Middle UV Research Articles

Quantum Study of the Optical Conductivity of Composite Films Formed by Bilayer Graphene and Single-Walled Carbon Nanotubes under Axial Stretching

In this article, quantum methods are used to study the optical properties of composite films formed by AB-stacked bilayer graphene and chiral single-walled carbon nanotubes (SWCNT) (12, 6) with a diameter of 1.2 nm. The analysis of optical properties is carried out on the basis of the results of calculating the diagonal elements of complex optical conductivity tensor in the wavelength range of 0.2–2 μm. Two cases of electromagnetic radiation polarization are considered: along the X axis (along the graphene bilayer) and along the Y axis (along the nanotube axis). The calculations are performed for three topological models (V1, V2, V3) of composite films, which differ in the width of the graphene bilayer and in the value of the shift between graphene layers. It is found that in the case of polarization along the X axis, the profile of the real part of optical conductivity in the region of extremal and middle UV radiation is determined by SWCNT (12, 6), and in the region of near UV and visible radiations, it is determined by bilayer graphene. In the case of polarization along the Y axis, the profile of the real part of optical conductivity in the region of extremal, near UV, and visible radiation is determined by SWCNT (12, 6), and in the region of the mid-UV range, it is determined by bilayer graphene. Regularities in the change in the profile of the surface optical conductivity of bilayer graphene-SWCNT (12,6) composite films under the action of stretching deformation along the Y axis are revealed. For models V1 (width of the graphene nanoribbon is 0.5 nm, the shift between layers is 0.48 nm) and V2 (width of the graphene nanoribbon is 0.71 nm, the shift between layers is 0.27 nm), the shift of the conductivity peaks in the region of extreme UV radiation along the wavelength to the right is shown. For the model V3 (width of the graphene nanoribbon is 0.92 nm, the shift between layers is 0.06 nm), the shift of the conductivity peaks to the right along the wavelength is observed not only in the region of extreme UV radiation, but also in the region of visible radiation. It is assumed that graphene-SWCNT (12,6) composite films with island topology are promising materials for photodetectors in the UV-visible and near-IR ranges.

Optical Reflection from Unforbidden Diffraction of Block Copolymer Templated Gyroid Films.

We present material substitutions and optical characterization of block copolymer (BCP)-templated gyroid structures that are obtained from a volume-asymmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA). In addition to the structural analyses reported earlier, we elucidate the optical responses to the nonaffine gyroid planes, in which the PMMA channels are complexed with Al2O3 by sequential infiltration synthesis and the organic components are further eliminated to produce an inorganic air-Al2O3 gyroid film. Grazing-incidence small-angle X-ray scattering measurements show that three-dimensional gyroid lattices are retained in both in-plane and out-of-plane directions through these material substitution processes. Our BCP-templated gyroid films respond to the middle UV wavelength from 200 to 300 nm, and peculiar optical reflectance peaks correlate with the unforbidden {110} diffraction spots. Together with the red- and blue-shifts of the reflectance peaks by the component substitutions, the air-Al2O3 gyroid structure reveals the high-amplitude spectrum due to the large refractive-index difference between channel and matrix.

Multilayer ultraviolet reflective coating based on atomic layer deposited aluminum oxide and fluoride

Ultraviolet optical coatings employ wide bandgap dielectric materials due to their characteristic low absorption. High-reflectivity and antireflective coatings are essential for optical devices, which can be achieved by alternately depositing two dielectrics with different refractive indices. In this research, a multilayer high-reflectivity coating has been designed for middle UV wavelengths using Al2O3 and AlF3 layers on a sapphire (0001) substrate, and the initial two-layer structure has been fabricated by atomic layer deposition. The surface morphology and roughness of the coating was measured by atomic force microscopy after each deposition step. Ultraviolet spectroscopy and spectroscopic ellipsometry were used to characterize the optical performance of the single and multilayer coatings. Monochromatic x-ray photoemission spectroscopy was used to study the film composition, bonding, and impurities. A bilayer reflective coating was demonstrated, with a smooth surface (Rq < 1 nm) and peak reflectance of 25%−30% at a wavelength of 196 nm. The measured reflectance deviated from the simulations in the middle UV range, and an analysis of the AlF3 layer prepared by plasma enhanced atomic layer deposition indicated the presence of Al-rich clusters, which were associated with the UV absorption. A thermal atomic layer deposition process for AlF3 deposition showed reduced absorption, which could be more effective for shorter wavelength designs.

