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  • 1
    Electronic Resource
    Electronic Resource
    Hyphal transport by a vesicular-arbuscular mycorrhizal fungus of N applied to the soil as ammonium or nitrate (1993)
    Springer
    Biology and fertility of soils 16 (1993), S. 66-70 
    Details
    ISSN: 1432-0789
    Keywords: VA mycorrhiza ; Glomus intraradices ; Hyphal N transport ; Cucumis sativus ; 15N recovery ; Root compartment
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Summary Transport of N by hyphae of a vesicular-arbuscular mycorrhizal fungus was studied under controlled experimental conditions. The N source was applied to the soil as 15NH inf4 sup+ or 15NO inf3 sup- . Cucumis sativus was grown for 25 days, either alone or in symbiosis with Glomus intraradices, in containers with a hyphal compartment separated from the root compartment by a fine nylon mesh. Mineral N was then applied to the hyphal compartment as 15NH inf4 sup+ or 15NO inf3 sup- at 5 cm distance from the root compartment. Soil samples were taken from the hyphal compartment at 1, 3 and 5 cm distance from the root compartment at 7 and 12 days after labelling, and the concentration of mineral N in the samples was measured from 2 M KCl extracts. Mycorrhizal colonization did not affect plant dry weight. The recovery of 15N in mycorrhizal plants was 38 or 40%, respectively, when 15NH inf4 sup+ or 15NO inf3 sup- was applied. The corresponding values for non-mycorrhizal plants were 7 and 16%. The higher 15N recovery observed in mycorrhizal plants than in non-mycorrhizal plants suggests that hyphal transport of N from the applied 15N sources towards the host plant had occurred. The concentration of mineral N in the soil of hyphal compartments was considerably less in mycorrhizal treatments than in controls, indicating that the hyphae were able to deplete the soil for mineral N.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00336518
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  • 2
    Electronic Resource
    Electronic Resource
    Polarized emission spectroscopy of photodissociating nitromethane at 200 and 218 nm (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3958-3969 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the polarized emission spectra from photodissociating nitromethane excited at 200 and 218 nm. At both excitation wavelengths, the emission spectra show a strong progression in the NO2 symmetric stretch; at 200 nm a weak progression in the NO2 symmetric stretch in combination with one quantum in the C–N stretch also contributes to the spectra. We measure the angular distribution of emitted photons in the strong emission features from the relative intensity ratio between photons detected perpendicular to versus along the direction of the electric vector of the excitation laser. We find the anisotropy is substantially reduced from the 2:1 ratio expected for the pure CH3NO2 X(1A1)→1B2(ππ*)→X(1A1) transition with no rotation of the molecular frame. The intensity ratios for the features in the NO2 symmetric stretching progression lie near 1.5 to 1.6 for 200 nm excitation and 1.7 for 218 nm excitation. The analysis of the photon angular distribution measurements and consideration of the absorption spectrum indicate that the timescale of the dissociation is too fast for molecular rotation to contribute significantly to the observed reduction in anisotropy. The detailed analysis of our results in conjunction with electron correlation arguments and previous work on the absorption spectroscopy and final products' velocities results in a model which includes two dissociation pathways for nitromethane, an electronic predissociation pathway and a vibrational predissociation pathway along the 1B2(ππ*) surface. Our analysis suggests a reassignment of the minor dissociation channel, first evidenced in photofragment velocity analysis experiments which detected a pathway producing slow CH3 fragments, to the near threshold dissociation channel CH3 + NO2(2 2B2).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458781
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  • 3
    Electronic Resource
    Electronic Resource
    Probing dynamics in the Franck–Condon and exit channel regions of dissociating H2S: Emission spectra upon tunable excitation from 199–203 nm (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5652-5664 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This study uses emission spectroscopy of H2S at excitation energies near 200 nm to probe the dissociation dynamics from a conical intersection in the Franck–Condon region to the H+SH product exit channel. Photoexcitation accesses these coupled surfaces near the transition state region of the lower adiabat, a potential surface for the excited state H+SH→HS+H reaction. Excitation wavelengths from 199–203 nm tune through the first of the resonances in the absorption spectrum assigned to recurrences in the motion along the symmetric stretch orthogonal to the reaction coordinate and also access energies just above and at the conical intersection. We disperse the emission from the dissociating molecules at each of five excitation wavelengths in