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- Besnard, Simon, et al.
(författare)
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Quantifying the effect of forest age in annual net forest carbon balance
- 2018
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Ingår i: Environmental Research Letters. - : IOP Publishing. - 1748-9326. ; 13:12
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Tidskriftsartikel (refereegranskat)abstract
- Forests dominate carbon (C) exchanges between the terrestrial biosphere and the atmosphere on land. In the long term, the net carbon flux between forests and the atmosphere has been significantly impacted by changes in forest cover area and structure due to ecological disturbances and management activities. Current empirical approaches for estimating net ecosystem productivity (NEP) rarely consider forest age as a predictor, which represents variation in physiological processes that can respond differently to environmental drivers, and regrowth following disturbance. Here, we conduct an observational synthesis to empirically determine to what extent climate, soil properties, nitrogen deposition, forest age and management influence the spatial and interannual variability of forest NEP across 126 forest eddy-covariance flux sites worldwide. The empirical models explained up to 62% and 71% of spatio-temporal and across-site variability of annual NEP, respectively. An investigation of model structures revealed that forest age was a dominant factor of NEP spatio-temporal variability in both space and time at the global scale as compared to abiotic factors, such as nutrient availability, soil characteristics and climate. These findings emphasize the importance of forest age in quantifying spatio-temporal variation in NEP using empirical approaches.
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- Busto, David, et al.
(författare)
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Fano's Propensity Rule in Angle-Resolved Attosecond Pump-Probe Photoionization
- 2019
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Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 123:13
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Tidskriftsartikel (refereegranskat)abstract
- In a seminal article, Fano predicts that absorption of light occurs preferably with increase of angular momentum. We generalize Fano's propensity rule to laser-assisted photoionization, consisting of absorption of an extreme-ultraviolet photon followed by absorption or emission of an infrared photon. The predicted asymmetry between absorption and emission leads to incomplete quantum interference in attosecond photoelectron interferometry. It explains both the angular dependence of the photoionization time delays and the delay dependence of the photoelectron angular distributions. Our theory is verified by experimental results in Ar in the 20-40 eV range.
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- Dahlström, Jan Marcus, et al.
(författare)
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Attosecond transient absorption of a bound wave packet coupled to a smooth continuum
- 2017
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Ingår i: Journal of Optics. - : IOP Publishing. - 2040-8978 .- 2040-8986. ; 19:11
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the possibility of using transient absorption of a coherent bound electron wave packet in hydrogen as an attosecond pulse characterization technique. In a recent work, we have shown that photoionization of such a coherent bound electron wave packet opens up for pulse characterization with unprecedented temporal accuracy-independent of the atomic structure-with maximal photoemission at all kinetic energies given a wave packet with zero relative phase (Pabst and Dahlstrom Phys. Rev. A 94 13411 (2016)). Here, we perform numerical propagation of the time-dependent Schrodinger equation and analytical calculations based on perturbation theory to show that the energy-resolved maximal absorption of photons from the attosecond pulse does not uniquely occur at a zero relative phase of the initial wave packet. Instead, maximal absorption occurs at different relative wave packet phases, distributed as a non-monotonous function with a smooth -pi/2 shift across the central photon energy (given a Fourier-limited Gaussian pulse). Similar results are also found in helium. Our finding is surprising, because it implies that the energy-resolved photoelectrons are not mapped one-to-one with the energy-resolved absorbed photons of the attosecond pulse.
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- Dahlström, Jan Marcus, et al.
(författare)
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Pulse analysis by delayed absorption from a coherently excited atom
- 2019
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Ingår i: Apl Photonics. - : AIP Publishing. - 2378-0967. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- In this tutorial, we provide a short review of attosecond pulse characterization techniques and a pedagogical account of a recently proposed method called Pulse Analysis by Delayed Absorption (PANDA) [S. Pabst and J. M. Dahlstrom, Phys. Rev. A 94, 013411 (2016)]. We discuss possible implementations of PANDA in alkali atoms using either principal quantum number wave packets or spin-orbit wave packets. The main merit of the PANDA method is that it can be used as a pulse characterization method that is free from atomic latency effects, such as scattering phase shifts and long-lived atomic resonances. Finally, we propose that combining the PANDA method with angle-resolved photoelectron detection should allow for experimental measurements of attosecond delays in photoionization from bound wave packets on the order of tens of attoseconds.
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- Hari, Pertti, et al.
(författare)
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Prediction of photosynthesis in Scots pine ecosystems across Europe by a needle-level theory
- 2018
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Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 18:18, s. 13321-13328
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Tidskriftsartikel (refereegranskat)abstract
- Photosynthesis provides carbon for the synthesis of macromolecules to construct cells during growth. This is the basis for the key role of photosynthesis in the carbon dynamics of ecosystems and in the biogenic CO2 assimilation. The development of eddy-covariance (EC) measurements for ecosystem CO2 fluxes started a new era in the field studies of photosynthesis. However, the interpretation of the very variable CO2 fluxes in evergreen forests has been problematic especially in transition times such as the spring and autumn. We apply two theoretical needle-level equations that connect the variation in the light intensity, stomatal action and the annual metabolic cycle of photosynthesis. We then use these equations to predict the photosynthetic CO2 flux in five Scots pine stands located from the northern timberline to Central Europe. Our result has strong implications for our conceptual understanding of the effects of the global change on the processes in boreal forests, especially of the changes in the metabolic annual cycle of photosynthesis.
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| 8. |
- Lindroth, Eva, et al.
(författare)
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Attosecond delays in laser-assisted photodetachment from closed-shell negative ions
- 2017
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Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - 2469-9926 .- 2469-9934. ; 96:1
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Tidskriftsartikel (refereegranskat)abstract
- We study laser-assisted photodetachment time delays by attosecond pulse trains from the closed-shell negative ions F- and Cl. We investigate the separability of the delay into two contributions: (i) the Wigner-like delay associated with one-photon ionization by the attosecond pulse train and (ii) the delay associated with the exchange of an additional laser photon in the presence of the potential of the remaining target. Based on the asymptotic form of the wave packet, the latter term is expected to be negligible because the ion is neutralized, leading to a vanishing laser-ion interaction with increasing electron-atom separation. While this asymptotic behavior is verified at high photoelectron energies, we also quantify sharp deviations at low photoelectron energies. Further, these low-energy delays are clearly different for the two studied anions, indicating a breakdown of the universality of laser-ion-induced delays. The fact that the short-range potential can induce a delay of as much as 50 as can have implications for the interpretation of delay measurements also in other systems that lack long-range potential.
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- Vinbladh, Jimmy, et al.
(författare)
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Many-body calculations of two-photon, two-color matrix elements for attosecond delays
- 2019
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Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - 2469-9926 .- 2469-9934. ; 100:4
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Tidskriftsartikel (refereegranskat)abstract
- We present calculations for attosecond atomic delays in the photoionization of noble-gas atoms based on the full two-color two-photon random-phase approximation with exchange in both the length and velocity gauge. Gauge-invariant atomic delays are demonstrated for the complete set of diagrams. The results are used to investigate the validity of the common assumption that the measured atomic delays can be interpreted as a one-photon Wigner delay and a universal continuum-continuum contribution that depends only on the kinetic energy of the photoelectron, the laser frequency, and the charge of the remaining ion, but not on the specific atom or the orbital from which the electron is ionized. Here, we find that although effects beyond the universal IR-photoelectron continuum-continuum transitions are rare, they do occur in special cases such as around the 3s Cooper minimum in argon. We conclude also that in general the convergence in terms of many-body diagrams is considerably faster in the length gauge than in the velocity gauge.
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