First-principles investigation of electronic, optical, mechanical and heat transport properties of pentadiamond: A comparison with diamond

Pentadiamond is a carbon allotrope consisting of hybrid sp2 and sp3 atoms, which has been predicted to be stable and synthesizable. In this work we employ first-principles calculations to explore the electronic structure, optical characteristics, mechanical response and lattice thermal conductivity of pentadiamond, performing a direct comparison with the corresponding properties in diamond. The HSE06 density functional predicts indirect electronic band gaps for pentadiamond and diamond with values of 3.58 eV and 5.27 eV, respectively. Results for optical characteristics reveal pentadiamond’s large absorption in the middle UV region, where diamond does not absorb light, consistent with the smaller band gap of pentadiamond. The elastic modulus and tensile strength of pentadiamond are found to be 496 GPa and 60 GPa, respectively, considerably lower than the corresponding values for diamond. The lattice thermal conductivity is examined by solving the Boltzmann transport equation, with anharmonic force constants evaluated via state-of-the-art machine-learning interatomic potentials. We predict a thermal conductivity of 427 W/m-K for pentadiamond, less than one fifth of the corresponding quantity for diamond. Our results provide a useful vision of the intrinsic properties of pentadiamond, but also highlight some of its disadvantages in mechanical strength and heat conduction when compared to diamond.

Extended UV detection bandwidth: h-BN/Al powder nanocomposites photodetectors sensitive in a middle UV region due to localized surface plasmon resonance effect

The development of high-effective photodetectors operating in a wide spectral range is an important technological task. In this work we have demonstrated that the detection bandwidth of h-BN photodetectors in the UV range can be extended due to the surface plasmon resonance (SPR) effect. Theoretical calculations showed that, among Al, Au, Ag, and Cu, Al is the most suitable metal for the h-BN UV sensible detectors due to the SPR effect in the middle UV range. Based on the theoretical predictions, a simple and highly efficient method for obtaining h-BN/Al nanocomposites for localized SPR-based UV detectors was developed. It was demonstrated that the h-BN/Al material is sensitive to UV radiation with a wavelength of 266 nm that is far away of the detection limit of 240 nm inherent for pure h-BN.

Synthesis, supramolecular structure, spectral properties and correlation between nonlinear optic, thermochemistry and thermal behavior of an α-Hydroxyphosphonate acid ester, dual experimental and DFT approaches

First, we note in this paper that α-Hydroxyphosphonate acid ester was synthesized before; we confirmed it by a new method and its various properties are discussed experimentally and theoretically. The structure of Diethyl [hydroxy(4-methoxyphenyl) methyl] phosphonate (DH4MPMP) resolved by SXRD is compared to the optimized geometry, the FT-IR, NMR, UV-Vis spectroscopies results were compared and discussed experimentally and theoretically. The Hirshfeld surface analyses were employed to determine qualitative and quantitative intermolecular interactions present in the solid state and through, the supramolecular 3D-network was discussed. The UV-Vis spectrum exhibit a strong absorption in middle UV domain and an optical transmittance in the visible one. To be close from the experimental results, all DFT calculations based on B3LYP/6-31G (d,p) were detailed on monomer and dimer models. The DFT calculation of the second-order nonlinear optical properties is based on the first static hyperpolarizability (β), the results show that the material might have nonlinear optical properties. The calculated frontier molecular orbitals and their energies were calculated. The research was extended to the calculations of the molecular electrostatic potential map; it shows negative potential areas localized around oxygen atoms as well as the positive localized around the hydrogen atoms. Finally, the thermodynamic functions (entropy, heat capacity and enthalpy) were determined from spectroscopic data by statistical methods in the range 100–1000 K and the experimental thermal decomposition behavior was analyzed by TGA.

