I. Morrison

A computational study of photoisomerization in Al3O3- ­clusters

Ab initio calculations are employed to understand the photoisomerization process in small Al3O3− clusters. This process is the first example of a photoinduced isomerization observed in an anion cluster gas-phase system. Potential energy surfaces for the ground state and the excited state (S1 and T1) are explored by means of B3LYP, MP2, CI-singles, and CASSCF methods. We demonstrate that the isomerization process occurs between the global minimum singlet state Book structure (C2v,1A1) and the triplet state Ring structure (C2v,3B2). The calculated vertical excitation energy is 3.62 eV at the CASSCF level of approximation, in good agreement with the experimental value (3.49 eV). A nonplanar conical intersection, which hosts the intersystem crossing between the S1 and T1 surfaces is identified at the region of around R(1,6)=2.4 A. Beyond the experimental results, we predict, that this isomerization is reversible upon absorption of a phonon with energy of 1.92 eV. Our results describe a unique system, whose st...

First principles lattice dynamics studies of the vibrational spectra of ice

Abstract We present studies of the vibrational properties of a number of ice structures as evaluated by `first principles’ lattice dynamics. Calculations are performed within the generalized gradient approximation to density functional theory using the `ab initio’ pseudopotential method. Dynamical properties are determined within the harmonic approximation by finite difference evaluation of the dynamical matrix from atomic forces. The resulting normal modes of vibration are analyzed in detail by projection onto pure intra- and inter-molecular modes. The importance of configurational disorder is assessed by comparison of results from different water molecule orientations in similar supercells. All structures considered here comprise of tetrahedral hydrogen-bonded networks and include low-pressure hexagonal and cubic structures and high-pressure cubic structures. Calculated vibrational density of states are compared with results from quasielastic neutron scattering and the microscopic origin of various features in the spectra discussed.

The dependence on structure of the projected vibrational density of states of various phases of ice as calculated by ab initio methods

We determine the structural data of seven of the polymorphs of ice (ice Ih, ice Ic, ice IX, ice II, ice VI, ice VII and ice VIII) from ab initio calculations. The dynamical properties have been analysed within the harmonic approximation via a finite-difference evaluation of dynamical matrices from atomic forces. Supercells are used to model the various ordered and disordered phases considered. Calculations are done at zero pressure in order to compare directly with neutron scattering studies performed on recovered phases. The normal modes are resolved into projections chosen to display their intra- and inter-molecular character. Further projections are performed for ice VI, ice VII and ice VIII to probe the interactions between sub-lattices. Trends in the dynamical results are discussed in terms of changes in the structural complexity of the various phases considered.

The pressure?temperature phase diagram of MgH2 and isotopic substitution

Computational thermodynamics using density functional theory ab initio codes is a powerful tool for calculating phase diagrams. The method is usually applied at the standard pressure of p = 1 bar and where the Gibbs energy is assumed to be equal to the Helmholtz energy. In this work, we have calculated the Gibbs energy in order to study the release temperature and phase modifications of MgH(2) at high pressures up to 10 GPa (100 kbar). The isotopic substitution of hydrogen with deuterium (or tritium) does not bring about any strong effects on the phase diagram. These considerations are of extreme importance for (i) the synthesis of novel substitutional magnesium based materials at high pressure and (ii) the determination of the correct reference states for the calculation of phase diagrams at high pressure. The calculated results are compared with experimental data obtained with an in situ neutron diffraction measurement.

The influence of the spatial structure on the electronic properties of porous silicon: quantum chemical study

In order to study the electronic properties of porous silicon (por-Si) we have investigated silicon nanoclusters with atomic configurations satisfying the general features of the radial and angular distribution functions of the por-Si structure modelled using the diffusion-limited aggregation model. The electronic structure calculations were performed within the intermediate neglect of differential overlap approximation of the Hartree-Fock-Roothaan scheme, while the restricted configuration interaction was employed in calculating excited states and transition rates. Both the electronic structure and optical transition strength are found to depend strongly on the cluster size and local environment. Importantly, the inclusion of many-electron effects is crucial in determining the transition rates. The low reactivity of some nanoclusters, together with their intermediate size and well-determined electronic properties, make them prospective initial blocks for the manufacturing of optoelectronic materials with desired characteristics.

Large optical nonlinearities in semiconductor superlattices

We show that the nonlinear response of a direct‐gap semiconductor superlattice is greatly enhanced when the separation of the lowest two conduction minibands approaches the magnitude of the principal gap. Two structures exhibiting this enhanced nonlinear response are described.

Electronic structure of SiSi1−xGex and SiSi1−xSnx strained layer superlattices

Abstract We present a quantitative study of the electronic structure and zone folding in Si SiGe and Si SiSn strained layer superlattices. We find that the effect of strain is to increase the degree of electronic confinement and to enhance the optical matrix element across the fundamental superlattice gap, although this latter effect is small in the systems studied.

Symmetry classification of the projected vibrational density of states in ice VIII from ab initio methods

We have determined the normal modes of ice VIII, a 16-molecule supercell, using ab initio calculations. The dynamical matrices so obtained are diagonalized and the eigenvectors projected on to the stretching, bending and libration modes of each water molecule in turn. We can therefore accurately correlate the symmetry assignments of the zone centre modes and the corresponding frequencies of these dynamical results with Raman, infrared and neutron scattering data. In particular, using neutron scattering data, we obtain excellent agreement with the frequencies of the Ry B1u phonon mode which is inactive in Raman or infrared spectroscopy.

Band structure engineering of a nonlinear optical response in semiconductor superlattices

The authors show that the nonlinear response of certain direct gap semiconductor superlattices is greatly enhanced when the separation of the lowest conduction bands is comparable with the magnitude of the principal gap. The mechanism which results in this enhancement is virtual transitions to higher subbands. Examples of (InAs)1-x(GaSb)x/AlSb superlattices expected to exhibit the enhanced nonlinear response in the near-infrared region of the spectrum are presented. They also investigate nonlinear response in semiconductor superlattices at ultrashort times ( approximately 100 fs). In the authors scheme a full account is given fro the first time of band structure effects upon transients. They then use a three-level model to determine the conditions under which predictions, based on the steady-state response theory and the golden rule, break down in structures with closely spaced energy levels.

Strain-induced electron states in Si0.75Ge0.25(Si/Si0.5Ge0.5)(001) superlattices

The authors have carried out a pseudopotential calculation of the electronic structure of a strained Si/Si0.5Ge0.5 superlattice in which the effect of strain peculiar to the choice of substrate (i.e. Si0.75Ge0.25) and that of the microscopic potential of the individual layers are fully accounted for. They report several strain-induced well confined electron states localised in the silicon layers. They find that the optical matrix element for the transition across the fundamental superlattice gap is quite small.