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Direct probing of low-energy intra d-band transitions in gas-phase cobalt clusters

By: Kevin A. Kaw, Rick J. Louwerse, Joost M. Bakker, Peter Lievens, Ewald Janssens & Piero Ferrari



In the interstellar medium, many molecules and clusters are found in isolation. Suppose they get excited by the absorption of high-energy stellar radiation or collisions with cosmic particles. In that case, their isolation prevents them from transferring their heat to other species by collisions. Instead, they do so by fragmentation, electron emission, or radiation, with the latter process coming in two flavors, recurrent fluorescence (RF) and vibrational cooling (VC). Photon emission in VC happens through a cascade of vibrational transitions, making it a slower mechanism than fragmentation. RF photon emission takes place from an electronic excited state populated by the conversion of vibrational energy into electronic energy. A key difference between both types of radiation is their timescales: if the RF mechanism is available, it can be much faster, providing the molecule or cluster with a rapid relaxation mechanism protecting it from fragmentation. Such a mechanism may determine which molecules survive in the harsh interstellar conditions, and which do not.

The time-dependent density functional theory (TD-DFT) formalism was employed, for which the resources from the Flanders Supercomputing Center (VSC) were paramount.

In the past, our laboratory experiments have – indirectly – shown that RF cooling is a much more widespread phenomenon than was earlier accepted, and also prevalent in small metal clusters. However, a smoking gun for the presence of RF was not shown. For that mechanism to be viable, low-lying electronic states need to be available.


In this work, we directly probed such low-lying electronic transitions from which excited cobalt clusters radiate, using a technique called infrared multiple-photon dissociation (IRMPD) spectroscopy. Here, clusters were prepared, labeled with a weakly bound noble gas atom, and irradiated by the light of the free-electron laser FELIX in Nijmegen (The Netherlands). Upon resonant light absorption through low-energy electronic states, the noble gas atoms were desorbed from the clusters, allowing the construction of their optical absorption spectra at low excitation energies.


Accurate quantum chemical calculations were critical to understanding the nature of the probed transitions. The time-dependent density functional theory (TD-DFT) formalism was employed, for which the resources from the Flanders Supercomputing Center (VSC) were paramount. To properly describe the complex electronic structure of the cobalt clusters it was necessary to (implicitly) treat all the electrons of the system, using large basis sets and direct inclusion of relativistic effects. Because such calculations scale non-linearly with the number of electrons, they are computationally demanding and can only be performed on supercomputers. A comparison between experimental data and theoretical calculations obtained with the use of VSC is presented in Figure 1.

Figure 1. Schematic of the experimental instrument at FELIX (left), the experimental results for Co8+-Kr complex (top-right), compared to the calculated electronic absorption spectra of Co8+-Kr and its geometry (bottom-right) obtained using the VSC facilities.

Figure 1. Schematic of the experimental instrument at FELIX (left), the experimental results for Co8+-Kr complex (top-right), compared to the calculated electronic absorption spectra of Co8+-Kr and its geometry (bottom-right) obtained using the VSC facilities.


Computational results reveal the presence of electronic transitions in the energy ranges probed experimentally, which are assigned as intraband d-transitions arising from the high spin multiplicity of the clusters. We hypothesize that many other transition metal clusters, with high spin states and open d-shells, should have similar low-lying electronic states.


 

For further information, please read the open-access paper published in Communications Chemistry (https://www.nature.com/articles/s42004-024-01206-2).

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