Dy3+ and Dy3+/Ce3+ doped Al2O3 coatings obtained by plasma electrolytic oxidation: Photoluminescence and energy transfer from Ce3+ to Dy3+

In this paper, Dy3+ and Dy3+/Ce3+ doped Al2O3 coatings were synthesized by plasma electrolytic oxidation process of aluminum. Structural and photoluminescence (PL) properties of obtained coatings as well as energy transfer mechanism from Ce3+ to Dy3+ were studied. The PL excitation and emission spectra of Al2O3:Dy3+ featured weak bands attributed to f-f transitions of Dy3+. The PL emission spectrum of Al2O3:Dy3+ exhibits three emission bands in blue-yellow-red regions, assigned to 4F9/2→6H15/2, 4F9/2→6H13/2, and 4F9/2→6H11/2 transitions of Dy3+, respectively. Under middle UV excitation the Ce3+ in Al2O3 emits broad band, peaked at about 330 nm, attributed to the 5d→4f transition of Ce3+. Because of the strong overlap between the PL emission of Ce3+ and the PL excitation of Dy3+ in Al2O3, energy transfer from Ce3+ to the Dy3+ in Al2O3:Dy3+/Ce3+ is observed upon 285 nm excitation. PL emissions due to 4F9/2→6HJ (J = 11/2, 13/2, and 15/2) transitions of Dy3+ in Al2O3:Dy3+/Ce3+ is two to three orders of magnitude larger than in Al2O3:Dy3+. The values of CIE chromaticity coordinates for Al2O3:Dy3+/Ce3+ suggest their application for cold white light emission.

Unveiling Excited-State Chirality of Binaphthols by Femtosecond Circular Dichroism and Quantum Chemical Calculations.

Time-resolved circular dichroism (TR-CD) is a powerful tool for probing conformational dynamics of biomolecules over large time scales that are crucial for establishing their structure-function relationship. However, such experiments, notably in the femtosecond regime, remain challenging due to their extremely weak signals, prone to polarization artifacts. By using binol and two bridged derivatives (PL1 and PL2) as chiral prototypes, we present here the first comprehensive study of this type in the middle UV, combining femtosecond TR-CD and quantum mechanical calculations (TD-DFT). We show that excitation of the three compounds induces large variations of their transient CD signals, in sharp contrast to those of their achiral transient absorption. We demonstrate that these variations arise from both the alteration of the electronic distribution and the dihedral angle in the excited state. These results highlight the great sensitivity of TR-CD detection to signals hardly accessible to achiral transient absorption.

Monte Carlo simulation of skin multispectral autofluorescence

This work is devoted to the simulation of human skin autofluorescence in different spectral ranges. Analytical review was performed for selecting the main endogenous fluorophores with the greatest contribution to the skin fluorescence: tryptophan, tyrosine, collagen, melanin, elastin, lipofuscin, protoporphyrin IX, NADH, FAD. It was necessary to set parameters for autofluorescence modeling, such as the absorption/emission spectra of fluorophores, molar concentration, molar extinction coefficient, and quantum yield. The six-layer skin model was designed in the TracePro software and autofluorescence was simulated when excited at different wavelengths in the middle UV (270-300 nm), near UV (330-360 nm) and visible (400-450 nm) spectral ranges. The simulation results were compared with the experimental results of other authors. The principal distinctive factor of this work is the simulation of the human skin autofluorescence excited in different spectral ranges.

The dependence of the [FUV-MUV] colour on solar cycle

Solar UV variability is extremely relevant for the stratospheric ozone. It has an impact on Earth's atmospheric structure and dynamics through radiative heating and ozone photochemistry. Our goal is to study the slope of the solar UV spectrum in two UV bands important for the stratospheric ozone production. In order to investigate the solar spectral variability, we use SOLSTICE (the Solar Stellar Irradiance Comparison Experiment) data onboard Solar Radiation and Climate Experiment (SORCE) satellite. Data sets used are far UV (115-180nm) and middle UV (180-310nm), as well as the Mg II index (the Bremen composite). We introduce the SOLSTICE [FUV - MUV] colour to study the solar spectral characteristics, as well as analysis of the colour versus Mg II index. To isolate the 11-year scale variation, we used the Empirical Mode decomposition (EMD) on the data sets. The [FUV - MUV] colour strongly correlates with the Mg II index. More in detail, the [FUV - MUV] colour shows a time dependent behavior when plotted versus Mg II index. To explain this dependence we hypothesize an efficiency reduction of SOLSTICE FUV irradiance using an exponential aging law.

