Absorption Of Ultraviolet Photons Research Articles

A plausible molecular mechanism to explain near-infrared continuum emission: Recurrent fluorescence

Context. Very small grains and large hydrocarbon molecules are known to convert a fraction of the ultraviolet (UV) and visible stellar radiation to near- and mid-infrared (IR) photons via stochastic heating and subsequent radiative de-excitation. However, no convincing explanation for the near-IR continuum emission observed in some reflection nebulae and planetary nebulae has been provided so far. Aims. We aim to investigate the extent that recurrent fluorescence originating from stellar photon absorption by Cn (n = 24, 42, 60) carbon clusters can account for the IR emission detected in various interstellar environments. To this aim, we modelled the collective emission signature of a carbon cluster sample induced by irradiation from a 20 000 K blackbody source. From the obtained results, we set out to determine the fraction of interstellar carbon locked up in the emitting objects. Methods. The collective emission signature was computationally determined for different structural families encompassing cages, flakes, pretzels, and branched isomers by means of a kinetic Monte Carlo stochastic approach based on harmonic vibrational densities of states. The collective emission spectra result from the overall radiative cooling of a large population of neutral carbon clusters, during which recurrent fluorescence and vibrational emission compete with each other. Results. Our modelling shows that recurrent fluorescence from C60 cages and flakes (with little or no sp1 carbon atoms) and C42 cages are able to explain the near-IR continuum emission observed in several reflection nebulae and planetary nebulae. Assuming that the continuum emission observed towards NGC 7023 is due to recurrent fluorescence induced by UV or visible photon absorption in neutral cage carbon clusters containing about 30–60 atoms, the carriers contain about 0.1–1.5% of the interstellar carbon abundance.

Perception of solar UV radiation by plants: photoreceptors and mechanisms.

About 95% of the ultraviolet (UV) photons reaching the Earth’s surface are UV-A (315–400 nm) photons. Plant responses to UV-A radiation have been less frequently studied than those to UV-B (280–315 nm) radiation. Most previous studies on UV-A radiation have used an unrealistic balance between UV-A, UV-B, and photosynthetically active radiation (PAR). Consequently, results from these studies are difficult to interpret from an ecological perspective, leaving an important gap in our understanding of the perception of solar UV radiation by plants. Previously, it was assumed UV-A/blue photoreceptors, cryptochromes and phototropins mediated photomorphogenic responses to UV-A radiation and “UV-B photoreceptor” UV RESISTANCE LOCUS 8 (UVR8) to UV-B radiation. However, our understanding of how UV-A radiation is perceived by plants has recently improved. Experiments using a realistic balance between UV-B, UV-A, and PAR have demonstrated that UVR8 can play a major role in the perception of both UV-B and short-wavelength UV-A (UV-Asw, 315 to ∼350 nm) radiation. These experiments also showed that UVR8 and cryptochromes jointly regulate gene expression through interactions that alter the relative sensitivity to UV-B, UV-A, and blue wavelengths. Negative feedback loops on the action of these photoreceptors can arise from gene expression, signaling crosstalk, and absorption of UV photons by phenolic metabolites. These interactions explain why exposure to blue light modulates photomorphogenic responses to UV-B and UV-Asw radiation. Future studies will need to distinguish between short and long wavelengths of UV-A radiation and to consider UVR8’s role as a UV-B/UV-Asw photoreceptor in sunlight.

Graphite to diamond transition induced by photoelectric absorption of ultraviolet photons

The phase transition from graphite to diamond is an appealing object of study because of many fundamental and also, practical reasons. The out-of-plane distortions required for the transition are a good tool to understand the collective behaviour of layered materials (graphene, graphite) and the van der Waals forces. As today, two basic processes have been successfully tested to drive this transition: strong shocks and high energy femtolaser excitation. They induce it by increasing either pressure or temperature on graphite. In this work, we report a third method consisting in the irradiation of graphite with ultraviolet photons of energies above 4.4 eV. We show high resolution electron microscopy images of pyrolytic carbon evidencing the dislocation of the superficial graphitic layers after irradiation and the formation of crystallite islands within them. Electron energy loss spectroscopy of the islands show that the sp2 to sp3 hybridation transition is a surface effect. High sensitivity X-ray diffraction experiments and Raman spectroscopy confirm the formation of diamond within the islands.

