near-UV Spectral Region Research Articles

Daytime operation of a pure rotational Raman lidar by use of a Fabry–Perot interferometer

We propose to use a Fabry-Perot interferometer (FPI) in a pure rotational Raman lidar to isolate return signals that are due to pure rotational Raman scattering from atmospheric nitrogen against the sky background. The main idea of this instrumental approach is that a FPI is applied as a frequency comb filter with the transmission peaks accurately matched to a comb of practically equidistant lines of a pure rotational Raman spectrum (PRRS) of nitrogen molecules. Thus a matched FPI transmission comb cuts out the spectrally continuous sky background light from the spectral gaps between the PRRS lines of nitrogen molecules while it is transparent to light within narrow spectral intervals about these lines. As the width of the spectral gaps between the lines of the PRRS of nitrogen molecules is -114 times the width of an individual spectral line, cutting out of the sky background from these gaps drastically improves the signal-to-background ratio of the pure rotational Raman lidar returns. This application of the FPI enables one to achieve daytime temperature profiling in the atmosphere with a pure rotational Raman lidar in the visible and near-UV spectral regions. We present an analysis of application of the FPI to filtering out the pure rotational Raman lidar returns against the solar background. To demonstrate the feasibility of the approach proposed, we present temperature profiles acquired during a whole-day measurement session in which a Raman lidar equipped with a FPI was used. For comparison, temperature profiles acquired with Vaisala radiosondes launched from the measurement site are also presented.

Diode-pumped, ultrafast, multi-octave supercontinuum source at repetition rates between 500 kHz and 20 MHz using Yb:glass lasers and tapered fibers

We present a compact, all diode-pumped supercontinuum source based on a SESAM mode-locked Yb:glass oscillator at 1040 nm and a tapered fiber. The oscillator has a repetition rate of 20 MHz, a pulse duration of 200 fs, and a maximum pulse energy of about 15 nJ. This system delivers an 1100 nm broad spectrum with an output power of more than 100 mW. Decreasing the repetition rate to 500 kHz by cavity-dumping results in a supercontinuum with a high pulse energy of about 50 nJ. Furthermore, using the frequency-doubled output of this laser at 520 nm with 300 fs pulse duration resulted in supercontinua in the near-UV and visible spectral region. We compare the experimental spectra with theoretical simulations.

Light-induced fluorescence changes in Phycomyces: evidence for blue light-receptor associated flavo-semiquinones.

Light-induced fluorescence changes (LIFCs) were detected in sporangiophores of the blue-light-sensitive fungus Phycomyces blakesleeanus (Burgeff). The LIFCs can be utilized as a spectrophotometric assay for blue-light photoreceptors and for the in vivo characterization of their photochemical primary reactions. Blue-light irradiation of sporangiophores elicited a transient decrease and subsequent regeneration of flavin-like fluorescence emission at 525 nm. The signals recovered in darkness in about 120 min. In contrast to blue light, near-UV (370 nm) caused an increase in the fluorescence emission at 525 nm. Because the LIFCs were altered in a light-insensitive madC mutant with a defective photoreceptor, the fluorescence changes must be associated with early photochemical events of the transduction chain. Action spectra for the fluorescence changes at 525 nm showed major peaks near 470 and 600 nm. Double-pulse experiments involving two consecutive pulses of either blue and near-UV, blue and red, or near-UV and red showed that the responses depended on the sequence in which the different wavelengths were applied. The results indicate a blue-light receptor with intermediates in the near-UV, blue and red spectral regions. We explain the results in the framework of a general model, in which the three redox states of the flavin photoreceptor, the oxidized flavin (Fl), the flavo-semiquinone (FlH*), and the flavo-hydroquinone (FlH2) are each acting as chromophores with their own characteristic photochemical primary reactions. These consist of the photoreduction of the oxidized flavin generating semiquinone, the photoreduction of the semiquinone generating hydroquinone, and the photooxidation of the flavo-hydroquinone regenerating the pool of oxidized flavins. The proposed mechanism represents a photocycle in which two antagonistic photoreceptor forms, Fl and FlH2, determine the pool size of the biological effector molecule, the flavo-semiquinone. The redox changes that are associated with the photocycle are maintained by redox partners, pterins, that function in the near-UV as secondary chromophores.

Spectral Classification of the Hot Components of a Large Sample of Stars with Composite Spectra, and Implication for the Absolute Magnitudes of the Cool Supergiant Components.