Development of express method of microgravimetric determination of actual resins in petrols

A new approach is proposed for a quick, simple assessment of one of the quality indicators of gasoline - the concentration of actual pitches. Separate standard quality indicators for the gasoline samples (with octane number 92), which are associated with the content of the components of petroleum fractions and other non-volatile compounds, were measured. The method of piezoquartz micro-weighting with one-sided loading of the resonant electrode of the resonator, sensitive on mass (OAB-type) with a limiting contour, is used. The correlation between the results obtained by piezoquartz micro-weighing and by the standard method was studied. Critical points of the developed method are determined - the stage of applying the exact volume of samples to the resonator electrode with a micro syringe. The method of molecular absorption spectroscopy in the middle UV region relative to hexane revealed differences in the composition of the components of the petroleum fraction of gasoline samples. The principle of comparing "fingerprints" is used to compare and establish the degree of identity of spectra and composition for gasoline samples from several characteristic points on the spectra. To find and evaluate the tightness of the connection between the results of different methods, a nonparametric method for small samples with the Spearman rank correlation was used. It is established that the values of the Spearman's correlation coefficients for the piezoquartz microweighting and the peak optical density present in all gasoline samples at the characteristic wavelength have a direct and strong tightness of the connection. The dependence of the signs is statistically significant, there is a close linear relationship between the parameters. The possibility of applying the piezoquartz micro-weighting method to develop an express method for estimating the non-volatile residue of gasoline was positively evaluated.

Accelerated UV ageing studies of acrylic, alkyd, and polyvinyl acetate paints: Influence of inorganic pigments

The stability of two types of acrylic binding media, alkyd and polyvinyl acetate (PVAc), four widely used synthetic binders in modern and contemporary art, to UV light also including the UV-B range (315–280nm, middle UV) for simulating sunlight outdoor conditions was studied and compared by double-shot and single-shot Py-GC/MS, FTIR-ATR, and colour measurements. Thermally assisted hydrolysis and methylation (THM-GC/MS) analyses were used for the alkyd. Additionally, the influence of inorganic pigments on the photo-oxidative stability of the binding media was also considered. For this purpose, the binders in their pure form as well as mixed with eight different inorganic pigments (titanium white—anatase and rutile, cadmium yellow, cadmium red, hydrated chromium oxide green, ultramarine blue, raw umber Cyprus, and ivory black) were exposed to the accelerating artificial UV ageing for different periods of time and analysed before and after UV exposure. After UV ageing, the double-shot Py-GC/MS detected the effect of photo-oxidative processes of the binders in a much more detailed way than by the single shot. For instance, photo-oxidation of the acrylics resulted in a production of oligomers even during the thermal desorption step of double-shot Py-GC/MS and in a decrease of the EA and nBA main monomers in the pyrolysis second step. Contrarily, the aged alkyd samples were mostly characterised by a decrease of unsaturated fatty acids and especially by the increase of free ortho-phtalic acid detected by double-shot Py-GC/MS and THM-GC/MS, which is also reflected in the FTIR-ATR results. On the other hand, an increase of free acetic acid was observed for the aged polyvinyl acetate by double-shot Py-GC/MS. Colour measurements recorded a greater sensitivity of the alkyd paints, shown by to the bigger shift of L*, a*, and b* coordinates and change of E* values. Of additional interest was the higher sensitivity to the UV light for synthetic binding media in combination with pigments.

Production, HPLC analysis, and in situ apoptotic activities of swainsonine toward lepidopteran, Sf-21 cell line.

Swainsonine, a secondary metabolite from Metarhizium anisopliae has been extensively studied in the complementary areas of therapeutics and toxicology. This work aims to develop a simple UV-HPLC method of analyses for swainsonine in Metarhizium fermentation broth and to explore its in situ entomotoxic activities. The partially purified broth was quantitatively analyzed using middle UV (205 nm)-reverse phase HPLC method with different mobile phases and gradient programmes. Swainsonine was eluted as single peak at (te ) 6.0-6.9 min with average concentration of 4.04 ± 0.52 μg/mL using optimal mobile phase (0.1% trifluoroacetic acid in water and acetonitrile). The mass spectrometry analysis further indicated the characteristic MS1 species for swainsonine, [M+H](+) 174.30 in corresponding HPLC peaks. The antiproliferative effects of swainsonine on lepidopteran, Sf-21 cells were determined through 3-(4, 5-dimethylthia-zol-2-yl)-2, 5-diphenyl tetrazolium bromide (IC50 standard = 3.90 μM and IC50 purified = 5.27 μM) and trypan blue dye exclusion (IC50 standard = 6.91 μM and IC50 purified = 8.67 μM) assays. The fluorescence activated cell sorting evaluation of Sf-21 cells showed nearly 35% and 42% of population in various apoptotic stages at 36 h, when treated with standard and purified swainsonine, respectively. The morphodimensional field emission scanning electron and atomic force microscopic analyses further confirmed the characteristic apoptotic features like membrane blebbings, ruptures and volume shrinkage in the lepidopteran cells after 24-36 h of post-treatment incubation. The study describes the potential entomotoxic activities of swainsonine and its role in the virulence of Metarhizium spp.

