed to correctly describe various properties of this multimode problem: the absorption spectrum; the energy exchange between the system and the bath; and the rate of inter-state crossing. It was found that, despite the relatively weak system–bath coupling, a multiconfigurational wave function was necessary to describe the interaction between the two parts of the problem. While it was not possible to treat the full 24-mode problem with such a wave function, the spectrum for a 14-mode system, which includes all the important bath modes, has been calculated in this way. The results, in agreement with the path integral calculations of Krempl et al. [J. Chem. Phys. 100, 926 (1994)], show that the effect of a model bath linearly coupled to the system is to reduce the vibrational structure of the spectrum, so as to produce a broad envelope analogous to that observed experimentally. The details of the spectrum are however different for the two methods. The effect of introducing anharmonicity to the bath was also studied, with the result that this leads to a yet broader spectrum.

When an electron is suddenly removed, a universal response of the system is shown to occur on an attosecond (10(-18) s) time scale. During this response time, which lasts about 50 attoseconds, the density of the created hole changes in a characteristic way. Explicit examples are shown. The results are analyzed in terms of the eigenstates of the residual ion and related to the filling of the exchange-correlation hole associated with the electron in the ground state of the system by the remaining electrons.