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Physical Chemistry

Revealing Pictures Of Molecular Orbitals

In a new use, photoelectron spectroscopy provides distribution maps of electrons in molecular orbitals

by Jyllian N. Kemsley
September 14, 2009 | A version of this story appeared in Volume 87, Issue 37

Credit: Science
ARPES allowed experimental mapping of p-sexiphenyl’s LUMO (top) and HOMO (bottom).
Credit: Science
ARPES allowed experimental mapping of p-sexiphenyl’s LUMO (top) and HOMO (bottom).

Photoelectron spectroscopy can provide information not just on electron binding energies but also on the structure of the molecular orbitals from which the electrons come, reports a group led by Peter Puschnig of the University of Leoben, in Austria (Science, DOI: 10.1126/science.1176105). Angle-resolved photoelectron spectroscopy (ARPES) normally involves measuring the kinetic energy and angular distribution of electrons emitted from solids to determine the electron binding energies. Puschnig and colleagues show that scientists can also use the emission intensity to map the distribution of electrons, providing a snapshot of the molecular orbital structure. In experiments using a crystalline pentacene film, the researchers mapped the highest occupied molecular orbital (HOMO) with good agreement to the same orbital computed for gas-phase pentacene. And in experiments on a chemisorbed monolayer of p-sexiphenyl, the researchers mapped both the HOMO and the lowest unoccupied molecular orbital (LUMO). ARPES could become a valuable tool for investigating organic films and monolayers, complementing techniques such as scanning tunneling microscopy and near-edge X-ray absorption fine structure spectroscopy, the researchers say.


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