EVALUATION OF TECHNOLOGIES FOR PURIFICATION OF GREENHOUSE HORTICULTURAL DISCHARGE WATER

The demands of the European Water Framework Directive for ecological and chemical good water quality, resulted in the need for the greenhouse horticultural sector to decrease the emission of nutrients and plant protection products (PPP) drastically in the coming years. To reach this goal, our research follows a dual approach by 1) developing techniques and methods for further recirculation of nutrient solutions, and 2) end-of-pipe solutions for emission via discharge of the nutrient solutions. Here we present our research on an end-of-pipe solution for PPPs, which is the use of water purification techniques. The technologies are tested on effectiveness of removing PPPs from so-called ‘Standardised Water’, which contains specified amounts of nutrients, dissolved organic matter and twelve active ingredients of PPPs. Also applicability in a greenhouse environment and economic feasibility are examined. Four technologies were chosen to be investigated: 1) Ozone with and without activated carbon, 2) Hydrogen peroxide (H2O2) with middle pressure UV-radiation, 3) H2O2 with low pressure UV-radiation, and 4) Electrochemical flocculation. Ozone and H2O2 – UV (both low and middle pressure) proved to be able to remove the PPPs with an effectiveness of approximately 80%. Activated carbon filter after ozone treatment improved effectiveness up to 100% (short run). Electrochemical flocculation showed an effectiveness of around 40%, but it should be noted that a suboptimal pilot device was tested. Cost calculations of the three most promising technologies were carried out using four case studies. The calculations showed that ozone treatment has the highest cost per m3 of treated water, followed by H2O2 with middle pressure and low pressure UV treatment. In general, the applicability of the three technologies (ECF excluded) in greenhouse horticulture was assessed as good.

Calculation of UV, IR, and NMR Spectra of Diethyl 2,2′-[(1,1′-Biphenyl)-4,4′-Diylbis(Azanediyl)]Diacetate

Introduction. We previously developed o lm polarizers based on polyvinyl alcohol (PVA) and organic dichroic dyes that absorbed over a broad spectral range (350–680 nm) [1]. The development of novel technical processes in the electronics industry has made it necessary to expand the operating range of polarizers to shorter wavelengths that cover the middle UV range (i 300 nm). Biphenyl derivatives absorb light in the UV range [2]. This is a decisive factor in fabricating o lm UV polarizers. We modeled a series of compounds based on biphenyl that contained various functional groups. Preliminary calculations showed that only diethyl 2,2�c -[(1,1�c -biphenyl)-4,4�c -diylbis(azanediyl)]diacetate could absorb light near ~300 nm. Theoretical quantum-chemical methods based on the Hartree–Fock (HF) and density functional theory (DFT) methods have been broadly applied to calculate UV, IR, and NMR spectra of organic molecules [3]. Traditional DFT methods that are based on consideration of electron correlation effects in a local density approximation (LDA) or on a generalized gradient approximation (GGA) use effective functionals for the exchange-correlated energy that depend on the electron density or on the density and its gradient for a GGA [4]. Excited electronic states of molecular systems can be described in terms of a time-dependent density functional theory (TDDFT) [5]. Despite much progress, several features, e.g., van-derWaals interactions and electronic excitations with charge transfer, still cannot be accurately modeled in terms of traditional DFT methods [5]. It was noted [5, 6] that this method describes more completely polymeric systems or molecules in which the number of atom is >100. The geometries of molecules with C=O, –NO2, –CONH2, and >S=O. It is noteworthy that semi-empirical methods (AM1 and PM3) were used to optimize the compound structures in order to economize resources and machine time. Undoubtedly, this affected the accuracy of the calculated absorption spectra. The structure and vibrational spectrum of carnosine was calculated by an ab initio HF method in basis set 6-31G [10]. The geometric and energy parameters of carnosine in two tautomeric forms of the imidazole ring

