Electronic excitations lie at the origin of most of the commonly measured spectra. However, the first-principles computation of excited states requires a larger effort than ground-state calculations, which can be very efficiently carried out within density-functional theory. On the other hand, two theoretical and computational tools have come to prominence for the description of electronic excitations. One of them, many-body perturbation theory, is based on a set of Green’s-function equations, starting with a one-electron propagator and considering the electron-hole Green’s function for the response. Key ingredients are the electron’s self-energy S and the electron-hole interaction. A good approximation for S is obtained with Hedin’s GW approach, using density-functional theory as a zero-order solution. First-principles GW calculations for real systems have been successfully carried out since the 1980s. Similarly, the electron-hole interaction is well described by the Bethe-Salpeter equation, via a functional derivative of S. An alternative approach to calculating electronic excitations is the time-dependent density-functional theory (TDDFT), which offers the important practical advantage of a dependence on density rather than on multivariable Green’s functions. This approach leads to a screening equation similar to the Bethe-Salpeter one, but with a two-point, rather than a four-point, interaction kernel. At present, the simple adiabatic local-density approximation has given promising results for finite systems, but has significant deficiencies in the description of absorption spectra in solids, leading to wrong excitation energies, the absence of bound excitonic states, and appreciable distortions of the spectral line shapes. The search for improved TDDFT potentials and kernels is hence a subject of increasing interest. It can be addressed within the framework of many-body perturbation theory: in fact, both the Green’s functions and the TDDFT approaches profit from mutual insight. This review compares the theoretical and practical aspects of the two approaches and their specific numerical implementations, and presents an overview of accomplishments and work in progress.

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Electronic excitations: density-functional versus many-body Green's-function approaches

The spectroscopic properties, structure and interconversions of optically active oxygen-deficiency-related point defects in vitreous silica are reviewed. These defects, the E′-centers (oxygen vacancies with a trapped hole or 3-fold-coordinated silicons), different variants of diamagnetic `ODCs' (oxygen-deficiency centers), and their Ge-related analogs play a key role in the fiber-optic Bragg grating writing processes. The controversy surrounding the structural models for the Si- and Ge-related ODCs is discussed and the similarity between the bulk and surface point defects in silica is emphasized. The possible interconversion mechanisms between 2-fold-coordinated Si, neutral oxygen vacancies and E′-centers are discussed. The effects of glassy disorder have a profound effect on defect properties and interconversion processes in silica.

Optically active oxygen-deficiency-related centers in amorphous silicon dioxide

X-ray absorption and dichroism of transition metals and their compounds

Micelle-templated mesoporous silica materials are rapidly becoming important in many fields of chemistry for hosting reactants or catalysts in confined space. Fine control of the pore size, wall structure, surface functionalization, defects, and morphology is needed for fine-tuning the pores as nanoreactors. We review the physical chemistry of solution silicate species and surfactants in the synthesis of mesoporous silicas. Controls in surfactant packing and liquid crystalline phase transformation can lead to various tailored synthesis strategies. Postsynthesis treatments further make more stable mesoporous materials.

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Structural and morphological control of cationic surfactant-templated mesoporous silica

This review examines surface alignment of liquid crystals by exposure to light. Two distinct effects are discussed: illumination with polarized light induces a surface anisotropy to an alignment layer and hence a preferred in-plane orientation of the overlying liquid crystal director. Alternatively, a photochemical reaction of the alignment surface changes the liquid crystal anchoring conditions from homogeneous to homeotropic. We discuss how cis/trans isomerization, crosslinking and photodegradation are used to produce photoalignment layers and we show how the performance of the materials can be optimized by molecular design. Pretilted alignment is discussed and theoretical models are introduced to explain the photoalignment process. A range of display and non-display applications for photoalignment is presented.

https://iopscience.iop.org/article/10.1088/0022-3727/33/10/201/pdf

Photoinduced surface alignment for liquid crystal displays

The vacuum ultraviolet absorption cross sections of SiH4, GeH4, Si2H6, and Si3H8 are reported for the wavelength region 107–220 nm using synchrotron radiation as a light source. Absorption maxima of these compounds were found at the exciting wavelengths of 115–119 nm. Broad peaks observed were mostly assigned as primarily Rydberg transitions of the σSiH and σSiSi bonding electrons to the 4s, 4p, and 4d orbitals. The absorption features of germane resemble those of monosilane. In the photoexcitation of monosilane, the emission of the SiH(A 2Δ→X 2Π) transition was observed and its onset was found to be 132±2nm. The absorption spectrum of disilane showed five peaks. They were mostly assigned as 2a1g→4s, 2a1g→np(n=4−6) transitions and the strongest band was overlapped by 1eg→4d and 1eu→4p Rydberg transitions. In trisilane molecules three very weak and broad peaks were recognized and assigned as 3b2→4s, 4p and 4a1→4s, 4d Rydberg transitions. The strongest band was tentatively assigned as the superposition of...

