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  • 1
    Electronic Resource
    Electronic Resource
    The use of weak magnetic fields in collisions of polarized atoms (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 729-737 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have experimentally and theoretically investigated the behavior of excited Na atoms under the combined influence of resonant laser light and a weak magnetic field. The use of weak magnetic fields gives another possibility to vary the polarization of the atoms, additional to variation of the polarization of the exciting laser. We calculated in detail the influence of weak magnetic fields on the density matrix describing the excited atoms, using generalized rate equations. We checked the results of our model experimentally, at the Na(3 2 S1/2, Fl=2) →(3 2 P3/2, Fu =3) transition, excited with circularly polarized light. We determined the polarization of the excited atoms, by measuring the fluorescence light at various angles, as a function of both magnetic field strength and laser irradiance. Model and experiment agreed very well. We applied our model to the associative ionization of the excited Na atoms. We measured the ion signal as a function of magnetic field. Analyzing the results, we could determine a specific associative ionization cross section, which is not accessible in our experimental geometry without the use of magnetic fields. Thereby we showed that the application of magnetic fields in polarization dependence studies can provide us with additional information about the process.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456151
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  • 2
    Electronic Resource
    Electronic Resource
    Cross sections and coherence terms for associative ionization of two differently excited Na(3p) atoms (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 738-753 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The associative ionization process in thermal Na(3p)–Na(3p) encounters has been studied in a series of crossed-beam experiments where the light polarization of the two laser beams preparing the excited atoms before collision was varied independently. It is shown how in this way maximum possible information for our geometry is extracted about the dependence of the ion formation process on the shape and spatial orientation of the electron clouds of the two approaching atoms, including all coherence terms. The experimental findings are discussed in the light of recent theoretical results for the states of the Na2 molecule. It is concluded that just a few of the possible geometrical approaches are favorable for molecular–ion formation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456152
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  • 3
    Electronic Resource
    Electronic Resource
    Land-use related organic matter dynamics in North Cameroon soils assessed by 13C analysis of soil organic matter fractions (2005)
    Oxford, UK; Malden, USA : Blackwell Science Ltd
    European journal of soil science 56 (2005), S. 0 
    Details
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Topsoil samples from cultivated and adjacent non-cultivated fields on three major agricultural soils in North Cameroon were fractionated into particle-size fractions that were analysed subsequently for their C and 13C contents. The aim was to obtain further insight into the dynamics of soil organic matter (SOM) in relation to land use in Cameroon. Since organic carbon contents of the fractions were often very small, samples and analyses were extensively replicated to obtain robust statistical estimates of observed differences. For each soil type, differences in δ13C values between fields could be related to changes in the input and decomposition of organic matter arising from soil type, land management and, for example, the nature and abundance of weeds. Turnover of organic matter appeared to be fastest in the sand fraction, which is in line with results from earlier studies. In the finer fractions, clear differences in reaction to changes in input and decomposition were observed, that seem to be linked to differences in clay mineralogy. The results illustrate that SOM in the various fractions is much less stable and more strongly affected by changes in land use than might be assumed on the basis of changes in total SOM contents alone. At the same time, they demonstrate the relevance of 13C isotope analyses of SOM for studies on the impact of land use on these savannah soils with little SOM that are highly susceptible to degradation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2389.2004.00649.x
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  • 4
    Electronic Resource
    Electronic Resource
    Validation of the doubly labeled water method in Japanese Quail Coturnix c. japonica chicks: is there an effect of growth rate? (2000)
