UV Transitions Research Articles

Intensity of the 250 nm Transition of the Pyridine Chromophore and of Some Methyl Derivatives

The intensity of the near UV transition of pyridine, pyrazine, and some of their methyl derivatives, is studied on the ground of the Interaction Vector Model (IVM), assuming that this transition is similar to the secondary one of the benzene chromophore.

D-2-Deoxyribose and D-Arabinose, but not D-Ribose, Stabilize the Cytosine Tetrad (i-DNA) Structure

Described here are studies exploring the effect of the sugar-phosphate backbone on the stability of i-tetrads in solution [K. Gehring et al. Nature 363, 561–565 (1993)]. In the accompanying paper, branched oligonucleotides are shown to be effective probes for organizing oligodeoxycytidine strands into I-motif structures (C-tetrads). Specifically, the joining of a pair of parallel deoxycytidylate strands with a riboadenosine “linker” leads to marked enhancement in stability of the tetrad structure. To further characterize the nature of the sugar-sugar interactions in this novel structure, branched oligonucleotides containing D-arabinocytidine and D-ribocytidine were synthesized and their association properties examined. The ribo oligomers were prepared in two regioisomeric forms differing only in the connectivities of the deoxycytidine strands, i.e., 3′-to-5′ versus 2′-to-5′ linked dC5 strands. The branched D-deoxycytidine analogue, rA(2′,5′-dC5)3′,5′-dC5, which previously has been shown to fold into a bimolecular I-motif, served as model system. It is found that the arabinose substitution leads to hypochromic structures that are characteristic of four-stranded intercalated DNA and has little, if any, effect on the stability of the complex formed. Parallel experiments with the branched ribocytidine analogs gave very weak or no discernible UV transitions, consistent with no strand association in this case [Lacroix et al., Biochemistry 35, 8715–8722 (1996)]. These results are discussed in relation to expected steric interactions of oligocytidine strands within the I-structure. The findings increase our understanding of the impact of the sugar and intemucleotide connectivity on the stability of this higher-order nucleic acid structure.

Conformational and Energetic Analysis of Saturated Organic Ring Compounds by 2 + 1 Resonance-Enhanced Multiphoton Ionization Spectroscopy

The UV transition to the 3s Rydberg state in a number of saturated cyclic compounds has yielded a wealth of structural and energetic information. The HOMO → 3s spectra of pulsed molecular beam cooled samples can be conveniently investigated by 2 + 1 resonance-enhanced multiphoton ionization (REMPI) using pulsed laser light in the 360−420 nm region. The transition origins for a variety of methyl- and ethyl-substituted cyclohexanes, cyclohexanones, cyclopentanones, and tetrahyrdopyrans, as well as bicyclic compounds such as decalone and various camphors, are highly sensitive to the location of alkyl substitution as well as the orientation of the alkyl group (axial vs equatorial). In addition, the orientation of the ethyl rotor can be determined from the spectra. This technique is especially useful for investigating molecules in which several equilibrating conformations make the analysis of room-temperature spectra difficult. By rapid cooling in the molecular beam, higher energy conformers can be frozen out ...

Luminescence spectra of matrix isolated N2 at high pressure and low temperature

N2 molecules with a concentration of ca. 1% were isolated in Xe, Kr, and Ar. X-ray stimulated luminescence A ∑u+2→X ∑g+1) was investigated as a function of pressure (<30 GPa), temperature (300 K and 77 K), and matrix material. Observed UV transitions were interpreted as rovibronic transitions of the impurity molecule along with phonon sidebands as lattice excitations. Spectra were analyzed due to band maxima of vibron progressions, bandwidth, and bandshape, Franck–Condon profiles, and molecule constants, each as a function of pressure. The main result was a shift of band maxima with pressure to lower energies at a rate of 100 cm−1/GPa in Xe, 50 cm−1/GPa in Kr, and 10 cm−1/GPa in Ar. We modeled and explained these pressure-induced shifts of electronic bands by two mechanisms: a dielectric effect and a volume effect.