A sample of 135 stars with composite spectra has been observed in the near-UV spectral region with the Aurélie spectrograph at the Observatoire de Haute-Provence. Using the spectral classifications of the cool components previously determined with near infrared spectra, we obtained reliable spectral types of the hot components of the sample systems. The hot components were isolated by the subtraction method using MK standards as surrogates of the cool components. We also derived the visual magnitude differences between the components using Willstrop's normalized stellar flux ratios. We propose a photometric model for each of these systems on the basis of our spectroscopic data and the Hipparcos data. We bring to light a discrepancy for the G supergiant primaries between the visual absolute magnitudes deduced from Hipparcos parallaxes and those tabulated by Schmidt-Kaler for the GIb stars: we propose a scale of Mv-values for these stars in composite systems. By way of statistics, about 75% of the hot components are dwarf or subgiant stars, and 25% should be giants. The distribution in spectral types is as follows: 41% of B-type components, 57% of type A, and 2% of type F; 68% of the hot components have a spectral type in the range B7 to A2. The distribution of the ΔMv-values shows a maximum near 0.75 mag.

Short-wavelength solar-blind detectors-status, prospects, and markets

Recent advances in the research work on III-nitride semiconductors and Al/sub x/Ga/sub 1-x/N materials in particular has renewed the interest and led to significant progress in the development of ultraviolet (UV) photodetectors able to detect light in the mid- and near-UV spectral region (/spl lambda//spl sim/200-400 nm). There have been a growing number of applications which require the use of such sensors and, in many of these, it is important to be able to sense UV light without detecting infrared or visible light, especially from the Sun, in order to minimize the chances of false detection or high background. The research work on short-wavelength UV detectors has, therefore, been recently focused on realizing short-wavelength "solar-blind" detectors which, by definition, are insensitive to photons with wavelengths longer than /spl sim/285 nm. In this paper the development of Al/sub x/Ga/sub 1-x/N-based solar-blind UV detectors will be reviewed. The technological issues pertaining to material synthesis and device fabrication will be discussed. The current state-of-the-art and future prospects for these detectors will be reviewed and discussed.

A Dendritic Antenna for Near-Infrared Emission of Nd3+ Ions

An interior of 18 amide groups and a periphery functionalized with 24 dansyl groups forms a light-harvesting dendrimer which features intense absorption bands in the near-UV spectral region and a strong fluorescence band in the visible region. Upon encapsulation of Nd(3+) ions, the fluorescence of the dansyl groups is quenched and an intense sensitized near-infrared emission of Nd(3+) is observed. The associated energy transfer is shown in the cartoon.

Optical characterization of chalcogenide thin films

In this paper, the optical characterization of a film of amorphous As–S chalcogenides evaporated on glass substrates will be performed using variable angle of incidence spectroscopic ellipsometry (VASE) and near-normal incidence spectroscopic reflectometry (NNSR). The spectral dependences of the ellipsometric parameters and reflectance of the chalcogenide thin film mentioned is measured within the near-UV, visible and near-IR spectral regions. For interpreting these optical quantities the new dispersion model of the spectral dependences of the optical constants of amorphous solids is employed. This model is based on the modified Lorentz oscillator. Within this model the concepts of the band gap and Urbach tail are respected.

High-frequency Bragg gratings in a photothermorefractive glass

Holographic UV mirrors were recorded in a volume of photothermorefractive glass. This photosensitive silicate glass doped with silver, cerium, and fluorine is transparent in the near-UV, visible, and near-IR spectral regions, and its induced refractive-index change reaches 10(-3). UV radiation of a He-Cd laser at 325 nm was used for Bragg grating recording with a spatial frequency of approximately 9200 mm(-1). The absolute diffraction efficiency of the recorded mirror reached 12.6% at 325 nm and did not deteriorate under the long-time effects of optical irradiation and heating up 400 degrees C.

Spectroscopic resolution of the picosecond reduction kinetics of the secondary electron acceptor A 1 in photosystem I

Forward electron transfer in photosystem I from Synechocystis sp. PCC 6803 has been studied in the picosecond time range with transient absorption spectroscopy in the blue and near-UV spectral regions. From the direct measurement, at 380–390 nm, of the reduction kinetics of the phylloquinone secondary acceptor A 1 and from the absence of spectral evolution between 100 ps and 2 ns, we conclude that electron transfer, from the chlorophyll a primary acceptor A 0, to A 1 occurs directly and completely with a time constant of about 30 ps.

Photochemical properties of 9-(4-azidophenyl)acridine

Syntheses are reported for 9-(azidophenyl)acridine and the corresponding N-methylacridine derivative, which have absorption bands in the near-UV and visible spectral regions. The photodissociation quantum yields upon irradiation of these azides at the long-wavelength band (365 nm) are 0.88 and <10−2, respectively. An MNDO-PM3 semiempirical analysis was carried out on the change in the bond orders, NNN bond angle, and charge redistribution upon excitation of these molecules to the lowest excited singlet state, which accounted for the experimentally observed photochemical activity of 9-(4-azidophenyl)acridine and inertness of its quaternerized derivative.

