Book of Abstracts Book of Abstracts - Universität Konstanz

Oxidation Effects of the Small Chromium and Manganese Cluster
Ions
K. Tono, 1,2 A. Terasaki, 2 T. Ohta, 1 and T. Kondow 2
1 Department of Chemistry, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
2 Cluster Research Laboratory, Toyota Technological Institute in East Tokyo Laboratory, Genesis
Research Institute, Inc., 717-86 Futamata, Ichikawa, Chiba 272-0001, Japan
Properties of transition-metal clusters are affected by reaction with foreign elements such as
nitrogen and oxygen. For example, it has been predicted that the magnetic property of a
chromium dimer is drastically changed by either nitridation or oxidation [1,2]. Motivated by
these theoretical studies, we investigated the electronic structures of the chromium- and
manganese-oxide cluster anions through measurements of photoelectron spectra and analyses
with the aid of the density functional theory (DFT). It was found that the spin magnetic moment
of Cr2O – is as large as 9 µB, while that of Cr2 – is 1 µB. This result shows that the spin coupling
between the chromium sites becomes ferromagnetic upon oxidation of Cr2 – whose chromium
sites are coupled antiferromagnetically [3,4]. The mechanism of the ferromagnetic spin coupling
is explained on the basis of the geometric and electronic structures obtained by the DFT
calculations. The oxygen atom at the bridge site weakens the strong bond between the
chromium atoms, which is responsible for the antiferromagnetic coupling in Cr2 – . In addition,
significant hybridization between chromium 3d and oxygen 2p orbitals allows electrons in the
same spin state to be transferred from the two chromium atoms to the oxygen atom.
Consequently the local spins on the chromium sites are bound to form a ferromagnetic coupling
in a similar scheme of the superexchange interaction encountered in solids. An oxidation effect
of a similar kind was found for Mn2O – , in which a ferromagnetic coupling between the local
spins (about 5.5 µB) at the manganese sites stabilizes its electronic structure [5,6].
Manganese clusters differ distinctly from chromium clusters; the manganese atoms are very
weakly bound because of the closed shell of 4s electrons. This is also true to singly charged
manganese cluster cations, MnN + (N=3–7), where the binding energy is found to be less than 1
eV/atom as reported in Ref. [7] and as shown further by the present measurements. The
structural specificity results from a weak van-der-Waals like interaction between the constituent
atoms, and is likely to change greatly if one introduces an oxygen atom into MnN + , because the
oxygen could glue manganese atoms in its vicinity. Actually we elucidated how the introduced
oxygen changes the geometrical and electronic structures of MnN + in photodissociation
experiments of MnNO + (N=3–5). In practice, the mass spectra of photodissociation products
from MnNO + were measured as a function of the photon energy of an excitation laser. The
results show that the oxygen atom and two manganese atoms form a Mn2O + core ion and the
other manganese atoms are weakly bound to the core ion. Analyses of the photodissociation
action spectra of MnNO + are now in progress in order to obtain detailed information on the
electronic and geometric structures.
References
[1] S. E. Weber, B. V. Reddy, B. K. Rao, and P. Jena, Chem. Phys. Lett. 295, 175 (1998).
[2] B. V. Reddy and S. N. Khanna, Phys. Rev. Lett. 83, 3170 (1999).
[3] K. Tono, A. Terasaki, T. Ohta, and T. Kondow, Phys. Rev. Lett. 90, 133402 (2003).
[4] K. Tono, A. Terasaki, T. Ohta, and T. Kondow, J. Chem. Phys. 119, 11221 (2003).
[5] K. Tono, A. Terasaki, T. Ohta, and T. Kondow, Chem. Phys. Lett. 388, 374 (2004).
[6] S. N. Khanna, P. Jena, W.-J. Zheng, J. M. Nilles, and K. H. Bowen, Phys. Rev. B 69, 144418
(2004).
[7] A. Terasaki, S. Minemoto, and T. Kondow, J. Chem. Phys. 117, 7520 (2002).
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