Spatial and Temporal Behavior of a Laser Generated Titanium Plasma

Optical emission studies have been performed on the laser ablation of Titanium metallic targets. The high intensity radiation of a pulsed N2 laser (337.1 nm; 8 J/cm2) was focused onto the target to generate a short living (150 ns) plasma cloud that expands away from the surface into vacuum at high velocities. Time resolved measurements were taken as a function of the distance from the studied surface with a spatial rsolution of 20 µm, by collecting the emitted light in a direction perpendicular to the plasma expansion axis. A simple diffusion model with a “source” term is proposed to demonstrate the interaction of the laser beam with the plasma plume, as derived from analysis of the space-time behavior of several UV and visible atomic transitions.

Experimental evidence for strong UV transition contribution in the resonant nonlinearity of doped fibers

We report a new theoretical interpretation of the strong resonantly-enhanced third-order nonlinearity observed in fibers doped with Nd/sup 3+/, Er/sup 3+/, or Yb/sup 3+/. For each fiber, the absolute contributions of the near resonance (IR-visible) and far from resonance (UV) transitions to the nonlinearity are calculated and compared to experimental values. We show that in all cases, a large fraction of the measured nonlinearity is due to strong UV transitions. The consequences of this observation on the bandwidth, switching power and response time of switches based on this effect are discussed.

96/05685 Quantitative electron spectroscopic analysis of the surface chemistry of bituminous coal

The combined effects of elevated free-stream turbulence intensity (FSTI) and streamwise acceleration on flow and thermal structures in transitional boundary layers were investigated experimentally on a heated flat plate with elevated FSTI levels ranging from 3 to 7%. The acceleration strength, based on the parameter, K [≡ (ν/U∞2)dU∞/dx], rangedfrom 0.39 × 10−6 to 4.1 × 10−6. A three-wire probe was used to measure detailed momentum and thermal boundary layer structures, including the streamwise and cross-stream velocity fluctuations, temperature fluctuation, Reynolds stresses, Reynolds heat fluxes, eddy viscosity, turbulent thermal diffusivity, and turbulent Prandtl number. The results show that FSTI has a greater effect than streamwise acceleration on flow and thermal structures in transitional boundary layers. However, the effect of streamwise acceleration on increasing the transition length is significant. Furthermore, streamwise acceleration suppresses the magnitude of uv and slows down the uv transition process relative to u′ even when elevated FSTI seems to have a dominant effect. Acceleration is also shown to suppress ϵH more than ϵM in the transitional region.

Combined effects of elevated free-stream turbulence and streamwise acceleration on flow and thermal structures in transitional boundary layers

The combined effects of elevated free-stream turbulence intensity (FSTI) and streamwise acceleration on flow and thermal structures in transitional boundary layers were investigated experimentally on a heated flat plate with elevated FSTI levels ranging from 3 to 7%. The acceleration strength, based on the parameter, K [≡ (ν/ U ∞ 2)d U ∞/dx] , rangedfrom 0.39 × 10 −6 to 4.1 × 10 −6. A three-wire probe was used to measure detailed momentum and thermal boundary layer structures, including the streamwise and cross-stream velocity fluctuations, temperature fluctuation, Reynolds stresses, Reynolds heat fluxes, eddy viscosity, turbulent thermal diffusivity, and turbulent Prandtl number. The results show that FSTI has a greater effect than streamwise acceleration on flow and thermal structures in transitional boundary layers. However, the effect of streamwise acceleration on increasing the transition length is significant. Furthermore, streamwise acceleration suppresses the magnitude of uv and slows down the uv transition process relative to u′ even when elevated FSTI seems to have a dominant effect. Acceleration is also shown to suppress ϵ H more than ϵ M in the transitional region.

Light force cooling, focusing, and nanometer-scale deposition of aluminum atoms

We deposited neutral-aluminum atoms in thin parallel lines to form a grating with a line separation of 154.7 nm by using near-resonant laser light and direct-write optical lithography techniques. We did this by using the single-frequency closed UV transition from the second ground state 3p2P3/2 (F = 4) to 3d2D5/2 (F = 5) at 309.4 nm. The aluminum features were analyzed with an atomic-force microscope.

