The nuclear dynamics accompanying the excitation to and the subsequent decay of an electronic state is discussed. Particular attention is paid to cases, in which the whole process cannot be divided into two steps (excitation and decay) since the excitation and the decay times are of the same order of magnitude. The recently introduced time-dependent formulation of the theory describing the wave packets' dynamics is extended to include the excitation process. The wave packets can be related to the intensity of the emitted particles. Most of the resulting integrals can actually be performed by employing eigenstates of the Hamiltonians corresponding to the involved potential energy surfaces. This leads to the so called ''timeindependent'' formulation of the theory. Computational details of the implementation of the corresponding ''timedependent'' and ''time-independent'' methods are presented. Illustrative applications are given to illuminate both the influence of the excitation process and the lifetime of the decaying state. It emerges that the intuitive interpretation of the spectra (within the above two step model) may fail. Insight into the process is gained by studying the evolution of the spectra as a function of time. The appearance of ''atomic lines'' due to dissociative decaying and final states is investigated in some detail.PACS: 33.80.!b; 32.80.Hd; 42.55.Vc I IntroductionThe spectroscopic methods which involve the appearance of an excited decaying electronic state can roughly be divided into absorption or emission spectroscopies. In the first class the excitation process, i.e. the transition between the initial and the intermediate, decaying state is observed. Examples are conventional optical absorption spectroscopies and electron energy loss spectroscopy (EELS). In the emission spectroscopies the decay of the excited electronic state is studied. Under these spectroscopies come methods like X-ray emission spectroscopy (XES), Auger electron spectroscopy (AES) or the autoionization of coreexcited states.This classification is useful if the effects of nuclear dynamics on the detected spectra are considered. In absorption spectroscopies the nuclear dynamics leads in combination with the finite lifetime of the excited state only to a broadening of the observed bands. In emission spectroscopies, however, additional effects due to interference phenomena may occur. Apart from band broadening the bands can be energetically shifted and asymmetric band forms may be introduced [1, 2]. These effects are the strongest if the lifetime of the decaying state is in the range of the typical times of internal vibrations in this state.If the excitation time is of the same order-of-magnitude as both the lifetime of the decaying state and the typical time of internal vibrations, further effects in the spectra may appear. Then, the observed spectra are also influenced by the duration and other details of the excitation. In these cases the excitation process cannot be regarded as an instantaneous process, which can be separated...

The C–Cl bond activation by Au/Pd bimetallic alloy nanocatalysts has been investigated with regard to the oxidative addition of chlorobenzene (PhCl). Fifteen stable structures of the Au10Pd10 nanocluster (NC) obtained by a genetic algorithm were examined by DFT calculations using the M06-L, TPSS, and B3LYP functionals. Triplet states of cage-like C1 and Cs structures are found to be relevant reflecting the quasi-degenerate nature of the Pd moiety, while several other low-lying structures and spin states may also contribute to the oxidative addition. For all examined cluster structures, the oxidative addition step is exothermic, and internal conversion and/or spin crossing are expected to occur as several states are close in energy and geometry. Based on an energetic analysis of a model system consisting of the Au10Pd10 NC surrounded by four poly(n-vinylpyrrolidone) (PVP) molecules, the PVP units activate the system as electron donors and stabilize it. While a neutral NC model overestimates the energy barrier slightly, the opposite holds for an anionic NC model. In the oxidative addition, the interaction between the phenyl group and the Pd atom on the NC surface as well as a dissociation taking place at the Pd site are found to be essential. This indicates the importance of direct coordination effects in the Au/Pd bimetallic NC. NBO analysis shows that a π back-donation of the M(dπ) to σ*(C–Cl) orbital is relevant for the C–Cl bond activation and the interaction energy explains the favorable dissociation at the Pd site compared to the Au site.

The cohesive energies of argon in its cubic and hexagonal closed packed structures are computed with an unprecedented accuracy of about 5 J mol(-1) (corresponding to 0.05 % of the total cohesive energy). The same relative accuracy with respect to experimental data is also found for the face-centered cubic lattice constant deviating by ca. 0.003 Å. This level of accuracy was enabled by using high-level theoretical, wave-function-based methods within a many-body decomposition of the interaction energy. Static contributions of two-, three-, and four-body fragments of the crystal are all individually converged to sub-J mol(-1) accuracy and complemented by harmonic and anharmonic vibrational corrections. Computational chemistry is thus achieving or even surpassing experimental accuracy for the solid-state rare gases.

Although studied experimentally for centuries, the melting of solids is still a fascinating phenomenon whose underlying mechanisms are not yet well understood.[1] Predicting melting points is a nontrivial task: The standard computational method relies on analyzing the free energies of the solid and liquid phases obtained independently by thermodynamic [2] or Gibbs-Duhem [3] integration; this approach suffers from the difficulty of calibration. Alternatively, in coexistence methods the interface between the two phases must be described explicitly; [4] this interface is often hard to stabilize.[5]An alternative idea, which we pursue herein, is to obtain information about the melting transition by studying finite clusters and extrapolating the results to infinitely large systems. Here we present for the first time calculated melting temperatures reaching experimental accuracy obtained from Monte Carlo simulations of Ne N and Ar N clusters consisting of a "magic number" N of atoms (N = 13, 55, 147, 309, 561, 923) and of bulk samples. This was achieved by the use of accurate interaction potentials obtained from precise ab initio data having the same computational efficiency as the widely used empirical Lennard-Jones (LJ) potential, and without any experimental input whatsoever. Argon and neon adopt the face-centered cubic (fcc) periodic packing in the solid state, but their clusters with N < 1000 are most stable as complete Mackay icosahedra. The number of atoms in the first six shells of the cluster corresponds to the "magic numbers": N = 1 + 2 P n k¼1 (5k 2 +1) = 13, 55, 147, 309, 561, and 923.[6] These "magic numbers" have been provided by mass spectra in free-jet expansions of rare-gas clusters. [7,8] The unusual stability of these clusters is explained by the structure in which one to six completed shells of atoms surround a central atom (see the inset of Figure 1). [6] Previous work on determining melting curves for rare-gas clusters include Monte Carlo (MC) simulations by Labastie and Whetten [9] (N = 13, 55, 147). They found a well-defined single peak in the heat capacity, which becomes higher more intense and narrower as the cluster size increases. The apparent convergence towards the bulk limit was, however, questioned by Noya and Doye, [10] who found an additional premelting peak for the 309-atom cluster. As we show below, even though complicated premelting phenomena remain for all clusters with more than 309 atoms, the melting peak itself always turns out to be well defined. Furthermore, and to our knowledge, the 309-atom system is the largest rare-gas cluster studied so far for melting. To determine the bulk melting point T m by extrapolation to the macroscopic limit, inclusion of two more shells is required, as we demonstrate in this work.

An old problem solved: Monte Carlo simulations using the diatomic-in-molecule method derived from accurate ground- and excited-state relativistic calculations for Hg2 show that the melting temperature for bulk mercury is lowered by 105 K, which is due to relativistic effects.