3,438 research outputs found
Double Ionisation in R-Matrix Theory Using a 2-electron Outer Region
We have developed a two-electron outer region for use within R-matrix theory
to describe double ionisation processes. The capability of this method is
demonstrated for single-photon double ionisation of He in the photon energy
region between 80 eV to 180 eV. The cross sections are in agreement with
established data. The extended RMT method also provides information on
higher-order processes, as demonstrated by the identification of signatures for
sequential double ionisation processes involving an intermediate He state
with .Comment: 5 pages, 4 figure
Quasiclassical double photoionization from the 2^{1,3}S excited states of helium including shakeoff
We account for the different symmetries of the 2^{1,3}S helium excited states
in a quasiclassical description of the knockout mechanism augmented by a
quantum shakeoff contribution. We are thus able to formulate the separate
contribution of the knockout and shakeoff mechanisms for double photoionization
for any excess energy from the 2^{1,3}S states. Photoionization ratios and
singly differential cross sections calculated for the 2^{1,3}S excited states
of helium are found to be in very good agreement with recent theoretical
results.Comment: 9 pages, 5 figure
Electron correlation and short-range dynamics in attosecond angular streaking
We employ the R matrix with time-dependence method to study attosecond angular streaking of Fâ. Using this negative ion, free of long-range Coulomb interactions, we elucidate the role of short-range electron correlation effects in an attoclock scheme. Through solution of the multielectron time-dependent Schrödinger equation, we aim to bridge the gap between experiments using multielectron targets, and one-electron theoretical approaches. We observe significant negative offset angles in the photoelectron momentum distributions, despite the short-range nature of the binding potential. We show that the offset angle is sensitive to the atomic structure description of the residual F atom. We also investigate the response of co- and counter-rotating electrons, and observe an angular separation in their emissio
Use of partial-wave decomposition to identify resonant interference effects in the photoionizationâexcitation of argon
We have studied simultaneous photoionization and excitation of Ar in the range of incident photon energies between 36.00 and 36.36 eV, where the resonant production of doubly excited neutral Ar states imbedded in the ionization continuum is dominant. By measuring the relative Stokes parameters of the fluorescence from residual Ar+â (3p4 [3P] 4p) ions (2P1/2, 465.8 nm transition; 2P3/2, 476.5 nm; 2D3/2, 472.7 nm; 2D5/2, 488.0 nm; 4P5/2, 480.6 nm; 4D5/2, 514.5 nm) we demonstrate a technique for determining individual partial-wave cross sections in photoionizing collisions. This procedure is shown to be important in sorting out competing dynamical ionization mechanisms, particularly with regard to resonant production of intermediate doubly excited autoionizing states. Comparison with theoretical photoionization cross sections demonstrates that spinâorbit coupling between different states of Ar II needs to be accounted for in the calculations
Time delay between photoemission from the 2p and 2s subshells of Neon
The R-Matrix incorporating Time (RMT) method is a new method for solving the
time-dependent Schroedinger equation for multi-electron atomic systems exposed
to intense short-pulse laser light. We have employed the RMT method to
investigate the time delay in the photoemission of an electron liberated from a
2p orbital in a neon atom with respect to one released from a 2s orbital
following absorption of an attosecond XUV pulse. Time delays due to XUV pulses
in the range 76-105 eV are presented. For an XUV pulse at the experimentally
relevant 105.2 eV, we calculate the time delay to be 10.2 +/- 1.3 attoseconds,
somewhat larger than estimated by other theoretical calculations, but still a
factor two smaller than experiment. We repeated the calculation for a photon
energy of 89.8 eV with a larger basis set capable of modelling
correlated-electron dynamics within the neon atom and the residual Ne(+) ion. A
time delay of 14.5 +/- 1.5 attoseconds was observed, compared to a 16.7 +/- 1.5
attosecond result using a single-configuration representation of the residual
Ne(+) ion.Comment: 4 pages, 3 figures, 1 tabl
R-matrix-with-time-dependence theory for ultrafast atomic processes in arbitrary light fields
We describe an ab initio and non-perturbative -matrix with time-dependence
theory for ultrafast atomic processes in light fields of arbitrary
polarization. The theory is applicable to complex, multielectron atoms and
atomic ions subject to ultrashort (particularly few-femtosecond and attosecond)
laser pulses with any given ellipticity, and generalizes previous
time-dependent -matrix techniques restricted to linearly polarized fields.
We discuss both the fundamental equations, required to propagate the
multielectron wavefunction in time, as well as the computational developments
necessary for their efficient numerical solution. To verify the accuracy of our
approach, we investigate the two-photon ionization of He, irradiated by a pair
of time-delayed, circularly polarized, femtosecond laser pulses, and compare
photoelectron momentum distributions, in the polarization plane, with those
obtained from recent time-dependent close-coupling calculations. The predictive
capabilities of our approach are further demonstrated through a study of
single-photon detachment from F in a circularly polarized, femtosecond
laser pulse, where the relative contribution of the co- and counter-rotating
electrons is quantified.Comment: 16 pages, 3 figure
Three-photon detachment of electrons from the fluorine negative ion
Absolute three-photon detachment cross sections are calculated for the
fluorine negative ion within the lowest-order perturbation theory. The Dyson
equation of the atomic many-body theory is used to obtain the ground-state 2p
wavefunction with correct asymptotic behaviour, corresponding to the true
(experimental) binding energy. We show that in accordance with the adiabatic
theory (Gribakin and Kuchiev 1997 {Phys. Rev. A} {\bf 55} 3760) this is crucial
for obtaining absolute values of the multiphoton cross sections. Comparisons
with other calculations and experimental data are presented.Comment: 10 pages, two figures, Latex, IOP styl
Molecular diversity of arbuscular mycorrhizal fungi and patterns of host association over time and space in a tropical forest
We have used molecular techniques to investigate the diversity and distribution of the arbuscular mycorrhizal (AM) fungi colonizing tree seedling roots in the tropical forest on Barro Colorado Island (BCI), Republic of Panama. In the first year, we sampled newly emergent seedlings of the understory treelet Faramea occidentalis and the canopy emergent Tetragastris panamensis, from mixed seedling carpets at each of two sites. The following year we sampled surviving seedlings from these cohorts. The roots of 48 plants were analysed using AM fungal-specific primers to amplify and clone partial small subunit (SSU) ribosomal RNA gene sequences. Over 1300 clones were screened for random fragment length polymorphism (RFLP) variation and 7% of these were sequenced. Compared with AM fungal communities sampled from temperate habitats using the same method, the overall diversity was high, with a total of 30 AM fungal types identified. Seventeen of these types have not been recorded previously, with the remainder being similar to types reported from temperate habitats. The tropical mycorrhizal population showed significant spatial heterogeneity and nonrandom associations with the different hosts. Moreover there was a strong shift in the mycorrhizal communities over time. AM fungal types that were dominant in the newly germinated seedlings were almost entirely replaced by previously rare types in the surviving seedlings the following year. The high diversity and huge variation detected across time points, sites and hosts, implies that the AM fungal types are ecologically distinct and thus may have the potential to influence recruitment and host composition in tropical forests
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