Multiscale Conformational Sampling Reveals Excited-State Locality in DNA Self-Repair Mechanism.

Ultraviolet (UV) irradiation is known to be responsible for DNA damage. However, experimental studies in DNA oligonucleotides have shown that UV light can also induce sequence-specific self-repair. Following charge transfer from a guanine adenine sequence adjacent to a cyclobutane pyrimidine dimer (CPD), the covalent bond between the two thymines could be cleaved, recovering the intact base sequence. Mechanistic details promoting the self-repair remained unclear, however. In our theoretical study, we investigated whether optical excitation could directly lead to a charge-transfer state, thereby initiating the repair, or whether the initial excited state remains localized on a single nucleobase. We performed conformational sampling of 200 geometries of the damaged DNA double strand solvated in water and used a hybrid quantum and molecular mechanics approach to compute excited states at the complete active space perturbation level of theory. Analysis of the conformational data set clearly revealed that the excited-state properties are uniformly distributed across the fluctuations of the nucleotide in its natural environment. From the electronic wavefunction, we learned that the electronic transitions remained predominantly local on either adenine or guanine, and no direct charge transfer occurred in the experimentally accessed energy range. The investigated base sequence is not only specific to the CPD repair mechanism but ubiquitously occurs in nucleic acids. Our results therefore give a very general insight into the charge locality of UV-excited DNA, a property that is regarded to have determining relevance in the structural consequences following absorption of UV photons.

Photochemistry from low-lying states of HOSO.

Using configuration interaction ab initio methods, the evolution of the lowest electronic states of singlet and triplet spin multiplicities of HOSO+ along the stretching and bending coordinates of is investigated. Equilibrium geometries, rotational constants, and harmonic vibrational frequencies of the lowest electronic states are calculated, i.e., X1A', 11A″, 13A', and 13A″. The global minimum of the 11A″ state is located below the first dissociation limit and its calculated lifetime is predicted to be 0.40 µs, making it suitable for detection by laser-induced fluorescence. According to the potential energy surfaces, HOSO+ should produce SO2 + and H after ultraviolet photon absorption to the 21A' state. This work opens the door to investigate the branching ratio and the production rates of SO2 +, SO+, and OH from HOSO+. These insights can help understand the SO2 cycle in the earth's atmosphere and its effect on cooling our planet.

Hexagonal MASnI3 exhibiting strong absorption of ultraviolet photons

MASnI3, an organometallic halide, has great potential in the field of lead-free perovskite solar cells. Ultraviolet photons have been shown to generate deep trapping electronic defects in mesoporous TiO2-based perovskite, affecting its performance and stability. In this study, the structural, electronic, and optical properties of the cubic, tetragonal, and hexagonal phases of MASnI3 were studied using first-principles calculations. The results indicate that the hexagonal phase of MASnI3 possesses a larger indirect bandgap and a larger carrier effective mass along the c-axis compared to the cubic and tetragonal phases. These findings were attributed to enhanced electronic coupling and localization in the hexagonal phase. Moreover, the hexagonal phase exhibited high absorption of ultraviolet photons and high transmission of visible photons, particularly along the c-axis. These characteristics demonstrate the potential of hexagonal MASnI3 for application in multijunction perovskite tandem solar cells or as coatings in mesoporous TiO2-based perovskite solar cells to enhance ultraviolet stability and photon utilization.

Radiation feedback in dusty clouds

Abstract We have investigated the impact of photoionization and radiation pressure on a dusty star-forming cloud using one-dimensional radiation hydrodynamic simulations, which include absorption and re-emission of photons by dust. We find that, in a cloud of mass 105 M⊙ and radius 17 pc, the effect of radiation pressure is negligible when star formation efficiency is 2 per cent. The importance of radiation pressure increases with increasing star formation efficiency or an increasing dust-to-gas mass ratio. The net effect of radiation feedback, however, becomes smaller with the increasing dust-to-gas mass ratio, since the absorption of ultraviolet photons by dust grains suppresses photoionization and hence photoheating.