Luminescence spectroscopy of atomic zinc in rare-gas solids. I

Steady-state and time-resolved luminescence spectroscopy of atomic zinc isolated in thin film samples of the solid rare gases, prepared by the cocondensation of zinc vapor with argon, krypton, and xenon has been recorded at 6.3 K using synchrotron radiation. Pairs of emission bands result from photoexcitation of the singlet 4p 1P1←4s 1S0 resonance transition of atomic zinc, even in annealed samples. In Zn/Ar the pair of emission bands were observed in the uv at 218.9 and 238 nm and for Zn/Xe in the near-uv at 356 and 399 nm. For the Zn/Kr system two emission bands were observed in the uv region at 239.5 and 259 nm but in addition, a weaker band was present in the near-uv at 315.6 nm. In a given annealed rare-gas host, the excitation profiles recorded for all the emission bands are identical, exhibiting the threefold splitting characteristic of Jahn-Teller coupling in the triply degenerate excited P11 state. These excitation profiles are identified as the solid phase equivalent of the 4p 1P1←4s 1S0 resonance transition of atomic zinc occurring at 213.9 nm in the gas phase. Based on their spectral positions and temporal decay characteristics, the emission bands observed in the uv and near-uv spectral regions have been assigned as the singlet and triplet transitions, respectively, of atomic zinc. The origin of the pairs of emission bands is ascribed to the Jahn-Teller coupling between noncubic vibronic modes of the lattice and the excited 4p orbital of the P11 state of atomic zinc, resulting in the coexistence of two energy minima. In Zn/Ar, the effects of slow vibrational relaxation in the excited singlet state were evident in the relative intensities and temporal decay profiles of the pair of emission bands. Specifically, the lower energy emission band was favored with excitation of the highest energy component of the threefold split Jahn-Teller absorption band, while the higher-energy emission was favored with excitation of the lowest-energy component. The intensity of the triplet state emission was observed to be enhanced in the heavier rare gases, being completely absent in Ar, weak in Kr, and the only emission observed in Xe.

Electronic Structure of the Lowest Excited States of Cr(CO)(4)(2,2'-bipyridine): A CASSCF/CASPT2 Analysis.

The visible and UV absorption spectra of Cr(CO)(4)(bpy) are interpreted according to CASPT2 calculations based on CASSCF reference wave functions using atomic natural orbital (ANO) basis sets. The excitation energies of the lowest singlet MLCT (metal-to-ligand-charge-transfer) states range between 12 630 and 17 580 cm(-)(1). They originate in the excitation of chromium d electrons to the lowest pi(bpy) orbital of b(2) local symmetry. Dipole transition moments calculated for individual MLCT transitions show that the only transition expected to contribute significantly to the intense absorption band in the visible spectral region is the a(1)A(1) --> b(1)A(1) transition calculated at 17 580 cm(-)(1). This result agrees very well with the experimental spectra recorded in weakly solvating C(2)Cl(4), characterized by a band at 17 700 cm(-)(1). The low-energy emission band at 12 850 cm(-)(1) has been attributed to the lowest a(3)B(2) state calculated at 12 560 cm(-)(1). The next set of excited states corresponding to 3d --> pi(bpy,a)()2 excitations range between 25 370 and 26 200 cm(-)(1) for the singlets and between 24 630 and 25 750 cm(-)(1) for the triplets. The singlet excited states corresponding to d --> d excitations are calculated between 29 650 and 37 360 cm(-)(1). These results show that the intense absorption in the near-UV spectral region originates in strongly overlapping absorption bands due to closely spaced transitions into MLCT (a(2)) and dd excited states, respectively. The c(1)B(2) excited state corresponding to the 3d(xz)() --> 3d(z)()()2 excitation, proposed as photoactive in the mechanism of the efficient CO loss under irradiation at 27 630 cm(-)(1), is calculated at 36 320 cm(-)(1) and is far too high to be directly populated in these photochemical studies. Its mixing with one or several low-lying (singlet) MLCT states at the early stage of the reaction path could be responsible for the observed primary reaction. A comparison between the excitation energies of the lowest singlet states of Cr(CO)(4)(bpy) and Cr(CO)(4)(dab) is reported. The most significant feature is the lowering of the MLCT excited states on going from the bpy-containing molecule to its dab (1,4-diaza-1,3-butadiene) analog.

