Many-body effects in weakly bound anion and neutral clusters: Zero electron kinetic energy spectroscopy and threshold photodetachment spectroscopy of Ar n Br (n2–9) and Ar n I (n2–19 (original) (raw)
The anion zero electron kinetic energy ZEKE spectra of the van der Waals clusters Ar 2-3 Br and Ar 2-3 I have been measured, and partially discriminated threshold photodetachment PDTP experiments have been performed on Ar 4-9 Br and Ar 8-19 I. The experiments yield size-dependent adiabatic electron affinities EAs and electronic state splittings of the halogen atom in the neutral clusters formed by photodetachment. These results are compared with simulated annealing calculations using model potentials for the anion and neutral clusters, making use of the neutral and anion pair potentials determined from previous work on the diatomic rare gas–halide atom complexes Y. Zhao, I. Yourshaw, G. Reiser, C. C. Arnold, and D. M. Neumark, J. Chem. Phys. 101, 6538 1994. A simple first-order degenerate perturbation theory model W. G. Lawrence and V. A. Apkarian, J. Chem. Phys. 101, 1820 1994 of the neutral cluster potentials was found to agree well with the size-dependent splitting of the halogen 2 P 3/2 state observed in the ZEKE spectra. However, the binding energies calculated from the pair potentials alone were found to be inconsistent with the experimental electron affinities, and it was necessary to include various nonadditive terms in the simulated annealing calculations to obtain reasonable agreement with experiment. Many-body induction in the anion clusters was found to be the dominant nonadditive effect. The exchange quadrupole effect—i.e., the interaction of the exchange induced electron charge distribution distortion among argon atoms with the halide charge—was also found to be important. This comparison between experiment and theory provides a sensitive probe of the importance of nonadditive effects in weakly bound clusters.
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2001
Ar n Cl Ϫ clusters have been investigated by anion zero electron kinetic energy ͑ZEKE͒ and partially discriminated threshold photodetachment spectroscopy. The experiments yield size-dependent electron affinities ͑EAs͒ and electronic state splittings for the X, I, and II states accessed by photodetachment. Cluster minimum energy structures have been determined from calculations based on a ''simulated annealing'' approach employing our recently presented Ar-Cl ͑Ϫ͒ pair potentials from anion ZEKE spectroscopy ͓T. Lenzer, I. Yourshaw, M. R. Furlanetto, G. Reiser, and D. M. Neumark, J. Chem. Phys. 110, 9578 ͑1999͔͒ and various nonadditive terms. The EAs calculated without many-body effects overestimate the experimental EAs by up to 1500 cm Ϫ1 . Repulsive many-body induction in the anion clusters is found to be the dominant nonadditive effect. In addition, the attractive interaction between the chloride charge and the Ar 2 exchange quadrupole is important. These findings are consistent with our earlier results for Xe n I Ϫ , Ar n I Ϫ , and Ar n Br Ϫ clusters and highlight again the necessity of an adequate implementation of many-body effects to describe the energetics of such systems. For Ar n Cl Ϫ clusters with nϾ12 we find some deviations between experimental and calculated ͑0 K͒ EA which can be explained by the population of less stable anion structures due to the finite temperatures of the clusters in our experiments. This results in lower EAs than predicted for the corresponding global minimum energy structures.
Effects of three-body interactions on the structure of clusters
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The structure of 54 and 147 atom clusters was studied using molecular dynamics at constant temperature and incorporating three-body interactions to the usual pairwise additive potentials among atoms. The effects of three-body interactions of the triple-dipole and exchange ...
Electron attachment to strongly polar clusters
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Electron localization is studied in formamide cluster anions. The isolated formamide molecule has a large dipole moment and its clusters can give birth to multipole-bound anions as well as valence anions. The vertical valence electron affinity of the isolated molecule is determined by electron transmission spectroscopy. The anion formation process is studied as a function of cluster size with Rydberg electron transfer spectroscopy. DFT calculations of the neutral and negatively-charged cluster structures show that the anion excess electron localizes on a single molecule. The adiabatic valence electron affinity of isolated formamide is deduced from the observation of the cluster size threshold for valence attachment. PACS. 34.80.Gs Molecular excitation and ionization by electron impact -36.40.Mr Spectroscopy and geometrical structure of clusters -87.15.By Structure and bonding
Journal of Physical Chemistry A, 1997
An experimental spectroscopic study of the van der Waals aniline-Ar n clusters (n ) 1-6) has been achieved via resonant two-photon two-color ionization, whose results improve the accuracy of the previously reported spectra and complete them by the observation of further structure in the spectra of well-defined structural isomers. These experimental results have been analyzed in detail in relation with molecular dynamics simulations including minimum energy structures calculations, direct spectral simulations, and isomerization dynamics. Two potential energy surfaces, in both the ground S 0 and the electronically excited S 1 states, have been built in order to recover the main experimental observables. They permit the site specific electronic shift additivity rule to be confirmed by the semiclassical spectral simulations and a global picture of the solvation phenomena in this aromatic-rare gas atom system to be provided. In particular, careful examination of the isomerization dynamics helps the general understanding of the structural behavior of these microsolvent clusters.
Angle-resolved photoelectron spectra of metal cluster anions within a many-body-theory approach
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A consistent many-body theory based on the jellium model is applied for the description of angular resolved photoelectron spectra of metal clusters anions. The results of calculations demonstrate the dominant role of the many-body effects in the formation of angular distributions of photoelectrons emitted from sodium clusters and are in a good agreement with recent experimental data. The concrete comparison of theory and experiment has been performed for the photoionization of Na 7 − and Na 19 − anions being characterized by the entirely closed shells of delocalized electrons.
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The 3p valence region of argon clusters has been investigated with photoemission near the photoionization threshold. A strong feature between 14.6 and 15.3 eV binding energy shows a photon-energy dependence indicative of electronic-energy band dispersion. A similar feature at approximately the same binding energy and with a similar dispersion occurs in photoemission spectra of both the ordered and disordered solids. The effect is already fully developed for scaling-law mean cluster sizes ͗N͘ of approximately 200 atoms, thus showing an early onset of bulklike electronic properties.
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The results of an experimental study of the ionisation threshold in the various structural isomers of the Van der Waals clusters aniline-(argon) n (n = 1-5) and 4-fluorostyrene-(argon) n (n = 1-2) using resonant two-photon ionisation are reported. The data validate and generalise the site-specific modified shift additivity rule. They show an interesting influence of the localisation of argon atoms at the surface of the chromophore on the value of the net shift of the ionisation potential. Interpretation of the results involves evaluation and balancing between the charge-induced-dipole interaction and dispersion interaction.
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