Retraction: Radiation feedback in dusty clouds

We have investigated the impact of photoionization and radiation pressure on a dusty star-forming cloud using one-dimensional radiation hydrodynamic simulations, which include absorption and re-emission of photons by dust. We find that, in a cloud of mass 10^5 Msun and radius 17 pc, the effect of radiation pressure is negligible when star formation efficiency is 2%. The importance of radiation pressure increases with increasing star formation efficiency or an increasing dust-to-gas mass ratio. The net effect of radiation feedback, however, becomes smaller with the increasing dust-to-gas mass ratio, since the absorption of ultra-violet photons by dust grains suppresses photoionization and hence photoheating.

Organic Compounds in Star Forming Regions

The influence of complex dust composition on the general chemical evolution of a prestellar core and the content of complex organic compounds is studied. It is shown that various component groups respond differently to the presence of a small dust population. At early stages the difference is determined primarily by changes in the balance of photo processes due to effective absorption of ultraviolet photons by small dust grains of the second population and collisional reactions with dust particles. At later stages differences are also caused by the growing dominance of additional reaction channels related to surface organic synthesis.

Optical Characterization and Computational Chemical Evaluation of Electronic Localized States in Polyolefin

SUMMARYOptical absorption spectra and photoluminescence spectra were obtained for eight kinds of polyolefin sheet samples using photons in a range from visible to vacuum ultraviolet (UV). Almost all samples were found to exhibit an absorption peak at around 6.5 eV and a luminescence band at around 4.3 eV. The luminescence was found to be induced by the absorption. Furthermore, it was found that successive absorption of UV photons weakened the luminescence intensity. It is assumed from these results that ‐unsaturated carbonyls are luminous and that the carbonyls are decomposed through the Norrish type II reaction by absorbing UV photons. Quantum chemical calculations were carried out using polyethylene models with and without an unsaturated carbonyl to verify the above‐mentioned assumption. As a result, the model with an unsaturated carbonyl was found to have localized electronic states in the forbidden band. One of the differential energies between the states is close to the photon energy, by which the luminescence is induced. The bond length of a double bond, which is next to the carbonyl, was found to be longer at the excited singlet state than at the ground state. These results obtained by computation support the above‐mentioned assumption of the luminescence center and its decomposition.

Optical Characterization and Computational Chemical Evaluation of Electronic Localized States in Polyolefin

SUMMARY Optical absorption spectra and photoluminescence spectra were obtained for eight kinds of polyolefin sheet samples using photons in a range from visible to vacuum ultraviolet (UV). Almost all samples were found to exhibit an absorption peak at around 6.5 eV and a luminescence band at around 4.3 eV. The luminescence was found to be induced by the absorption. Furthermore, it was found that successive absorption of UV photons weakened the luminescence intensity. It is assumed from these results that -unsaturated carbonyls are luminous and that the carbonyls are decomposed through the Norrish type II reaction by absorbing UV photons. Quantum chemical calculations were carried out using polyethylene models with and without an unsaturated carbonyl to verify the above-mentioned assumption. As a result, the model with an unsaturated carbonyl was found to have localized electronic states in the forbidden band. One of the differential energies between the states is close to the photon energy, by which the luminescence is induced. The bond length of a double bond, which is next to the carbonyl, was found to be longer at the excited singlet state than at the ground state. These results obtained by computation support the above-mentioned assumption of the luminescence center and its decomposition.

Radical Directed Dissociation for Facile Identification of Iodotyrosine Residues Using Electrospray Ionization Mass Spectrometry

Iodination of tyrosine residues in proteins has many uses in chemistry, biology, and medicine. Site specific identification of the sites of iodination is important for many of these uses. Reported herein is a facile method employing photodissociation and mass spectrometry to localize sites of iodination in whole proteins. Absorption of ultraviolet photons by iodotyrosine results in loss of iodine via homolytic bond dissociation. The resulting protein radical fragments in the vicinity of the iodotyrosine upon collisional activation. Analysis of the fragments within the vicinity of each tyrosine residue in the protein enables quantitative evaluation of the likelihood for iodination at each site. The results are compared with both traditional bottom up and top down mass spectrometric methods. Radical directed dissociation yields results in agreement with traditional approaches but requires significantly less effort and is inherently more sensitive. One limitation occurs when multiple tyrosine residues are in close proximity, in which case the extent of iodination at each residue may be difficult to determine. This limitation is frequently problematic for traditional approaches as well.