Vacuum ultraviolet absorption cross sections of SiH4, GeH4, Si2H6, and Si3H8

A high-resolution monochromator with varied line space plane gratings (VLSG) and spherical focusing mirrors was installed in one of three branches of BL27SU in SPring-8. The performance of the monochromator was roughly evaluated from the photo ion yield of nitrogen molecule. Furthermore, the kinetic energy of the photoelectron from Xe 5p3/2 orbit was also measured at the same photon energy with the N2 to avoid the influence of natural width. The resolving power over 104 has been confirmed at the N K-edge.

Monochromator for a soft X-ray photochemistry beamline BL27SU of SPring-8

Vacuum UV photolyses of silane and chlorinated silanes were investigated by using rare gas resonance lamps. Strong emissions, from the A 2Δ→X 2Π transition of SiH and the 1P0→1D2 transition of Si were observed in the photolysis of SiH4 by Ar and Kr resonance lamps. It was suggested that the threshold energies for the appearance of both emissions were lower than the values obtained by the electron impact of SiH4 reported by Perrin et al. A new broad unstructured emission band in the region 300–400 nm was observed in the photolysis of SiH2Cl2 and SiHCl3 by Ar, Kr, and Xe lamps. The band was attributed to the A 1B1→X 1A1 transition of SiCl2 radicals from the measurement of the appearance energy of the emission by using synchrotron orbital radiation.

Emission spectra of SiH(A 2Δ→X 2Π) and SiCl2(Ã 1B1→X̃ 1A1) in the VUV photolyses of silane and chlorinated silanes

In vacuum ultraviolet photolyses of CCl4 and CBrCl3 a diffuse emission band was observed in the region of 410–750 nm by Ar i resonance and H Lyman‐α line irradiation. The band was attributed to a CCl2(A 1B1 → X 1A1) transition from the measurements of the appearance energies of the emitters produced from photodissociative excitation of both CCl4 and CBrCl3 using synchrotron radiation. The fluorescence decay of the CCl2(A → X) transition showed a superposition of two lifetime components of 2.17±0.26 and 4.0±0.12 μs at pressures from 10 to 140 mTorr. The pressure dependence of the amplitudes for the two lifetimes suggests the occurrence of collision‐induced intersystem crossing between 1B1 and 3B1 states of CCl2 radicals. The absorption cross section of CBrCl3 was measured for the first time in the 106–200 nm wavelength region and tentative assignments of Rydberg transitions are presented.

CCl2(Ã 1B1) radical formation in VUV photolyses of CCl4 and CBrCl3

The Doppler profiles of the hydrogen atom photofragments from the photodissociation of HCl and HBr at 157 and 243 nm are observed to determine the symmetries of the photoexcited states by using a resonance enhanced multiphoton ionization technique. The phototransitions are perpendicular for HCl at 157 nm and 193 nm and for HBr at 157 nm and 243 nm. Isotope effects on the branching ratios of [Cl*(2P1/2)]/[Cl(2P3/2)] are observed for the photodissociation of HCl and DCl at 157 and 193 nm. Our results indicate that in the photodissociation processes of HCl (A 1Π) the dynamical curve crossings during the breakup of the molecule play important roles in the determination of the branching ratio.

Toshio Ibuki

Papers

Vacuum ultraviolet absorption cross sections of SiH4, GeH4, Si2H6, and Si3H8

Monochromator for a soft X-ray photochemistry beamline BL27SU of SPring-8

Emission spectra of SiH(A 2Δ→X 2Π) and SiCl2(Ã 1B1→X̃ 1A1) in the VUV photolyses of silane and chlorinated silanes

CCl2(Ã 1B1) radical formation in VUV photolyses of CCl4 and CBrCl3

Photodissociation of hydrogen chloride and hydrogen bromide