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Register a new userLigand field parameters and the ground state of Fe(II) phthalocyanine
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A judicious analysis of previously published experimental data leads one to conclude that the ground state of iron(II) phthalocyanine is an orbitally degenerate spin triplet $a_{1g}^2 e_g^{uparrowdownarrowuparrow} b_{2g}^{uparrow}$ ($^3E_g$). The ligand field parameters, in relation to Racahs $C$, are approximately as follows: $B_{20}/C=0.84$, $B_{40}/C=0.0074$. The uniqueness of this result is demonstrated by means of a special diagram in the $B_{20}/C-B_{40}/C$ plane (under additional conditions that $B_{44}/B_{40}=35/3$ and $B/C=0.227$). The system is in a strong-ligand-field regime, which enables the use of single-determinant techniques corrected for correlations within the 3d shell of Fe.
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We calculate the angular dependence of the x-ray linear and circular dichroism at the $L_{2,3}$ edges of $alpha$-Fe(II) Phthalocyanine (FePc) thin films using a ligand field model with full configuration interaction. We find the best agreement with the experimental spectra for a mixed ground state of $^3E_{g}(a_{1g}^2e_g^3b_{2g}^1)$ and $^3B_{2g}(a_{1g}^1e_g^4b_{2g}^1)$ with the two configurations coupled by the spin-orbit interaction. The $^3E_{g}(b)$ and $^3B_{2g}$ states have an easy axis and plane anisotropies, respectively. Our model accounts for an easy-plane magnetic anisotropy and the measured magnitudes of the in-plane orbital and spin moments. The proximity in energy of the two configurations allows a switching of the magnetic anisotropy from easy plane to easy axis with a small change in the crystal field, as recently observed for FePc adsorbed on an oxidized Cu surface. We also discuss the possibility of a quintet ground state ($^5A_{1g}$ is 250~meV above the ground state) with planar anisotropy by manipulation of the Fe-C bond length by depositing the complex on a substrate that is subjected to a mechanical strain.
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