THE PHOTOPHYSICS OF p‐AMINOBENZOIC ACID

Abstract— The lowest‐lying allowed UV transition in p‐aminobenzoic acid (PABA) is assigned Γ→1La based on quantitative absorption and fluorescence studies, as well as semiempirical PM3 multielec‐tron configuration interaction calculations. The oscillator strengths, fluorescence quantum efficiencies and lifetimes are reported for PABA in several polar, nonpolar, protic and aprotic solvents (aerated) at 296 K. Reasonable agreement is found between the observed radiative rate constant and that calculated from the absorption and fluorescence spectra. Shifts in the absorption and fluorescence spectra in aprotic solvents are analyzed in terms of the Onsager reaction field model; results are consistent with an increase in dipole moment of ca 4 D between the relaxed S0 and S1, states. No evidence is found for the emission from the amino‐twisted form of PABA in all solvents studied although calculations show that the amino‐twisted S, state is highly polar, but higher in energy by ca 35 kJ/mol (in vacuo). The fluorescence efficiency is excitation wavelength independent in both methylcyclohexane and water. The temperature dependence of the nonradiative rate constant (from S1) was studied in several solvents. Nonradiative decay may be due to intersystem crossing, which would be fast enough to compete with thermally activated intramolecular NH2 twisting. The phosphorescence spectrum and lifetime obtained in an EPA glass at 77 K are reported, and the triplet energy of PABA is estimated.

Recoil-induced lasing of cold atoms.

We present a review and extension of a recently made proposal [K. M. Gheri et al., Phys. Rev. Lett. 74, 678 (1995)] to utilize cold atoms in a Raman configuration as a uv-laser gain medium in the limit of the photon recoil shift exceeding the natural atomic linewidth and the Doppler broadening. A discussion of model assumptions is given as well as an analysis of the dependence of the gain on detunings and decay rates. Special emphasis is put on a calculation of the nonlinear gain that allows a self-consistent determination of the uv-laser intensity as a function of the model parameters. For a deep uv transition, the gain turns out to be of approximately the same order of magnitude as for comparable optical transitions including a significant enhancement through Raman coherences especially at weak laser intensities. Our calculations suggest the possibility of a cw deep uv laser, based on laser cooled metastable (e.g., helium) atoms.

Hopkins Ultraviolet Telescope observations of H2 toward the planetary nebula NGC 1535

We have observed the far-ultraviolet spectrum (912-1860 A) of the bright high-excitation planetary nebula NGC 1535 with approximately 3 A resolution using the Hopkins Ultraviolet Telescope (HUT) aboard the Astro-1 space shuttle pmission in 1990 December. We see strong continuum emission down to the Lyman limit and strong P Cygni profiles from high-excitation lines such as C IV wavelength 1549, N V wavelength 1240, O V wavelength 1371, and O VI wavelength 1035. Below 1150 A strong absorption bands of H2 are seen, which were unanticipated by us because of the low reddening and high galactic latitude of the object and the absence of detected H2 emission in the infrared. We construct model H2 spectra and convolve them to the HUT resolution for comparison with the NGC 1535 data. We find good agreement with a population distribution characterized by a single temperature (T = 300 K) or a two-temperature model (T = 144/500 K), and determine limits on the H2 column density. While both inter-stellar and circumstellar origins for the observed H2 absorption are plausible, we ascribe the material to the planetary nebula in order to estimate the conditions of excitation and place upper limits on the mass of both H2 and H1 in this system. Because the UV transitions are ground-state connected, we determine a stringent upper limit of 0.03 d(sup 2)(sub 1.6) solar mass on the mass of H2, where d(sub 1.6) is the distance relative to an assumed distance of 1.6 kpc. This value is less model-dependent than IR estimates. Along with the central star and nebular masses, these estimates allow us to limit the main-sequence mass of the progenitor star to less than 1.8 solar mass. This upper limit is consistent with a relatively low-mass extended thick disk or Population II progenitor, as expected for an object approximately 1 kpc off the galactic plane.