Ultraviolet photon absorption in single- and double-wall carbon nanotubes and peapods: Heating-induced phonon line broadening, wall coupling, and transformation

Ultraviolet photon absorption has been used to heat single- and double-wall carbon nanotubes and peapods in vacuum. By increasing the laser intensity up to $500\phantom{\rule{0.3em}{0ex}}\mathrm{mW}$, a downshift and a broadening of the optical phonons are observed corresponding to a temperature of $1000\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. The UV Raman measurements are free of blackbody radiation. We find that the linewidth changes for the ${G}_{+}$ and ${G}_{\ensuremath{-}}$ bands differ considerably in single-wall carbon nanotubes. This gives evidence that the phonon decay process is different in axial and radial tube directions. We observe the same intrinsic linewidths of graphite (highly oriented pyrolytic graphite) for the $G$ band in single- and double-wall carbon nanotubes. With increasing temperature, the interaction between the walls is modified for double-wall carbon nanotubes. Ultraviolet photon induced transformations of peapods are found to be different on silica and diamond substrates.

UV-VUV Excitation Luminescence Properties of Eu2 + -doped Ba2MSi2O7 ( M = Mg , Zn )

-doped were prepared by a solid-state reaction. All samples appeared white in body and remained with a single monoclinic phase. Monitored by emission, the excitation spectra were composed of three bands centered around 256, 290, and in the ultraviolet region and two bands at about 150 and in the vacuum ultraviolet (VUV) region. The excitation around was dominated in VUV region and the strength became reduced while it was enhanced in the ultraviolet (UV) region with the increasing -content. It could be ascribed to the tetrahedra associated with the absorption of UV photons and the tetrahedra of VUV photons. Green emissions peaking around due to 5d-4f transitions were observed both under 254 and excitations. The emission intensity increased under excitation as decreased with excitation along with the increasing -content and this was consistent with the excitation characteristics.

Ultraviolet-assisted pulsed laser deposition of thin oxide films

The properties of thin Y2O3, ZnO, and Ba0.5Sr0.5TiO3 films grown using an in situ ultraviolet (UV)-assisted pulsed laser deposition (UVPLD) technique have been studied. With respect to films grown by conventional PLD under similar conditions but without UV illumination, the UVPLD grown films exhibited better structural, optical, and electrical properties, especially for lower substrate temperatures. These improvements can be explained by the combined action of several processes. First, high energy UV photons and ozone ensure a better in situ cleaning of the substrate prior to the deposition. Second, the presence of ozone and atomic oxygen formed by photodissociation of molecular O2 promotes the growth of more oxygenated films. Finally, absorption of UV photons by adatoms could result in an increase of their surface mobility. All these factors have a beneficial effect upon crystalline growth.

New Measurements of Hydroxyl in the Middle Atmosphere Confound Chemical Models

The drive to measure, understand, and prevent ozone depletion over Antarctica has been so intense and attention-grabbing that the words “ozone” and “hole” now seem mated for life. But the ozone layer, which is densest at altitudes of 15–20 km, envelops the entire planet and extends into the mesosphere, where it plays a key role in the atmosphere's physics and chemistry. Ozone molecules, heated by their absorption of ultraviolet photons from the Sun, provide most of the energy that drives circulation in the upper stratosphere and mesosphere. (Figure 1 depicts the vertical structure of the atmosphere.)

