transition metal clusters; four-wave mixing; transition metal dimers; spectroscopy; free jet; laser-vaporization sources; catalysis
Beck M., Bornhauser P., Visser Bradley, Knopp G., Bokhoven J. A. van, Radi P. P. (2019), Spectroscopic disentanglement of the quantum states of highly excited Cu2, in Nature Communications
, 10(1), 3270-3270.
Visser Bradley, Beck Martin, Bornhauser Peter, Knopp Gregor, van Bokhoven Jeroen Anton, Radi Peter, Gourlaouen Christophe, Marquardt Roberto (2019), New experimental and theoretical assessment of the dissociation energy of C 2, in Molecular Physics
Visser B., Beck M., Bornhauser P., Knopp G., van Bokhoven J. A., Marquardt R., Gourlaouen C., Radi P. P. (2017), Identification of a new low energy 1 u state in dicopper with resonant four-wave mixing, in The Journal of Chemical Physics
, 147(21), 214308-214308.
Beck M., Visser B., Bornhauser P., Knopp G., van Bokhoven J. A., Radi P. P. (2017), Rovibrational Characterization of High-Lying Electronic States of Cu 2 by Double-Resonant Nonlinear Spectroscopy, in The Journal of Physical Chemistry A
, 121(44), 8448-8452.
Kouzov A.P., Radi P.P. (2017), Line space theory of Resonant Four-Wave Mixing: New prospects for all-optical studies of photofragment states, in Chemical Physics Letters
, 673, 103-107.
Based on the results obtained in the preceding proposal, we outline experiments that are focused on further studies of transition metal dimers. It is proposed to perform spectroscopic experiments on the homonuclear transition metal dimers Cu2 and Cr2 using the degenerate four wave mixing (DFWM) and two-color resonant four-wave mixing (TC-RFWM) experimental techniques, with a strong focus on low energy electronic structure in both species. Since it is the low energy excited states that are responsible for much of the reactivity and catalytic activity observed in reactive species, the proposed measurements will contribute significantly to the understanding of these properties in Cu2 and Cr2. Even the coinage metals, which are typified by filled shell ground states have a large density of states at relatively low energies due to ``d-hole'' atomic electronic configurations and these states interact in a complex way with those from ligands to lead to the formation of chemical bonds. In addition to dicopper and dichromium, studies of low-lying energy levels of heteronuclear dimers containing Zn, Cu, Ni, Pd, Pt are envisaged.Many of the states that are responsible for the chemical activity of these species are long lived and are difficult to probe in the gas-phase using traditional single resonance techniques such as laser induced fluorescence (LIF) or resonance enhanced multiphoton ionization (REMPI). States with long lifetimes normally arise when transitions to lower energy states are forbidden due to spectroscopic selection rules and this makes such states very difficult to probe from the ground state. The usefulness of single resonance spectroscopic techniques also tends to be limited due to the lack of spectral selectivity. This can often mean that the carriers of spectral features cannot be absolutely assigned and that strong transitions in other species can obscure those in the target.Two-color resonant four-wave mixing (TC-RFWM) is a double-resonance technique and does not have the shortcomings of the aforementioned spectroscopic methods. It has previously been employed by our research group to study C2, C3 and other small radicals and molecules and very recently in experiments on Cu2 and Cr2. These studies show that the technique is sensitive enough to make measurements on such species in the environment of a molecular (cluster) beam. As TC-RFWM is an absorption based technique, states that do not relax through radiative processes can still be measured. TC-RFWM is also highly specific and the spectra of different molecules and even isotopologues can easily be distinguished. This advantage is extremely important in disentangling spectrally dense regions, such as those proposed in this study. The experimental setup is, to the best of our knowledge, unique and comprises in addition to the laser-vaporization source and four-wave mixing setup, the possibility for sensitive (and simultaneous) laser-induced fluorescence (LIF)and cavity ring-down (CRD) measurements. Furthermore, the mass spectrometer integrated in the experiment is applicable for REMPI and fs-ionization studies.The transition metal dimers are to be produced with a laser vaporization source that has been recently commissioned in our laboratory. Initial experiments using copper disk targets have shown that sufficient quantities of the copper dimer are produced in the source to allow for DFWM and TC-RFWM measurements in the cluster beam. Chrome surfaces of multi-micrometer thickness have been produced by electroplating copper discs with chromium. Other transition metal targets that are not available commercially can be produced via sputtering processes. The cluster source is to be developed further to increase the production of dimers and to promote the growth of clusters. Improving the cluster source will be of great assistance for future experiments at the ATHOS beamline of the SwissFEL, which is currently being built at the Paul Scherrer Institut.The high experimental accuracy poses challenges and opportunities for theory. High level, state-of-the-art ab initio calculations for low lying states of dicopper have been performed in collaboration with the theoretical group at the University of Strasbourg lead by Prof. Roberto Marquardt. The results are in good agreement with the experiments and the computation will be continued for the species investigated in this proposal. The accurate electronic structure calculations will simplify the experimental search for unknown electronic states greatly and assist in the selection of TC-RFWM pumping schemes. Unfortunately, less demanding density functional theory computations have proven to be inaccurate for even the copper dimer.