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  • 1
    Electronic Resource
    Electronic Resource
    Analysis of longitudinal mode wave guiding in vertical-cavity surface-emitting lasers with long monolithic cavity (2001)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 78 (2001), S. 586-588 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Lasing mode switching between two longitudinal modes is observed in transverse single mode vertical-cavity surface-emitting lasers with an extended cavity. Near- and far-field analyses carried out on devices with 2, 4, and 8 μm cavity spacers show an inherent relation between the transverse mode diameter and the calculated oxide- or thermally induced index guiding for the oscillating modes. Depending on the designed alignment of the optical longitudinal standing wave patterns relative to the oxide layer, mode switching can either be promoted or suppressed. Record-high single mode output powers up to 5.5 mW obtained from devices with 8 μm spacers and 8μm active diameter indicate the potential of the given device concept for low-divergence fundamental mode emission as required for many applications. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1344569
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  • 2
    Electronic Resource
    Electronic Resource
    Spectroscopy and ultrafast dynamics of the 2A1 state of Z-hexatriene in gas phase (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 2205-2211 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: As in longer polyenes, the strong 1A1→1B2 band in the UV spectrum of 1,3Z,5-hexatriene (Z-hexatriene) has a weak precursor, the 1A1→2A1 transition. It was measured in this work by a conventional spectrometer. Whereas the wave number of the 0–0 transition is by 5400 cm−1 lower than that of the 1B2 origin, the vibrational contour indicates that the vertical transitions of thetwo bands nearly coincide. From the fast decay of the rotational anisotropy in the time-resolved measurements we conclude that this band is perpendicularly polarized. We measured the lifetime of the 2A1 state after pumping it directly by 250 fs pulses and probing the excited molecules by ionizing it by delayed pulses. The lifetimes decreased from several ps to 730 fs, when the excess energy was increased from near 0 to 4000 cm−1 and more. From the temperature dependence we infer a barrier of about 170 cm−1 (2 kJ/mol). © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474080
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  • 3
    Electronic Resource
    Electronic Resource
    Femtosecond photochemical ring opening of 1,3-cyclohexadiene studied by time-resolved intense-field ionization (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 9386-9389 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We found that hydrogen ion formation due to multielectron dissociative ionization by an intense-laser field is much less efficient with 1,3-cyclohexadiene than with its isomer 1,3Z,5-hexatriene (Z-HT). Moreover by suppressing the ionization barrier an intense-laser field ejects electrons most efficiently from molecular states of low ionization potential. After pumping 1,3-cyclohexadiene at 267 nm to its 1B2 state we probe the system by intense-laser field ionization with delayed 800 nm pulses. Monitoring of the parent ion C6H8+, of the main fragment C6H7+ and of H+ allows us to follow the motion from the 1B2 surface to the dark 2A1 state and from there towards the 2A1/1A1 conical intersection to the ground-state surface of the product. The measured 1B2 and 2A1 lifetimes are 43±3 and 77±7 fs, respectively, and the primary photoproduct cZc-HT is produced within 200 fs. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474009
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  • 4
    Electronic Resource
    Electronic Resource
    Time-resolved dissociative intense-laser field ionization for probing dynamics: Femtosecond photochemical ring opening of 1,3-cyclohexadiene (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 8347-8362 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The concerted photochemical ring opening of 1,3-cyclohexadiene was investigated in the gas phase by low-intensity pumping at 267 nm and subsequent probing by high-intensity photoionization at 800 nm and mass-selective detection of the ion yields. We found five different time constants which can be assigned to traveling times along consecutive parts of the potential energy surfaces. The molecule is first accelerated in the spectroscopic state 1B along Franck–Condon active coordinates, then alters direction before changing over to the dark state 2A. All constants including that for leaving the 2A surface are below 100 fs. These times are shorter than appropriate vibrational periods. Such a maximum speed is evidence that the pathway is continuous leading from surface to surface via real crossings (conical intersections) and that the molecule is accelerated right into the outlet of the 2A/1A funnel. On the ground state it arrives as a compact wave packet, indicating a certain degree of coherence. The experimental method promises a high potential for investigating dynamics, since many consecutive phases of the process can be detected. This is because the fragmentation pattern depends on the location on the potential energy surface, so that monitoring several different ions permits to conclude on the population flow through these locations. Ionization at the intensities used is normally considered to be an effect of the electric field of the radiation. But in our case it is enhanced by resonances in the neutral molecule and in particular in the singly positive ion, and it is not sensitive for the length of the molecule (different conformers of the product hexatriene). The ionic resonances explain why hexatriene has a much richer fragmentation pattern than cyclohexadiene. Coulomb explosion is observed from an excited state of a doubly positive ion. Its mechanism is discussed. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481478
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  • 5
    Electronic Resource
    Electronic Resource
    3W average power 4.3 μm CO2 laser (1991)
    Springer
    Optical and quantum electronics 23 (1991), S. 405-410 
    Details
    ISSN: 1572-817X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Notes: Abstract A 3 W average power CO2 laser oscillating in the range of 4.3μm (10°1 to 10°O transition) is described. At the same time, the laser can emit 100 W in the sequence band 00°2 to 10°1 (10.6μm). It is based on a commercial system with continuous-wave discharge of 12 m length and a slow gas flow. It operates in the Q-switched mode at pulse repetition rates up to 15 kHz. The pulse peak power is 1 kW and the pulse duration is 200 ns. The deviation from the theoretical efficiency limit has been decreased by a factor 2.5 in our device, due to saturation of the pumping (sequence band) radiation. We predict an improvement by another factor of 5 (possible average power of 10 to 20 W), if one avoids the absorption in the discharge-free zones.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00619613
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  • 6
    Electronic Resource
    Electronic Resource
    Synthesis of a symmetric multivalent molecule containing four carbohydrate substituents (1995)
    Springer
    Monatshefte für Chemie 126 (1995), S. 923-931 
    Details
    ISSN: 1434-4475
    Keywords: Carbohydrates ; Dendrimers ; Spacer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung Ausgehend von den Allyl- und Pent-4-enyl-β-D-glucopyranosiden4–7 können dieD-Glucosederivate8–15 dargestellt werden, welche am anomeren Zentrum unterschiedlich lange Spacereinheiten mit verschiedenen Endgruppen besitzen. Bei Reaktion von vier Äquivalenten des Caesiumsalzes von Propan-4-carboxy-1-yl-2,3,4,6-tetra-O-acetyl-β-D-glucopyranosid15 mit Pentaerythrityltetrabromid entsteht das tetravalente Molekül16.
