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Mikko Hakala 's List: Publications

  • Dec 31, 13

    I. Juurinen et al., J. Phys. Chem. B, 2013, 117 (51), pp 16506-16511
    We report a study on the hydrogen-bond network of water in aqueous LiCl solutions using X-ray Raman scattering (XRS) spectroscopy. A wide concentration range of 0–17 mol/kg was covered. We find that the XRS spectral features change systematically at low concentrations and saturate at 11 mol/kg. This behavior suggests a gradual destruction in the hydrogen-bond network until the saturation concentration. The surprisingly large concentration required for the saturation supports an interpretation in which the ions affect the structure of water only within their first hydration shell. The study is complemented by density-functional-theory calculations and molecular dynamics simulations.

  • Dec 31, 13

    F. Djurabekova et al., J. Appl. Phys. 114, 243302 (2013); http://dx.doi.org/10.1063/1.4856875
    Metal surfaces operated under high electric fields produce sparks even if they are held in ultra high vacuum. In spite of extensive research on the topic of vacuum arcs, the mystery of vacuum arc origin still remains unresolved. The indications that the sparking rates depend on the material motivate the research on surface response to extremely high external electric fields. In this work by means of density-functional theory calculations we analyze the redistribution of electron density on {100} Cu surfaces due to self-adatoms and in presence of high electric fields from −1 V/nm up to −2 V/nm (−1 to −2 GV/m, respectively). We also calculate the partial charge induced by the external field on a single adatom and a cluster of two adatoms in order to obtain reliable information on charge redistribution on surface atoms, which can serve as a benchmarking quantity for the assessment of the electric field effects on metal surfaces by means of molecular dynamics simulations. Furthermore, we investigate the modifications of work function around rough surface features, such as step edges and self-adatoms.

  • Mar 13, 14

    Simo Huotari et al 2014 J. Phys.: Condens. Matter 26 135501
    We report a study on charge-neutral crystal-field (dd) excitations in NiO as a function of applied pressure up to 55 GPa, using resonant inelastic x-ray scattering spectroscopy at the Ni K edge. We find distinct signatures of the pressure-induced modifications to the 3d orbital energies as a function of pressure. These modifications are experimentally evidenced by a subtle splitting of the dd-excitation resonance energies. We compare the experimental results to a charge-transfer cluster-model calculation, and a LSDA + U calculation of the ground state as a function of lattice constant. We thus show how resonant inelastic x-ray scattering spectroscopy is able to give insights into the manifold of excited states even in conditions that are difficult to access with many traditional experimental techniques.

    doi:10.1088/0953-8984/26/13/135501

  • Mar 31, 14

    J. Inkinen et al. J. Phys. Chem. A, 2014, 118 (18), pp 3288–3294
    We report a study on the temperature dependence of the valence-electron excitation spectrum of CO2, performed using non-resonant inelastic X-ray scattering spectroscopy. The excitation spectra were measured at the temperatures of 300 K and 850 K with momentum-transfer values of 0.4-4.8 Å-1, i.e. from the dipole limit to the higher-multipole regime, and were simulated using high-level coupled cluster calculations on dipole and quadrupole level. The results demonstrate the emergence of dipole-forbidden excitations owing to temperature-induced bending mode activation and finite momentum transfer.

    DOI: 10.1021/jp5019058

  • May 14, 14

    I. Juurinen et al. J. Phys. Chem. B, 2014, 118 (20), pp 5518–5523
    We report a Compton scattering study on the molecular-level structural changes of aqueous poly(N-isopropylacrylamide) (PNIPAM) across the conformational phase transition. PNIPAM is a thermoresponsive polymer that changes its conformation in water from the hydrophilic coil state to the collapsed hydrophobic globule state at 32 °C. Combined with density functional theory calculations, the Compton scattering experiments detect two type of changes in the phase transition. The amount of hydrogen bonds is found to reduce, and an elongation of the internal covalent bond lengths is observed. The elongation of the bonds indicates that not only the hydrogen bonding changes but there are other processes, most likely related to hydrophobic interaction, that should be taken into account in the phase transition.

  • Sep 19, 14

    I. Juurinen et al. J. Phys. Chem. B, 2014, 118 (29), pp 8750–8755
    The microscopic structure of the hydrogen-bond network of water-alcohol mixtures was studied using X-ray Raman scattering (XRS). To systematically examine how the hydrogen-bond network of water is affected by an increasing size of the hydrophobic group, small linear alcohols (methanol, ethanol, and propanol) in constant mole fractions were studied. The oxygen K-edge spectra were not altered upon hydration of the alcohols beyond a simple superposition of signals from alcohol and water. The experiment thus indicates that alcohols do not have a substantial effect on the structure of the hydrogen-bond network of water. In particular, no apparent breaking or forming of the hydrogen bonds is observed to take place in the overall structure. In addition, there is no indication of changes in the tetrahedrality of the hydrogen-bond network of water in the vicinity of alcohol molecules.

  • Sep 19, 14

    J. Hashemi et al. Phys. Rev. B 90, 075154 (2014)
    Intermediate-band (IB) photovoltaic materials are designed to absorb a wider range of the solar spectrum by dividing the gap with a set of bands that plays a mediating role in two-step absorption processes. Thus far, the conventional model of IB absorbers has been focused on a single half-filled IB. We show that a multiple-IB picture with filled and empty bands provides a more convincing explanation of the experimental findings in some cases, and it should be considered as a possible scenario in understanding and engineering IB solar cells. Toward that end, we report the formation of two sets of IBs in a Ti-substituted ${\mathrm{CuGaS}}_{2}$ compound. Using hybrid functional calculations within the density functional theory framework, we show that the lower IBs are occupied and mainly derive from the Ti $3{d}_{{x}^{2}$-${}{y}^{2}}$ states, while the higher ones originate from the Ti $3{d}_{{z}^{2}}$ states and are unoccupied. The positions of the IBs within the gap are found to depend weakly on the dopant concentration and the relative distances of the dopant ions. In addition, for a more pertinent comparison to real materials, we present a practical way to combine and analyze the densities of states resulting from the energetically most probable microscopic structures. This multi-intermediate-band picture provides a platform to comprehend unexplained features of the recent experimental study on the material [X. Lv et al., Solar Energy 103, 480 (2014)].

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