    Springer
    Journal of comparative physiology 170 (2000), S. 365-372 
    Details
    ISSN: 1432-136X
    Keywords: Key words Doubly labeled water ; Energy expenditure ; Japanese Quail Coturnix c. japonica ; Growth ; Fractional evaporative water loss ; AbbreviationsCb isotope concentration of background ; Cd isotope concentration of dose ; Ci isotope concentration at start ; Cf isotope concentration at end ; DLW doubly labeled water ; f1 fractionation factor ; k fractional turnover rate ; kd fractional turnover rate 2H ; ko fractional turnover rate 18O ; M average body mass ; Mf body mass at end ; Mi body mass at start ; N amount of body water ; Q dose of isotopes administered ; rCO2 rate of CO2 production ; rCO2-DLW rate of CO2 production measured with doubly labeled water ; rCO2-IR rate of CO2 production measured with respiration gas analysis ; Rdilspace ratio 2H and 18O body water dilution spaces ; rG fractional evaporative water loss ; RGR relative growth rate ; rH2O water efflux rate ; rH2O-uncorr water efflux rate uncorrected for fractionation effects ; t time ; TBW total amount of body water
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract The Doubly Labeled Water (DLW) method was validated against respiration gas analysis in growing Japanese Quail chicks (between 1 week and 3 weeks of age) as well as in birds after having achieved sexual maturity (7 weeks of age). A comparison was made between a strain selected for high growth rates (P-strain, n=18), and a non-selected strain (C-strain, n=18). Relative growth rates of individual chicks during the measurement ranged from −13.8% day−1 to 23.1% day−1. When employing a single-pool model (eq. 34, Lifson and McClintock 1966), it was found that the relative error of the DLW method was sensitive to assumptions concerning fractional evaporative water loss. The best fit was obtained after taking a fractional evaporative water loss value of 0.33. When applying this value for all chicks, it was found that neither strain, relative growth rate of the chick during measurement, nor age significantly contributed to the explained variance. When employing two-pool models, it was found that the DLW method significantly underestimated the true rates of CO2 production at all assumed levels of fractional evaporative water loss. Based on an evaluation of DLW validation studies in growing shorebirds, terns, and quail we recommend Speakman's Eq. 7.17 (Speakman 1997) for general use in young birds.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003600000112
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  • 5
    Electronic Resource
    Electronic Resource
    Polarization and collision velocity dependence of associative ionization in cold Na(3p)-Na(3p) collisions (1990)
    Springer
    The European physical journal 17 (1990), S. 257-270 
    Details
    ISSN: 1434-6079
    Keywords: 34.50.Lf ; 34.50.Fa ; 82.40.Dm
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We studied the polarization dependence of the associative ionization (AI) process Na(3p) + Na(3p) → Na 2 + at collision velocities between 100 and 700 m/s (5 and 200 K), using linearly and circularly polarized light for the excitation. We found that the polarization dependence varies strongly in the collision velocity range under study. At the high end of our scale preparation of both collision partners in the |m j |=1/2 substates yields the highest AI-rate, but at low velocities this ceases being so: the polarization dependence becomes less pronounced and at the low end of our scale the preparation |m j 2|, |m j 2|=3/2, 1/2 is most effective. All cross sections increase strongly at lower velocities. Combined with previous work this maps the detailed polarization dependence from 100–2400 m/s (5-2500 K). From these total results it is concluded that at collision velocities ≧500 m/s mainly one molecular potential curve leads to AI, but that this changes considerably at lower collision velocities. The findings are compared with recent theoretical results by Geltman, and we find large discrepancies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01437365
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  • 6
    Electronic Resource
    Electronic Resource
    The state dependence of the interaction of metastable rare gas atoms Rg*(ms3 P 2,3 P 0) (Rg=Ne, Ar, Kr, Xe) with ground state sodium atoms (1990)
    Springer
    The European physical journal 16 (1990), S. 237-249 
    Details
    ISSN: 1434-6079
    Keywords: 31.20Pv ; 31.50.+w ; 31.70.−f ; 34.20.−b ; 34.50.Gb ; 35.20.Gs ; 82.40.Dm