    Notes: Summary Starting with allyl- and pent-4-enyl-β-D-glucopyranosides4–7, differentD-Glucose units (8–15) containing alkyl spacers of variable lenght and with different end groups can be prepared. Reaction of four equivalents of the caesium salt derived from propan-4-carboxy-1-yl-2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside15 with pentaerythrityltetrabromide yields the tetravalent molecule16.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00811012
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  • 7
    Electronic Resource
    Electronic Resource
    Indium mediated allylation in carbohydrate chemistry: A convenient synthesis of 3-deoxy-D-arabino-2-heptulosonic acid (DAH) and 3-deoxy-D-arabino-2-heptulose (kamusol) (1996)
    Springer
    Monatshefte für Chemie 127 (1996), S. 1045-1050 
    Details
    ISSN: 1434-4475
    Keywords: 3-Deoxy-D-arabino-2-heptulosonic acid ; 3-Deoxy-D-arabino-2-heptulose (Kamusol) ; Indium mediated allylation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung Eine kurzer und ökonomischer Syntheseweg für 3-Desoxy-D-arabino-2-heptulosonsäure (4) und 3-Desoxy-D-arabino-2-heptulose (Kamusol,8) wurde entwickelt. Im Schlüsselschritt der Synthesesequenz wurde die indiumunterstützte Allylierung vonD-Erythrose in wäßrigen Reaktionsmedien angewendet. Das mittels dieser Methode auf sieben Kohlenstoffatome verlängerte Kohlenhydrat-Grundgerüst konnte einfach in die Titelverbindungen übergeführt werden.
    Notes: Summary A short and economical synthesis of 3-deoxy-D-arabino-2-heptulosonic acid (4) and 3-deoxy-D-arabino-2-heptulose (kamusol,8) has been developed. In the key step of the reaction sequence, the indium mediated allylation ofD-erythrose in an aqueous solvent system was utilized generating a seven carbon backbone which was further transformed into the title compounds.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00807577
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  • 8
    Electronic Resource
    Electronic Resource
    Multiwavelength Q-switched CO2 laser with continuous discharge (1992)
    Springer
    Applied physics 55 (1992), S. 65-70 
    Details
    ISSN: 1432-0649
    Keywords: 42.60.Da ; 42.60.Fc
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A low-pressure (20 mbar) CO2 laser allows to extract pulses at several selected wavelengths simultaneously from the same active medium. We demonstrated this, using an industrial laser modified by a Q-switch and a resonator with two branches. In one branch the wavelengths are spatially separated, whereas in the other they oscillate in one common transverse mode. We designed a multi-wavelength resonator which requires a single additional reflector compared to usual laser cavities. It provided tunable oscillation at six wavelength simultaneously.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00348616
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  • 9
    Electronic Resource
    Electronic Resource
    Eine einfache Synthese aller vier stereoisomeren 2,2,5-Trimethyl-1,3-dioxolan-4-carbaldehyde (1994)
    Springer
    Monatshefte für Chemie 125 (1994), S. 763-771 
    Details
    ISSN: 1434-4475
    Keywords: 2,2,5-Trimethyl-1,3-dioxolane-4-carbaldehydes ; aldopentose diethyl dithioacetals ; lead tetraacetate cleavage
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary A simple and efficient procedure for the syntheses of all four stereoisomers of the 2,2,5-trimethyl-1,3-dioxolane-4-carbaldehydes1a–1d has been developed. Starting with readily available aldopentose diethyl dithioacetals2,6,10 and14, the title compounds were obtained by a selective protecting group strategy and subsequentRaney-nickel reduction, followed by lead tetraacetate cleavage. This procedure allows an application on a multigram scale.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01277638
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  • 10
    Electronic Resource
    Electronic Resource
    On the Approximation of Certain Mass Distributions Appearing in Distance Geometry (2000)
    Springer
    Acta mathematica hungarica 87 (2000), S. 295-316 
    Details
    ISSN: 1588-2632
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract Let X be a compact subset of the n-dimensional Euclidean space R n . A theorem of G. Björck implies the existence of a unique probability measure μ0 which maximizes the value ∫ X ∫ X d 2(x, y) dμ(x) dμ(y), where μ ranges over all probability measures on X and d 2 denotes the Euclidean distance on R n . In this paper we introduce and investigate an algorithm which is easy to describe and which inductively constructs a sequence ω = x 1, x 2,... in X such that ω is uniformly distributed with respect to μ0. Geometrical and topological interpretations and applications, and concrete numerical examples are given.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006773702255
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