Electronic ground states of Fe2+ and Co2+ as determined by x-ray absorption and x-ray magnetic circular dichroism spectroscopy

Abstract

The 6 Π electronic ground state of the Co + 2 diatomic molecular cation has been assigned experimentally by x-ray absorption and x-ray magnetic circular dichroism

The carbon and oxygen K-edge NEXAFS spectra of CO+

“…1 Apart from in depth studies of the X-ray absorption process itself, effective and simplified rules have been proposed for the spectral analysis and applied for a wide range of species, like one-center rules, building block and bond length with ruler principles, and, for fixed in space species, orientational probing. [1][2][3][4] Owing to recently improved techniques to trap molecular cations in sufficient amount and time to be measurable by X-ray synchrotron beams, [5][6][7][8][9][10][11] highresolution NEXAFS of such species has now become a realistic proposition.…”

Large orbital magnetic moments of small, free cobalt cluster ions Co${_n^+}$ with n ${ \leqslant 9}$

“…[13,15] In t ho e calculation , the orbital magnetic moments per atom, f.LL :::; 0.7 f.LB , determined by XMCD, of small Co;t clusters with n 2: 8 and of the cobalt diatomic molecular cation (f.LL = 1 f.LB) were already used as a benchmark. [18,10,8] While in the meantime f.Ls and f.LL from XMCD measurements have also been reported for the cobalt trimer cation [1 9] with f.LL :::; 0.5 f.LB per atom, accura te experimental values for the cluster size range from n = 4 -9, where 3D cluster sLrucL ures appear, are sL ill missing. In the case of iron, already the smallest clusters have largely quenched orbital magnetic moments.…”

“…Instead, t he small orbital magnetic moment in Cot is due to its electronic configuration with three electrons in a doubly degenerate orbital of 1r symmetry with ml = ± 1. [18] The considerable difference between the orbital-to-spin magnetic moment ratio of Cot 5 and Cot indicaLes an abrupL change in the symmetry of Lhe 3d derived ' states between n = 5 and n = 6. Overall the ratio in the n = 2 15 cluster size range is considerably higher, by a factor of two to nine, than in bulk cobalt [3] , as indicated in figure 2.…”

“…Experimcntatly determined orbital-to-spin magnetic moment ratio as a function of cluster size. For comparison the values for Co;t clusters with n = 2, 3, 10 -15 [18,19,10] and t he value of 0.07 for bulk cobalt [3] are also shown. Note t hat the ratio for the cobalt cation in its [Ar]3d 8 3 F ground state is 1.5 and for the cobalt atom with [Ar]3d 7 4s 2 4 F ground state is 1, both of which ru·e outside the scale that is shown here.…”

“…For the neutral species, a recent theoretical swdy [13] predicted I-LL = 0.67 -0.74 I-LB per atom for Co 4 -g clust ers, in reasonable agreement with experimental results for cationic clusters Co;i with n = 2, 8, 9 available at the time. [8,18] It should be noted that orbital magnet ic moments;::: 0.5 /-LB per atom are only predicted by theory when explicitly taking into account the orbital dependence of the on-site Coulomb interactions. [15,13] In contrast to theoretical predictions for neutral clust ers [13], the results presented here indicate t hat the orbital magnetic moment per atom has a strong size-dependence, strongly increasing with increasing clust er size for Co;i 2 ~ n ~ 5 and decreasing for clusters with n ;::: 6.…”