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Using crossed beams of metastable rare gas atoms Rg*(ms3 P 2,3 P 0) (Rg=Ne, Ar, Kr, Xe) and ground state sodium atoms Na(3s 2 S 1/2), we have measured the energy spectra of electrons released in the respective Penning ionization processes at thermal collision energies. For Rg*(3 P 2)+Na(3s), the spectra are quite similar for the different rare gases, both in width and shape; they reflect attractive interactions in the entrance channel with well depthsD* e [meV] decreasing slowly from Rg=Ne to Xe as follows: 676(18); 602(23); 565(26); 555(30). For Rg*(3 P 0)+Na(3s), the spectra vary strongly with the rare gas, indicating a change in the character of the interaction from van der Waals type attraction (Ne) to chemical binding for Kr and Xe with well depthsD* e [meV] of: 51(19); 107(25); 432(30); 530(50). These findings are explained through model calculations of the respective potential curves, in which the exchange and the spin orbit interaction in the excited rare gas and the molecular interaction between the two valences-electrons in terms of suitably chosen singlet and triplet potentials are taken into account. These calculations also explain qualitatively the experimental finding that the ratiosq 2/q 0 of the ionization cross sections for Rg*(3 P 2)+Na and Rg*(3 P 0)+Na vary strongly with the rare gas from Ne to Xe as follows: 15.8(3.2); 2.6(4); 1.4(2); 1.6(4).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01437526
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  • 7
    Electronic Resource
    Electronic Resource
    Absolute detection of metastable rare gas atoms by a cw laser photoionization method (1991)
    Springer
    The European physical journal 21 (1991), S. 25-39 
    Details
    ISSN: 1434-6079
    Keywords: 35.80.+s ; 32.80.Rm ; 79.20.Rf
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A novel, accurate method for the absolute detection of metastable rare gas atoms is described and demonstrated. It involves a direct in situ determination of the electron emission coefficient γ for impact of the respective metastable atom on a conducting surface. γ is reliably obtained by a cw two-photon ionization — depletion technique: the reduction ΔI S in electron current from the detector surface due to efficient photoionization removal of the metastable flux is compared with the photoelectron current ΔI P (γ = ΔI S/ΔI P). The principle of the method, possible realization schemes for the different metastable rare gas atoms and the apparatus are described in detail. The method has been applied so far to metastable Ne* (3s 3 P 2), Ar* (4s 3 P 2), and Kr* (5s 3 P 2) atoms, and corresponding results for γ, obtained with five different chemically clean, polycrystalline surface materials and at two surface temperatures (300 K, 360 K) are reported. Whereas for Ne*, the value of γ (≈0.35) showed only a rather weak dependence on the surface material and temperature (as also found for a mixed He* (23 S, 21 S) beam), strong variations in γ, especially at 300 K, were detected for Ar* and Kr* (values between 0.25 and 0.003). Some applications of the described method, especially with regard to the determination of absolute reaction cross sections involving metastable rare gas atoms, are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01426615
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  • 8
    Electronic Resource
    Electronic Resource
    Polarization and velocity dependence of associative ionization in Na(3p)+Na(3p) collisions (1987)
    Springer
    The European physical journal 5 (1987), S. 299-308 
    Details
    ISSN: 1434-6079
    Keywords: 34.30.th ; 34.50.Fa ; 34.50.Rk
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have excited Na atoms of two counterrunning thermal beams by means of linearly polarized laser light and have investigated associative ionization processes. To this end we measured the total ionization signal as a function of the angle ϑ between light polarization and the relative collision velocity. Velocities of the excited atoms were selected by exploiting the Doppler effect. We found an increasing polarization dependence at increasing collision velocities. At all velocities the preparation of the collision partners in the |M j |=1/2 sublevel of the Na2 P 3/2 state is most efficient in producing ionization.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01385460
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  • 9
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    Ice cores from Arctic sub-polar glaciers: chronology and post-depositional processes deduced from radioactivity measurements (2003)
    In:  EPIC3Journal of Glaciology:, 49(164), pp. 149-158, ISSN: 0022-1430
    Details
    Publication Date: 2019-07-16
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
    Format: application/pdf
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  • 10
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    Modelling the Isotopic Composition of Antarctic Snow Using Backward Trajectories: Simulation of Snow Pit Records (2006)
    In:  EPIC3Journal of Geophysical Research, 111, No. D15, D15109.
    Details
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , peerRev
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