This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Molecules can be placed in a wide variety of quantum states. How can these states be distinguished in measurements when theoretical models are unreliable?
Which excitation process lies behind which measurement signal? These were precisely the questions facing Dr. Arnab Banerjee when he investigated individual cobaltocene molecules using tunneling spectroscopy.
The measured spectra showed a confusing variety of excitations. Together with four colleagues in Kiel and San Sebastian (Spain), he succeeded in deciphering the data in a new way. The findings were recently published in the journal Physical Review Letters and highlighted by the editors as an "Editors' Suggestion." The researchers investigated the molecule cobaltocene on a lead surface that becomes superconducting at very low temperatures, meaning that it conducts electricity without resistance.
To do so, the team used a scanning tunneling microscope. They brought its metallic tip so close to the molecule that electrons could cross the tiny gap between the tip and the molecule. By changing the energy of the electrons and recording the resulting current, the researchers obtained a spectrum: A sudden increase in current signals that a process is triggered in the molecule at that energy.
But a single jump does not reveal which process is actually behind it, and things become particularly confusing when many different excitations occur in the same spectrum. To distinguish the signals, the team moved electrons into and out of the molecule and also investigated the effect of a strong magnetic field. Together, the two approaches reveal the type of excitation, whether it is a vibration of the molecule, a flipping of an electron spin or an electron hopping between different orbitals.
The measurements show not only the energies at which excitations occur. The spatial distribution of the probability of excitation was also determined with submolecular resolution. A comparison with the results of elaborate calculations (density functional theory, time-dependent extension TDDFT) provided further confirmation of the assignment of the measurement signals to the molecular states involved.
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