Photoabsorption spectrum of helium trimer cation—Theoretical modeling

The photoabsorption spectrum of He3(+) is calculated for two semiempirical models of intracluster interactions and compared with available experimental data reported in the middle UV range [H. Haberland and B. von Issendorff, J. Chem. Phys. 102, 8773 (1995)]. Nuclear delocalization effects are investigated via several approaches comprising quantum samplings using either exact or approximate (harmonic) nuclear wavefunctions, as well as classical samplings based on the Monte Carlo methodology. Good agreement with the experiment is achieved for the model by Knowles et al., [Mol. Phys. 85, 243 (1995); Mol. Phys. 87, 827 (1996)] whereas the model by Calvo et al., [J. Chem. Phys. 135, 124308 (2011)] exhibits non-negligible deviations from the experiment. Predictions of far UV absorption spectrum of He3(+), for which no experimental data are presently available, are reported for both models and compared to each other as well as to the photoabsorption spectrum of He2(+). A simple semiempirical point-charge approximation for calculating transition probabilities is shown to perform well for He3(+).

Size‐dependent optical properties of transparent, spin‐coated polystyrene/ZnO nanocomposite films

AbstractZinc oxide (ZnO) nanoparticles are synthesized using a simple chemical method at room temperature. A variation in molar concentration of the precursor, potassium hydroxide, from 0.25 to 0.01 mol L−1 is accompanied by a decrease in the average size of the nanoparticles. These nanoparticles are used for the preparation of polystyrene/ZnO nanocomposite films using the spin‐coating technique. These films are found to be highly transparent throughout the visible region and absorb UV light in the region from 395 to190 nm, almost covering the near and middle UV ranges (400 to 200 nm). This observation highlights the possible prospects of these films in UV shielding applications. The wavelength corresponding to the onset of UV absorption is found to be blue shifted with a decrease in size of the ZnO particles in the composite films due to confinement effects. The photoluminescence spectra of the composite films also change as a function of particle size. The emissions at longer wavelength due to defects and impurity‐related states in ZnO are almost quenched as a result of surface modification by the polymer matrix. The observed band‐gap enlargement with a decrease in size of the ZnO particles in the composite films is significant for band‐gap engineering of nanoparticles for various applications. Copyright © 2011 Society of Chemical Industry

Photooxidation in Combination with Nanotechnologies - Principles, Developments and R&D Approaches of an Advanced Technology for Water and Air Treatment - Uviblox®

AbstractMany organic and oxidizable inorganic substances are main targets for oxidation and destruction during treatment, purification and disinfection of contaminated ground water, waste water, air, soil and waste gas and odor. Uviblox® technology uses the effects of photooxidation and photocatalytical processes by middle and low pressure UV lamps for technical systems. There are many possibilities for combination and optimisation of photooxidation with other technologies like nanotechnologies. Degradation processes can be enforced by nano structures of photocatalysts significantly. Different approaches are strongly pursued in research & development projects like NanoAqua, Fe-NANOSIT and nanoblox. These projects search for different ways of applying and handling the photocatalytical nano particles like TiO

Ultrafast acoustics in the middle UV range: coherent phonons at higher frequencies and in smaller objects

We show that the propagation of coherent acoustic phonons generated by femtosecond optical excitation can be clearly resolved using a probe laser in the middle UV (MUV) range. The MUV probe is easily produced from a high-repetition-rate femtosecond laser and a homemade frequency tripler. We present various experimental results that demonstrate efficient and high frequency detection of acoustic phonons. Thus, we show that the MUV range offers a unique way to reach higher frequencies and probe smaller objects in ultrafast acoustics.

The Role of Adenine Excimers in the Photophysics of Oligonucleotides

Energies and structures of different arrangements of the stacked adenine homodimer have been computed at the ab initio CASPT2 level of theory in isolation and in an aqueous environment. Adenine dimers are shown to form excimer singlet states with different degrees of stacking and interaction. A model for a 2-fold decay dynamics of adenine oligomers can be supported in which, after initial excitation in the middle UV range, unstacked or slightly stacked pairs of nucleobases will relax by an ultrafast internal conversion to the ground state, localizing the excitation in the monomer and through the corresponding conical intersection with the ground state. On the other hand, long-lifetime intrastrand stacked excimer singlet states will be formed in different conformations, including neutral and charge transfer dimers, which originate the red-shifted emission observed in the oligonucleotide chains and that will evolve toward the same monomer decay channel after surmounting an energy barrier. By computing the transient absorption spectra for the different structures considered and their relative stability in vacuo and in water, it is concluded that in the adenine homodimers the maximum-overlap face-to-face orientations are the most stable excimer conformations observed in experiment.