Excitation of vacuum UV transitions of alkali-metal atoms by charge-exchange reactions

We investigate the vacuum UV radiation spectra of atoms and ions of Rb, Cs, He, Ne, and Ar in a discharge plasma of their mixtures at different concentration of metal vapors in rare gases. The reactions leading to population and deactivation of resonant states of rare-gas atoms were studied. The observation of lines 109.1, 119.6, and 124.7 nm, corresponding to the radiation transitions from the 5p55d6s4P5/2 quasi-metastable autoionizing level of cesium, allowed us to estimate experimentally the cross section σ=(9±3) · 10−15 cm2 of the charge-exchange reaction of the metastable cesium ion on the cesium atom, giving as a result the above-mentioned quasi-metastable state of cesium.

Refinements of the crystal structures and UV-absorption properties of KBrO3, RbBrO3 and CsBrO3

Abstract The crystal structures of KBrO3, RbBrO3 and CsBrO3 at 295 K have been refined from single crystal MoKα x-ray data: Rhombohedral R3m, Z = 1:KBrO3, a = 4.4104(2) Å, α = 85.961(8)°, 2393 reflections (337 unique) resulted in R(F) = 0.040; RbBrO3, a = 4.4913(2) Å, α = 87.618(7)°, 2183 reflections (335 unique) resulted in R(F) = 0.047; CsBrO3, a = 4.6525(2) Å, α = 88.749(5)°, 2365 reflections (298 unique) resulted in R(F) = 0.023. The [BrO3]− ions form pyramids with Br–O of (3*) 1.651(2) [K], 1.653(2) [Rb] and 1.654(1) [Cs] Å and O–Br–O of (3*) 104.64(5)° [K], 104.31(6)° [Rb] and 104.50(4)° [Cs]. Including oxygens at (3*) 2.838(2) [K], 2.902(2) [Rb] and 3.055(2) [Cs] Å the Br coordinations can be described as trigonally distorted octahedral, and the overall structures as trigonally distorted perovskites, isostructural with RbClO3, TlClO3, NH4ClO3 and NH4BrO3. INDO calculations of the UV transition energies based on the refined bromate anion geometries were found in full agreement with experimental results, suggesting that the differences in ionic potentials cause constant shifts in the bromate energy levels, preserving their energy differences.

Excitation rate coefficients and line ratios for the optical and ultraviolet transitions in S II

view Abstract Citations (48) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Excitation Rate Coefficients and Line Ratios for the Optical and Ultraviolet Transitions in S II Cai, Wei ; Pradhan, Anil K. Abstract New calculations are reported for electron excitation collision strengths, rate coefficients, transition probabilities, and line ratios for the astrophysically important optical and UV lines in S II. The collision strengths are calculated in the close coupling approximation using the R-matrix method. The present calculations are more extensive than previous ones, including all transitions among the 12 lowest LS terms and the corresponding 28 fine-structure levels in the collisional-radiative model for S II. While the present rate coefficients for electron impact excitation are within 10-30 percent of the previous values for the low-lying optical transitions employed as density diagnostics of H II regions and nebulae, the excitation rates for the UV transitions 4S super 03/2 - 4Psub 1/2,3/2,5/2 differ significantly from earlier calculations, by up to factor of 2. We describe temperature and density sensitive flux ratios for a number of UV lines. The present UV results are likely to be of interest in a more accurate interpretation of S II emission from the Io plasma torus in the magnetosphere of Jupiter, as well as other UV sources observed from the IUE, ASTRO 1, and the HST. Publication: The Astrophysical Journal Supplement Series Pub Date: September 1993 DOI: 10.1086/191824 Bibcode: 1993ApJS...88..329C Keywords: Electron Scattering; H Ii Regions; Optical Transition; Sulfur; Transition Probabilities; Ultraviolet Spectra; Active Galactic Nuclei; Collision Parameters; Coulomb Collisions; Eigenvalues; Electron Transitions; Io; Jupiter (Planet); Nebulae; Optical Thickness; Planetary Magnetospheres; Tables (Data); Toruses; Astrophysics; ATOMIC DATA; ISM: H II REGIONS; GALAXIES: ACTIVE; ISM: GENERAL; ULTRAVIOLET: GENERAL full text sources ADS |

