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Doping is an efficient way to open the zero band gap of graphene. The control of the dopant domain size allows us to tailor the electronic structure and the properties of the graphene. We have studied the electronic structure of boron nitride doped graphenes with different domain sizes by simulating their near-edge X-ray absorption fine structure (NEXAFS) spectra at the N K-edge. Six different doping configurations (five quantum dot type and one phase-separated zigzag-edged type) were chosen, and N K-edge NEXAFS spectra were calculated with large truncated cluster models by using the density functional theory with hybrid functional and the equivalent core hole approximation. The opening of the band gap as a function of the domain size is revealed. We found that nitrogens in the dopant boundary contribute a weaker, red-shifted pi* peak in the spectra as compared to those in the dopant domain center. The shift is related to the fact that these interfacial nitrogens dominate the lowest conduction band of the system. Upon increasing the domain size, the ratio of interfacial atom decreases, which leads to a blue shift of the pi* peak in the total NEXAFS spectra. The spectral evolution agrees well with experiments measured at different BN-dopant concentrations and approaches to that of a pristine h-BN sheet., QC 20161019

We performed a combined experimental and theoretical study of the C1s Near-Edge X-ray Absorption Fine-Structure (NEXAFS) spectroscopy and X-ray Photoelectron Spectroscopy in the gas phase of two polycyclic aromatic hydrocarbons (phenanthrene and coronene), typically formed in combustion reactions. In the NEXAFS of both molecules, a double-peak structure appears in the C1s -> LUMO region, which differ by less than 1 eV in transition energies. The vibronic coupling is found to play an important role in such systems. It leads to weakening of the lower-energy peak and strengthening of the higher-energy one because the 0 - n (n > 0) vibrational progressions of the lower-energy peak appear in nearly the same region of the higher-energy peak. Vibrationally resolved theoretical spectra computed within the Frank-Condon (FC) approximation and linear coupling model agree well with the high-resolution experimental results. We find that FC-active normal modes all correspond to in-plane vibrations., QC 20140919

L-edge near-edge X-ray fine structure spectroscopy (NEXAFS) has become a powerful tool to study the electronic structure and dynamics of metallo-organic and biological compounds in solution. Here, we present a series of density functional theory calculations of Fe L-edge NEXAFS for spin crossover (SCO) complexes within the time-dependent framework. Several key factors that control the L-edge excitations have been carefully examined using an Fe(II) polypyridyl complex [Fe(tren(py)(3))](2+) (where tren(py)(3) = tris(2-pyridylmethyliminoethyl)amine) as a model system. It is found that the electronic spectra of the low-spin (LS, singlet), intermediate-spin (IS, triplet), and high-spin (HS, quintet) states have distinct profiles. The relative energy positions, but not the spectral profiles, of different spin states are sensitive to the choice of the functionals. The inclusion of the vibronic coupling leads to almost no visible change in the resulting NEXAFS spectra because it is governed only by low-frequency modes of less than 500 cm(-1). With the help of the molecular dynamics sampling in acetonitrile at 300 K, our calculations reveal that the thermal motion can lead to a noticeable broadening of the spectra. The main peak position is strongly associated with the length of the Fe-N bond., QC 20140130

The intensities of hydrogen Lyman-alpha and Balmer series emission lines as a result of photoexcitation of free CH4 and NH3 molecules at the C 1s and N 1s edges have been measured. For methane, the total fluorescence yield in the visible region ( 300 - 650 nm) was also recorded. Excitation functions of the Balmer lines show relative intensity enhancement of Balmer-beta emission in comparison with Balmer-alpha at higher core- to- Rydberg excitations. The Lyman-alpha emission intensity, in general, follows the relations observed in the corresponding total ion yield measurements. Additionally, the Balmer-gamma and -delta yields were measured for ammonia molecules and they show intensity maxima at photon energies shifted closer to the N 1s threshold than Balmer-alpha and -beta. A quantum defect analysis is performed to find out which core excitations are responsible for this enhanced intensity. Dissociation pathways leading to the emission in H atoms are discussed., QC 20100916