this region to probe several features of the reaction dynamics on the coupled potential energy surfaces. The resulting emission spectra cover the range of final vibrational eigenstates from 500 to 11 000 cm−1 above the initial ground vibrational state for all five excitation wavelengths, and go out to 16 500 cm−1 for the 199 and 201 nm excitation wavelengths. The resulting spectra, when considered in conjunction with recent scattering calculations by Heumann and Schinke on ab initio potential energy surfaces for this system, evidence a progression of emission features to low vibrational eigenstates in the SH stretch that result from coupling of the nuclear motion from the bound to the dissociative region of the potential energy surfaces.This emission, into local mode eigenstates such as 00+1, 11+0, 11+1, 21+0, 21+1, evidences the antisymmetric dissociative motion and bending induced near the conical intersection, and dominates the spectrum at excitation wavelengths only near 200 nm. We analyze the excitation wavelength dependence of these features and also of the n0+0 progression for n≥4, which reflect the exit channel dynamics. The excitation wavelength dependence shows that while the emission spectra do not reveal any dynamics unique to scattering states that access a symmetric stretch resonance in the Franck–Condon region, they do reveal the energy location of and the dynamics at the conical intersection. A reanalysis of other workers' measurements of the SH product vibrational state distribution shows that v=0 products are strongly favored at excitation wavelengths near the conical intersection.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467351
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  • 4
    Electronic Resource
    Electronic Resource
    Comparison of endogenous gibberellins in roots and shoots of elongating Salix pentandra seedlings (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 90 (1994), S. 0 
    Details
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Gibberellins GA1, GA8. GA19. GA29. GA20 and GA56 (2-epi-GA8). were identified by combined gas chromatography-mass spectrometry in root extracts of elongating Salix pentandra L. seedlings. The presence of GA8 was also demonstrated for the first time in S. pentandra shoots. The levels of GA1, GA8, GA19, GA20 in shoot tissue and in roots were estimated by selected ion monitoring. While the amounts of GA8 and GA19 were similar in both plant parts. the levels of the biologically active GA1 and its immediate precursor GA20. were found to be much lower in roots than in shoots.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1399-3054.1994.tb00402.x
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  • 5
    Electronic Resource
    Electronic Resource
    Dynamics of photodissociation of methyl iodide/silver (111) at 248 nm (1993)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 97 (1993), S. 2257-2261 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100112a029
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  • 6
    Electronic Resource
    Electronic Resource
    Interference effects in the polarized emission spectrum of methyl iodide at 248 nm: scattering through two coupled optically bright excited states (1991)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 95 (1991), S. 8096-8103 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100174a018
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  • 7
    Electronic Resource
    Electronic Resource
    Photochemistry of adsorbed molecules. XI. Charge-transfer photodissociation and photoreaction in chloromethanes on Ag(111) (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 5938-5953 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photoinduced charge-transfer (CT) dissociation of adsorbates is reported in this paper for a series of chloromethanes (RCl) adsorbed on Ag(111). The chloromethanes were CCl4, CHCl3, CH2Cl2, CH3Cl, and CCl3Br. The observation that Cl− ions were emitted following UV laser irradiation of the chloromethane covered metal surface gave direct evidence for CT photodissociation RCl/Ag(111)+hν→(RCl−)
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464888
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  • 8
    Electronic Resource
    Electronic Resource
    Dissociation dynamics of CH3SH at 222, 248, and 193 nm: An analog for probing nonadiabaticity in the transition state region of bimolecular reactions (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 2882-2890 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: These experiments use molecular photodissociation of CH3SH to probe the dynamics and the influence of nonadiabatic coupling in the transition state region of the CH3+SH→CH3S+H reaction. Photoexcitation at 222 and 248 nm in the first of two absorption bands accesses the lower of the two coupled potential energy surfaces near the saddle point of the excited state reaction coordinate. Measurement of the resulting photofragments' velocities and angular distributions determine the branching between the CH3+SH and the CH3S+H exit channels. At all wavelengths within the first absorption band, we observe preferential