Atomic data needs for the analysis of the ultraviolet spectrum of cool stars

A brief introduction to the UV spectra of cool stars is given, followed by a review of Astronomical Observatories and Instruments which are capable of acquiring UV spectral data on cool stars. Examples of observational data are presented, along with a description of typical analysis performed on such data. The basic atomic data needed to perform these analysis are summarized, with the most important UV multiplets or transitions of the most relevant ions identified.

IR depletion spectroscopy. A method for characterizing a microsolvation environment

A combination of IR depletion and one-color resonant two-photon ionization (1C-R2PI) spectroscopy is used for the first time to characterize mixed molecular clusters. We have studied the molecular aggregates (fluorobenzene)·(methanol) and (fluorobenzene)·(methanol) 2 in the 9.6 μm region. The IR absorption spectra obtained are compared with those of the free methanol aggregates. From this, the structure of the neutral precursor of the intra-cluster reaction product anisole + is identified as a complex of fluorobenzene and a methanol dimer. The disappearance of cluster-specific UV transitions in the R2PI spectra due to IR depletion is shown as a cross check of our results. We would like to recommend this method as a technique for the assignment of R2PI spectra of systems which decay quantitatively following ionization.

The optimization of preionization and the temporal behavior of stimulated uv emission of an N 2 laser

It is shown that the preionization conditions of a pulsed cross-field nitrogen laser can be optimized to produce a narrower laser pulse. The time intervals of the three stimulated second positive uv transitions, 0-0, 0-1, 1-0, are measured as a function of the gas pressure. The interval between the 0-0 and 0-1 emissions is approximately constant whereas the one between 0-0 and 1-0 is shown to decrease gradually with increasing pressure. Collisional depopulation of the B level as well as enhancement of the excitation cross-section of the C level are suggested to account for this pressure dependence.

On coenzyme Q orientation in membranes: A linear dichroism study of ubiquinones in a model bilayer

A general approach is developed to interpret linear dichroism (LD) spectra of ubiquinones (Qn) in host bilayers. Information is reported in terms of guest-host mutual orientation and localization. The overall orientational anisotropy of guest ubiquinone molecules is described by a basic set of limiting orientation/localization modes. Assignments of the UV transitions of the ubiquinone chromophore were obtained by the liquid crystal-linear dichroism technique and molecular orbital (CNDO/S) calculations. The LD spectra of Qn in the bilayers provided by the lyotropic nematic mesophase exhibited by water solutions of potassium laurate and decanol were interpreted on the basis of the above assignments. The resulting experimental evidence showed a multisite distribution in the host bilayer for the aromatic heads of all the investigated Qn derivatives except Q0. The orientational distribution suggested by the LD spectra fits the solubilization model recently proposed by G. Lenaz [J. Membrane Biol. (1988) 104:193-209] for ubiquinone in lipid membranes. Within this model Qn molecules are located in the midplane and their headgroups oscillate transversally across the membrane. Q0 instead has a single site location, close to the polar bilayer interface. Experimental evidence that the headgroup carbonyls tend to grasp the polar interface of the host bilayer was also obtained. Orientation and location distributions of Qn guest molecules are therefore likely to result from the tendency of their aromatic heads to grasp the polar heads of the host bilayer and from the concurrent tendency of their chains to settle into the hydrocarbon host interior.

Simultaneous oscillation on UV and IR transitions of the copper ion laser

Simultaneous oscillation of UV (248–270 nm) and IR lines (773–785 nm) in the copper ion laser with helical hollow cathode has been observed using high reflectivity combination mirrors. The output power characteristics of these laser lines are measured as functions of the discharge current and fill-gas pressure. A total laser power of about 100 mW with equal UV and IR contributions was obtained.