3,199 research outputs found
Multiphoton inner-shell ionization of the carbon atom
We apply time-dependent R-matrix theory to study inner-shell ionization of C
atoms in ultra-short high-frequency light fields with a photon energy between
170 and 245 eV. At an intensity of 10 W/cm, ionization is dominated
by single-photon emission of a electron, with two-photon emission of a
1s electron accounting for about 2-3\% of all emission processes, and
two-photon emission of contributing about 0.5-1\%. Three-photon
emission of a 1s electron is estimated to contribute about 0.01-0.03\%. Around
a photon energy of 225 eV, two-photon emission of a 1s electron, leaving C
in either 1s2s2p or 1s2p is resonantly enhanced by intermediate
1s2s2p states. The results demonstrate the capability of time-dependent
R-matrix theory to describe inner-shell ionization processes including
rearrangement of the outer electrons.Comment: 7 pages, 2 figures, 2 table
Angular distributions in two-colour two-photon ionization of He
We present R-Matrix with time dependence (RMT) calculations for the
photoionization of helium irradiated by an EUV laser pulse and an overlapping
IR pulse with an emphasis on the anisotropy parameters of the sidebands
generated by the dressing laser field. We investigate how these parameters
depend on the amount of atomic structure included in the theoretical model for
two-photon ionization. To verify the accuracy of the RMT approach, our
theoretical results are compared with experiment.Comment: 8 pages, 4 figures, 1 tabl
Electron dynamics in the carbon atom induced by spin-orbit interaction
We use R-Matrix theory with Time dependence (RMT) to investigate multiphoton
ionization of ground-state atomic carbon with initial orbital magnetic quantum
number =0 and =1 at a laser wavelength of 390 nm and peak intensity
of 10 W cm. Significant differences in ionization yield and
ejected-electron momentum distribution are observed between the two values for
. We use our theoretical results to model how the spin-orbit interaction
affects electron emission along the laser polarization axis. Under the
assumption that an initial C atom is prepared at zero time delay with ,
the dynamics with respect to time delay of an ionizing probe pulse modelled
using RMT theory is found to be in good agreement with available experimental
data.Comment: 8 pages, 5 figure
Population trapping in bound states during IR-assisted ultra-fast photoionization of Ne
We have investigated photoionization of Ne in the combined field of a
short infra-red laser pulse and a delayed ultra-short pulse of the infra-red
laser's 23 harmonic. We observe an ionization yield compatible with a
picture in which one electron gets excited into Rydberg states by the harmonic
laser field and is subsequently removed by the infra-red laser field.
Modulations are seen in the ionization yield as a function of time delay. These
modulations originate from the trapping of population in low members of the
Rydberg series with different states being populated at different ranges of
delay times. The calculations further demonstrate that single-threshold
calculations cannot reproduce the Ne photoionization yields obtained in
multi-threshold calculations.Comment: 7 pages, 5 figures, 1 tabl
Harmonic generation of noble-gas atoms in the Near-IR regime using ab-initio time-dependent R-matrix theory
We demonstrate the capability of ab-initio time-dependent R-matrix theory to
obtain accurate harmonic generation spectra of noble-gas atoms at Near-IR
wavelengths between 1200 and 1800 nm and peak intensities up to 1.8 X 10(14)
W/cm(2) . To accommodate the excursion length of the ejected electron, we use
an angular-momentum expansion up to Lmax = 279. The harmonic spectra show
evidence of atomic structure through the presence of a Cooper minimum in
harmonic generation for Kr, and of multielectron interaction through the giant
resonance for Xe. The theoretical spectra agree well with those obtained
experimentally.Comment: 6 pages, 5 figure
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
Extreme-Ultraviolet-Initated High-Order Harmonic Generation: Driving Inner-Valence Electrons Using Below-Threshold-Energy Extreme-Ultraviolet Light
We propose a novel scheme for resolving the contribution of inner- and outer-valence electrons in XUV-initiated high-harmonic generation in neon. By probing the atom with a low energy (below the 2s ionisation threshold) ultrashort XUV pulse, the 2p electron is steered away from the core, while the 2s electron is enabled to describe recollision trajectories. By selectively suppressing the 2p recollision trajectories we can resolve the contribution of the 2s electron to the high-harmonic spectrum. We apply the classical trajectory model to account for the contribution of the 2s electron, which allows for an intuitive understanding of the process
Pulse-shape control of two-color interference in high-order-harmonic generation
We report on calculations of harmonic generation by neon in a mixed (800-nm +
time-delayed 400-nm) laser pulse scheme. In contrast with previous studies we
employ a short (few-cycle) 400-nm pulse, finding that this affords control of
the interference between electron trajectories contributing to the cutoff
harmonics. The inclusion of the 400-nm pulse enhances the yield and cutoff
energy, both of which exhibit a strong dependence on the time delay between the
two pulses. Using a combination of time-dependent R-matrix theory and a
classical trajectory model, we assess the mechanisms leading to these effects
Time-dependent R-matrix theory applied to two-photon double ionization of He
We introduce a time-dependent R-matrix theory generalised to describe double
ionization processes. The method is used to investigate two-photon double
ionization of He by intense XUV laser radiation. We combine a detailed
B-spline-based wavefunction description in a extended inner region with a
single-electron outer region containing channels representing both single
ionization and double ionization. A comparison of wavefunction densities for
different box sizes demonstrates that the flow between the two regions is
described with excellent accuracy. The obtained two-photon double ionization
cross sections are in excellent agreement with other cross sections available.
Compared to calculations fully contained within a finite inner region, the
present calculations can be propagated over the time it takes the slowest
electron to reach the boundary.Comment: 6 pages, 4 figure
Use of partial-wave decomposition to identify resonant interference effects in the photoionization–excitation 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+∗ (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 spin–orbit coupling between different states of Ar II needs to be accounted for in the calculations
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