Abstract
The pathway toward the tailored synthesis of materials starts with precise characterization of the conformational properties and dynamics of individual molecules. Electron spin resonance based scanning tunneling microscopy can potentially address molecular structure with unprecedented resolution. Here, we determine the fine structure and geometry of an individual TiH molecule, utilizing a combination of a newly developed mK ESR-STM in a vector magnetic field and ab initio approaches. We demonstrate a strikingly large anisotropy of the g-tensor unusual for a spin doublet ground state, resulting from a non-trivial orbital angular momentum stemming from the molecular ground state. We quantify the relationship between the resultant fine structure, hindered rotational modes, and orbital excitations. Our model system provides new avenues to determine the structure and dynamics of individual molecules.
Highlights
Determining the fine structure, dynamics, and geometry of an individual molecule, with sub-molecular resolution, is a grand challenge in numerous fields of nanoscience
Starting from ab initio quantum chemistry, we consider the titanium-hydride molecule (TiH) molecule in the gas phase with C∞v symmetry, in order to fully account for the orbital angular momentum
We present results for a simple point-charge model of the surface, as well as embedded-cluster calculations on the complete-active-space multiconfigurational selfconsistent-field (CASSCF) level and the internally contracted multireference configuration interaction (MRCI) level, which we did with the Molpro quantum chemistry code [40]
Summary
Determining the fine structure, dynamics, and geometry of an individual molecule, with sub-molecular resolution, is a grand challenge in numerous fields of nanoscience. Complementary to imaging, SPM-based inelastic excitation spectroscopy (ISTS) has been successfully applied to infer the various intramolecular vibrational [5], rotational [6,7] or hindered rotational modes [8] These methods lack the precision to quantify the interplay between structure and molecular geometry like methods such as electron spin resonance (ESR) [9,10]. In the light of the inability of conventional density functional theory (DFT), as well as the mean-field DFT+U approach, to describe these experimental results, we adapted an approach based on quantum chemistry and exact quantum dynamics, to properly account for the correlations in this molecular system Using this approach, we include the Coulomb interactions generated by the ions of the surface, and illustrate how the spin quartet electronic ground state of the isolated TiH molecule transforms into a doublet state as it approaches the MgO surface. We quantify the g-tensor in embedded cluster calculations, which yield good agreement with the experiments, and enable determination of the structure and low-energy excitations of the molecule
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