Characteristics of Ultra violet-Assisted Pulsed Laser Deposited Thin Oxide Films

AbstractThe properties of thin oxide films such as Y2O3, ZnO and Ba0.5Sr0.5TiO3 grown using an in situ ultraviolet (UV)-assisted pulsed laser deposition (UVPLD) technique have been studied. With respect to films grown by conventional PLD under similar conditions but without UV illumination, the UVPLD grown films exhibited better structural and optical and electrical properties, especially for lower substrate temperatures. They also exhibited a better stoichiometry and contained less physisorbed oxygen than the conventional PLD grown layers. These improvements can be traced to several factors. Firstly, deep UV photons and ozone ensure a better in situ cleaning of the substrate prior to the deposition. Secondly, the presence of more reactive gaseous species like ozone and atomic oxygen formed by photodissociation of molecular O2 promotes the growth of more oxygenated films. Thirdly, absorption of UV photons by adatoms could result in an increased of their surface mobility. All these factors have a beneficial effect upon crystalline growth, especially for moderate substrate temperatures. For optimised growth conditions, the crystalline quality and properties of ultraviolet-assisted pulsed laser deposited films was similar to that of films grown using conventional PLD at substrate temperatures of at least 200°C higher.

Investigating the Near‐Infrared Properties of Planetary Nebulae. II. Medium‐Resolution Spectra

We present medium-resolution (R approximately 700) near-infrared (lambda = 1 - 2.5 micrometers) spectra of a sample of planetary nebulae (PNe). A narrow slit was used which sampled discrete locations within the nebulae; observations were obtained at one or more positions in the 41 objects included in the survey. The PN spectra fall into one of four general categories: H1 emission line-dominated PNe, H1 and H2 emission line PNe, H2 emission line-dominated PNe, and continuum-dominated PNe. These categories correlate with morphological type, with the elliptical PNe falling into the first group, and the bipolar PNe primarily in the H2 and continuum emission groups. The categories also correlate with C/O ratio, with the O-rich objects falling into the first group and the C-rich objects in the groups. Other spectral features were observed in all catagories, such as continuum emission from the central star, and warm dust continuum emission towards the long wavelength end of the spectra. H2 was detected in four PNe in this survey for the first time. An analysis was performed using the H2 line ratios in all of the PN spectra in the survey where a sufficient number of lines were observed to determine the ortho-to-para ratio and the rotational and vibrational excitation temperatures of the H-2 in those objects. One unexpected result from this analysis is that the H-2 is excited by absorption of ultraviolet photons in most of the PNe, although there are several PNe in which collisional excitation plays an important role. The correlation between bipolar morphology and H2 emission has been strengthened with the new detections of H2 in this survey.

Ultraviolet photoabsorption measurements in alkali iodide and caesium bromide evaporated films

Ultraviolet–photon absorption constants were measured for thin CsI, KI, RbI, NaI and CsBr evaporated films, in the 140–200 nm spectral range. The constant measured for CsI is consistent with literature data; the absorption data for the other materials are given here for the first time. The absorption lengths for all materials investigated vary between 10 and 40 nm. Post-evaporation thermal treatment of the films, known to enhance the photoemission properties of CsI, NaI and CsBr films, had no effect on their photoabsorption properties.

P-type electrical conduction in transparent thin films of CuAlO2

Optically transparent oxides tend to be electrical insulators, by virtue of their large electronic bandgap (⩾3.1 eV). The most notable exceptions are doped versions of the oxides In2O3, SnO2 and ZnO—all n-type (electron) conductors—which are widely used as the transparent electrodes in flat-panel displays1,2. On the other hand, no transparent oxide exhibiting high p-type (hole) conductivity is known to exist, whereas such materials could open the way to a range of novel applications. For example, a combination of the two types of transparent conductor in the form of a pn junction could lead to a ‘functional’ window that transmits visible light yet generates electricity in response to the absorption of ultraviolet photons. Here we describe a strategy for identifying oxide materials that should combine p-type conductivity with good optical transparency. We illustrate the potential of this approach by reporting the properties of thin films of CuAlO2, a transparent oxide having room-temperature p-type conductivity up to 1 S cm−1. Although the conductivity of our candidate material is significantly lower than that observed for the best n-type conducting oxides, it is sufficient for some applications, and demonstrates that the development of transparent p-type conductors is not an insurmountable goal.