Pressure dependence of electronic structures and spin states of iron-chalcogenide Fe 1.01 Se superconductors up to ∼66 GPa has been investigated with x-ray emission spectra and x-ray absorption spectra with partial-fluorescence yield. The intensity of the pre-edge peak at energy of ∼7112.7 eV of the Fe K-edge x-ray absorption spectrum of Fe 1.01 Se decreases progressively with pressure up to ∼10 GPa. A new prepeak at energy of ∼7113.7 eV develops for pressure above ∼13 GPa, indicating formation of a new phase. The experimental and the calculated Fe K-edge absorption spectra of Fe 1.01 Se using the FDMNES code agree satisfactorily. The larger compression accompanied by significant distortion around the Fe atoms along the c axis in Fe 1.01 Se upon applying pressure suppresses the Fe 3d-Se 4p and Fe 4p-Se 4d hybridization. The applied pressure suppresses the nearest-neighbor ferromagnetic superexchange interaction and enhances spin fluctuations on the Fe sites in Fe 1.01 Se. A discontinuous variation of the integrated absolute difference values of the Kβ emission line was observed, originating from a phase transition of Fe 1.01 Se for a pressure >12 GPa. Fe 1.01 Se shows a small net magnetic moment of Fe 2+ at ambient pressure, probably arising from strong Fe-Fe spin fluctuations. The satellite line Kβ was reduced in intensity upon applying pressure and became absent for pressure >52 GPa, indicating a continuous reduction of the spin moment of Fe in Fe 1.01 Se superconductors. The experimental results provide insight into the spin state of Fe 1.01 Se superconductors under pressure.

Mn 3d valence states and elementary electronic excitations in single-crystalline TbMnO 3 were probed with resonant inelastic x-ray scattering ͑RIXS͒ ͑or resonant x-ray emission spectroscopy, RXES͒, polarized x-ray absorption spectra and ab initio electron-structure calculations. Polarized Mn K-edge x-ray absorption spectra of TbMnO 3 crystals exhibit a significant anisotropy along three crystallographic directions, particularly for the white-line region. The 1s3p-RXES spectra obtained at the Mn K edge reveal that the quadrupolar Raman regime is restricted predominantly to below the pre-edge peak whereas the fluorescence regime starts from the first pre-edge peak of 1s → 3d transitions, indicating a relatively delocalized character of unoccupied Mn 3d states. The additional K␤ emission profile at energy loss ϳ62 eV is attributed to the off-site dipole Mn 1s-MnЈ 3d and/or Mn 1s-Tb 5d transitions, originating from hybridization between Mn 4p states and neighboring MnЈ 3d / Tb 5d states. The off-site dipole transition makes a considerable contribution to the pre-edge region of Mn K-edge spectrum. Three prominent RIXS features at ϳ2.9, 7, and 11 eV were observed. Based on GGA+ U calculations, the 7 eV region corresponds to transitions from O 2p states to unoccupied minority Mn 3d states, whereas the 11 eV band is ascribed to transitions from the O 2p band to the empty Tb 5d band. The broad 2.9 eV band is attributed to the coexistence of on-site Mn 3d-Mn 3d and off-site Mn 3d-MnЈ 3d transitions.

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In substituted toluenes, the potential energy barrier to internal methyl rotation and the preferred methyl conformation depend on the position of the fluorine, amino, or methyl substituents and also on the electronic state, either S0, S1, or ground state cation. We present a unified picture of the electronic factors controlling these effects. In S0 and cation, ab initio electronic structure calculations of modest scale produce rotor potentials in good agreement with experiment. The methyl group provides a sensitive probe of local ring geometry. When the geometry of the ring in the vicinity of the rotor has good local C2v symmetry, the barrier is invariably small. In S0 ortho-substituted toluenes, we use natural steric analysis to show that repulsive steric interactions between the halogen lone pair and the methyl CH bonds dominate over attractive donor–acceptor interactions to favor the pseudo-trans conformation. When steric interactions are unimportant, the key determinant of rotor barrier height is the difference in π-bond order between the two ring CC bonds nearest methyl. The barrier height is proportional to the calculated bond order difference, with slope of 950 cm−1 per bond. Attractive donor–acceptor interactions favor the conformation that places the rotor CH bond cis to the ring CC bond of higher order, analogous to the localized case of 2-methylpropene. In toluene cations, π-ionization creates a pattern of long and short ring CC bonds. Simple molecular orbital theory readily explains the coarse bond-order patterns calculated for ortho- and meta-substituted toluene cations. A localized picture of π-bonding from natural resonance theory explains more subtle details of the distribution of CC bond orders about the ring. When π-ionization places the methyl group between ring CC bonds of quite different order, a substantial barrier results. This explains the strong preference of m-fluorotoluene+ for the pseudo-cis conformation and contributes to the preference of o-fluorotoluene+ for the pseudo-trans conformation. Finally, we speculate that a similar molecular orbital argument applied to S1 might explain the observed characteristic changes in barrier height on S1–S0 excitation of ortho- and meta-substituted toluenes.

For 2,6-difluorotoluene (2,6-DFT), we determine the 6-fold potential for intemal methyl rotation in the first excited singlet state SI and the cation ground state Do. The sample is cooled internally by expansion in Ar from a pulsed nozzle. The &-SO absorption spectrum recorded by resonant two-photon ionization (R2PI) yields the effective rotational constant F ' = 5.1 f 0.1 cm-I and the potential parameter V6' = -10.5 f 2.0 cm-' (staggered minimum) in S I , We find clear evidence of potential energy coupling in S I between torsion and a low frequency out-of-plane bending mode b of a*'' symmetry under Gl2. In S I the unperturbed bending fundamental lies at only 78 cm-I; the magnitude of the torsion-bend coupling matrix element between the zeroth-order pure rotor state m4 and the combination b'ml is 7 cm-I. Threshold photoionization spectra detected by pulsed field ionization (PFI) through a number of SI intermediate states corroborate the SI-SO assignments and yield the rotor constants = 5.2 f 0.1 cm-' and v6+ = +15 f 2.0 cm-I (eclipsed minimum) in the ground state cation Do. The adiabatic ionization energy is 73 674 f 5 cm-l. We also present a b initio calculations indicating that for both 2,6-difluorotoluene (SO) and 2,6-dichlorotoluene (SO), the preferred conformation is eclipsed, which is unusual in SO. The calculated vibrationally adiabatic torsional potential parameter V/ is +14 cm-I in 2,6-difluorotoluene and +52 cm-' in 2,6-dichlorotoluene. In other words the calculated energetic preference for the eclipsed geometry increases from the difluoro to the dichloro species.At the same time, the optimized geometries vs rotor angle a show increasing in-plane methyl and halogen wagging from the difluoro to the dichloro species, consistent with stronger in-plane steric repulsion. This apparent paradox in the vibrationally adiabatic torsional potential can be explained by a simple model comprising free rotation coupled by the potential energy to in-plane and out-of-plane bending vibrations, with the in-plane coupling stronger.