fission of the stronger S–H bond over the weaker C–S bond. Fission of the C–S bond occurs only to a small degree at 222 nm and is not observable at 248 nm. Comparison with our earlier data at 193 nm, corresponding to excitation to the upper bound adiabat which is nonadiabatically coupled to the lower dissociative surface reached at 222 nm, shows that the branching ratio between C–S bond fission and S–H bond fission is a factor of eight larger at 193 nm.To probe the forces in the Franck–Condon region, we also measure the photoemission spectrum from dissociating CH3SH excited at 222 nm and compare it to the previous measurement at 193 nm. The 222 nm spectrum evidences emission into the S–H stretch and methyl stretch vibrations but not into C–S stretching modes, consistent with the dominance of S–H fission on the lower adiabat, while the 193 nm emission spectrum, reassigned here, has only a progression in the C–S stretch. The comparison of the spectra suggests a model in which stretching along the C–S coordinate on the bound upper state occurs as the amplitude couples nonadiabatically to the lower dissociative surface, allowing the molecule to access the region near the saddle point on the lower surface at extended C–S bond lengths. This results in better overlap with the C–S fission exit channel and thus an increased branching to C–S bond fission over that observed upon direct excitation to the lower dissociative surface at 222 nm. To further advance the experimental conclusions, we present collaborative calculations of the potential energy surfaces using the effective valence-shell Hamiltonian method developed by Freed and co-workers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464116
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  • 9
    Electronic Resource
    Electronic Resource
    Photochemistry of adsorbed molecules. XII. Photoinduced ion–molecule reactions at a metal surface for CH3X/RCl/Ag(111) (X=Br, I) (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 5954-5960 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A photoinduced ion–molecule reaction is reported between superimposed molecular layers of alkyl halides on a metal substrate CH3X/RCl/Ag(111) (where X=Br or I and R=CCl3, CHCl2, or CH2Cl) to form CH3Cl(ad) (wavelengths 193, 248, and 350 nm). The reaction is mediated by charge-transfer (CT) photodissociation, in which photoelectrons from the metal surface transfer to the lower layer of adsorbate RCl to form RCl−. These negative ions then react with the upper layer CH3X in an ion–molecule reaction to form CH3Cl+X−. The yield of product CH3Cl is found to be enhanced at ∼1 ML of adsorbed CH3X (upper layer) due to a decrease in the local potential in the region of the adsorbate–adsorbate interface that enhances the probability of CT to the lower layer. In addition to lowering the local potential at the interface, the adsorbed CH3X also lowers the surface work function; as a result changes in the microscopic local potential correlate (via the CT reaction rate) with changes in the observed macroscopic work function. The yield of CH3Cl decreases at still higher CH3X coverage in the upper layer as the work function increases. The ion–molecule reaction gives evidence of being a concerted process in which the Cl− reacts as it separates from RCl− rather than following separation. The reagent RCl−, as in the surface reaction discussed in the previous paper, is formed by CT from "hot'' electrons rather than free photoelectrons.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464889
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  • 10
    Electronic Resource
    Electronic Resource
    Selective bond fission in methyl mercaptan at 193 nm via radial derivative coupling between the 2 1A‘ and 1 1A‘ adiabatic electronic states (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 4324-4329 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We investigate the origin of the observed fission of the stronger S–H bond over the weaker C–S bond in CH3SH excited at 193 nm using the complementary techniques of mass-resolved photofragment time-of-flight spectroscopy and emission spectroscopy. The velocities and angular distributions of the CH3S and SH photofragments show that both C–S and S–H bond fission occur on a subpicosecond timescale and impart considerable energy to relative product translation. The dispersed emission from photoexcited CH3SH molecules in a flow cell evidences a progression in the CH3 umbrella mode and combination bands with one quantum in the C–S stretch, but no progression with S–H stretch. Examination of the results with reference to previous ab initio calculations of the excited state surfaces reveals the importance of nonadiabatic coupling in the dissociation dynamics. This is a clear example of selective bond fission upon excitation of an electronic state that is not repulsive in the bond that breaks. We discuss the implication of the work with respect to using the Born–Oppenheimer approximation in reactive collisions near a saddle point along the reaction coordinate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462825
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