Angle-dependent Mössbauer spectroscopy in the ground and metastable electronic states in Na2[Fe(CN)5NO]⋅2H2O single crystals

Abstract

Upon excitation of the metastable electronic state $S\mathrm{I}$ by irradiation with light in the blue-green spectral range at temperatures below $T=200\mathrm{}$ K the electron density in the $\left[\mathrm{Fe}\right(\mathrm{CN}{)}_5\mathrm{NO}{]}^{2-}$ anion is rearranged mainly in the region of the Fe-N-O bonds, resulting in the drastic increase of the quadrupole splitting and a more positive isomer shift. But the local symmetry of the first coordination sphere around the Fe central atom is conserved. The positive sign of the electric-field gradient is unaffected by the new electron density at the ${}^{57}\mathrm{Fe}$ central atom. The direction of the z component of the electric-field gradient with respect to the crystallographic a axis increases only by 0.4°. The degeneracy of the $\mathrm{Fe}{(3d}_{xz,yz})$ and $\mathrm{Fe}{(4p}_{x,y})$ orbitals is not changed. The optically excited anions are independent from each other and have no influence on the anions in the ground state. At every value of the population, the Mössbauer parameters of both states are not altered, only the number density of transfered anions is changed. These results are determined by measuring the sign and orientation $\alpha$ of the electric-field gradient, the asymmetry parameter $\eta ,$ the ${}^{57}\mathrm{Fe}$ Debye-Waller factors $f_a,$ $f_b,$ $f_c$ of the orthorhombic crystal and the mean-square displacement tensor in the ground and metastable state $S\mathrm{I}$ at $T=80\mathrm{}$ K by angle dependent Mössbauer spectroscopy. Additionally these parameters are also determined at $T=294\mathrm{}$ K for comparison. The dependence of the linewidth $\Gamma$ on the crystal thickness, measured at $T=294\mathrm{}$ K, yields again the Debye-Waller factors and at thickness zero the deviation from the natural linewidth ${\Gamma }_N,$ given by the linebroadening of the $\gamma$ source.