Allosteric intermediates in hemoglobin. 1. Nanosecond time-resolved circular dichroism spectroscopy.

Time-resolved circular dichroism (TRCD) studies performed on photolyzed hemoglobin-CO complex (HbCO) probe room temperature protein relaxations in Hb, including the R --> T allosteric transition. TRCD spectroscopy of photolysis intermediates in the near-UV (250-400 nm) spectral region provides a diagnostic for T-like structure at the alpha 1 beta 2 interface via the effect of quaternary structure on the UV CD of aromatic residues. The TRCD of porphyrin-based transitions in the UV and Soret regions, reflecting transition-dipole couplings between hemes and aromatic residues over a radius wide enough to permit heme-interface and inter-dimer interactions, is modulated by the tertiary and quaternary structure of photolysis intermediates. In the allosteric core model of Hb cooperativity, Fe-CO bond breakage initiates a heme structural change, thought to be heme doming, that is transmitted to the alpha 1 beta 2 interface via the F helix. The TRCD results, analyzed in light of kinetic information from time-resolved absorption studies, suggest specific features for the mechanism by which the ensuing tertiary and quaternary conformational changes propagate through the protein. In particular, the UV-TRCD indicates that the alpha 1 beta 2 interface responds within several hundred nanoseconds to initial events at the heme by shifting from an R toward a T-like interface. The appearance of T-like character at the alpha 1 beta 2 interface tens of microseconds before the appearance of equilibrated T state deoxyHb indicates that the R --> T transition in photolyzed HbCO is a stepwise process, as previously suggested by time-resolved resonance Raman studies.

Allosteric intermediates in hemoglobin. 2. Kinetic modeling of HbCO photolysis.

Nanosecond absorption spectra are measured in the Soret and near-UV spectral regions of human hemoglobin (Hb) after laser photolysis of the carbonyl adduct in order to study the dynamics of globin tertiary and quaternary conformational changes. Spectra and concentrations of physical intermediates, distinguished by extent of heme ligation and intraprotein relaxation, are obtained from a global analysis using a microscopic kinetic model that explicitly accounts for six observed relaxation and recombination processes. Three observed rate constants for CO rebinding and two intraprotein relaxation constants obtained are similar to constants determined by Hofrichter et al. [(1983) Proc. Natl. Acad. Sci. U.S.A. 80, 2235], the latter two comprising the 20-30-microseconds R --> T quaternary transition and a previously unassigned 1-microseconds intraprotein relaxation. On the basis of the modeled intermediate spectra, as well as UV circular dichroism results observed on this time scale [Björling, S.C., Goldbeck, R.A., Paquette, S.J., Milder, S.J., & Kliger, D.S. (1996) Biochemistry 35, 8619-8627], the 1-microsecond relaxation is assigned to heme conformational changes concomitant with a relaxation of protein conformation at the alpha 1 beta 2 interface corresponding to an initial step in a compound R --> T reaction path.

Magneto-optical enhancement in Pt/(Ni 1− xCo x) multilayers

Magneto-optical (MO) polar Kerr rotation (PKR) and ellipticity (PKE) spectra of Pt/(Ni 1− x ) multilayers with x = O, 0.3 and 1 nm prepared by evaporation under ultrahigh-vacuum conditions were studied. The thickness of the Pt layer was kept between 1.5 and 2 nm, and that of the ({Ni 1− x Co x }) alloy layer, t Ni 1− x Co x , was 0.45 and 1.4 nm Layers formed at the Pt—magnetic layer interfaces give rise to a surface-induced perpendicular magnetic anisotropy (PMA) and to an enhancement of PKR in the near-UV spectral region. Because the Curie temperature T c is close to 300 K for x = O a clear manifestation of these effects in the Ni/Pt system required cooling to low temperatures. The possibility to optimize the characteristics, like PMA, T C and the amplitude and position of the PKR peak in the spectra by proper choices of x, t Pt and t Pt and t Ni 1− x Co x makes Pt/(Ni 1− x Co x ) multilayers interesting for the realization of MO storage media capable of working in blue light.

A tyrosine-derived free radical in apogalactose oxidase.

Oxidation of apogalactose oxidase with ferricyanide leads to the formation of a stable free radical exhibiting distinctive optical absorption and EPR spectral features. The radical is associated with absorption in both near-UV and near-IR spectral regions, and its EPR spectrum is characteristic of an aromatic free radical with gav = 2.005. Reconstitution of both the apoenzyme and the free radical-containing form with copper substantially restores both the absorption spectra and the catalytic activity of the active enzyme, indicating that the preparation of the radical species does not significantly damage the protein. The absence of a free radical EPR signal in reconstituted and activated galactose oxidase containing nearly stoichiometric copper suggests the radical is an active site species relating to the free radical-coupled copper site previously proposed for this enzyme. Isotopic labeling experiments demonstrate that the radical derives from a tyrosine residue. The distinctive spectra associated with this radical indicate an environment which is different from that associated with the tyrosyl phenoxyl sites in other free radical enzymes.

Bronze Baby Syndrome: An Animal Model

We evaluated the appropriateness of an animal model for the bronze baby syndrome. Ligation of the common bile duct in adult Wistar rats induces an accumulation of porphyrins and copper in the liver and a 20% conversion of protoporphyrin IX into (Cu(II)-protoporphyrin IX. Upon irradiation of these animals with super-blue lamps, the plasma content of Cu(II)-protoporphyrin increases by about 30%. Cholestasis also increases the recovery of porphyrins in the urine, although light treatment of ligated rats further increases urinary porphyrin excretion. The spectroscopic changes induced by irradiation of sera of ligated rats are consistent with the formation of products that have the typical spectrum found in bronze baby syndrome patients, i.e. a reduced absorbance in the visible region and an increased absorption in near-UV and red spectral regions. The products responsible for the brown discoloration found in bronze baby syndrome seem to result from phototransformation of copper-porphyrins subsequent to an electron transfer between photoexcited bilirubin and the copper ion.

Optical anisotropy of YBa2Cu3O7-x.

The optical anisotropy of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$ in the 0.08--5.6 eV region is investigated by polarized reflectance measurements on single crystals, and by transmittance, reflectance, and ellipsometric measurements on oriented thin films. In the visible--near-uv spectral region the anisotropy is relatively mild, but a huge anisotropy is observed in the infrared. Here, the reflectance increases with decreasing frequency for both polraizations, consistent with metallic conductance both parallel and perpendicular to the ab plane. The c-axis plasma frequency is strikingly low, but there is strong damping which could originate from a continuum of low-energy interband transitions.

Development of a New Optical Wavelength Rejection Filter: Demonstration of its Utility in Raman Spectroscopy

A new tunable optical filter has been developed which rejects a narrow wavelength interval (&lt;7.5 nm) in the near-UV, visible, or near-IR spectral region and allows adjacent wavelengths to pass ( T &gt; 90%). This filter will be useful in optics, in spectroscopy, and for laser applications. The active element of the filter is a crystalline colloidal array of polystyrene spheres. The rejected wavelengths are Bragg diffracted from this ordered array. For a particular sphere concentration and scattering from a particular set of lattice planes, tunability can be achieved by the altering of the angle between the filter and incident light beam. Bragg diffraction and light rejection of these filters are monitored by transmission measurements. The utility of this filter for spectroscopic measurements is demonstrated for Raman spectroscopy. Raman measurements are shown for polypropylene, a highly scattering material with numerous low-frequency modes. The filter selectively attenuates the elastically scattered light and allows the low frequency peaks to be observed. Use of this wavelength rejection filter to reject the Rayleigh scattered light simplifies the instrumentation, decreases the cost, and increases the sensitivity of Raman spectral measurements. This filter also has the potential to replace dispersive elements such as gratings and prisms in a variety of spectroscopic and optical applications.

Spectral emissivity of tungsten: analytic expressions for the 340-nm to 26-μm spectral region

To correct emission spectra a standard radiance source is often used to determine the spectral responsivity of the detection system. In the near-UV, visible, and near-IR spectral regions the most common radiance standard is a tungsten strip lamp calibrated by a standards laboratory. For day-to-day experiments where slightly less accuracy is acceptable, a less expensive uncalibrated lamp is useful. In this case, the radiant temperature T/sub r/ of the lamp is measured with an optical pyrometer, generally at a single wavelength such as 650 nm, and the source spectral radiance L(lambda) is calculated from L(lambda) = tau(lambda)epsilon(lambda,T)L/sub B/(lambda,T). The transmittance of the source is tau(lambda), the spectral emissivity is epsilon(lambda,T), and L/sub B/(lambda,T) is the spectral distribution of blackbody radiation, Planck's radiation law. To obtain the true temperature T, Wien's approximation is employed. To conveniently calibrate a system, especially one which utilizes a microcomputer, it is advantageous to have analytic expressions for the spectral emissivity of tungsten. Although Larrabee has published such expressions, they are limited to the 450-800-nm spectral region. To obtain analytic expressions from 340 nm to 2.6 ..mu..m they have used the measurements of DeVos. Although DeVos's results differ by 2% from those of Larrabee, this differencemore » is assumed to be acceptable.« less