Criterion for Distinguishing Sequential from Nonsequential Contributions to the Double Ionization of Helium in Ultrashort Extreme-Ultraviolet Pulses
Date
2015-10-28
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Citation: Liu, A. H., & Thumm, U. (2015). Criterion for Distinguishing Sequential from Nonsequential Contributions to the Double Ionization of Helium in Ultrashort Extreme-Ultraviolet Pulses. Physical Review Letters, 115(18), 5. doi:10.1103/PhysRevLett.115.183002
We quantify sequential and nonsequential contributions in two-photon double ionization of helium atoms by intense ultrashort extreme-ultraviolet pulses with central photon energies (h) over bar omega(ctr) near the sequential double-ionization threshold. If the spectrum of such pulses overlaps both the sequential ((h) over bar omega > 54.4 eV) and nonsequential ((h) over bar omega < 54.4 eV) double-ionization regimes, the sequential and nonsequential double-ionization mechanisms are difficult to distinguish. By tracking the double-ionization asymmetry in joint photoelectron angular distributions, we introduce the two-electron forward-backward-emission asymmetry as a measure that allows the distinction of sequential and nonsequential contributions. Specifically, for <($)over bar>omega(ctr) = 50 eV pulses with a sine-squared temporal profile, we find that the sequential double-ionization contribution is the largest at a pulse length of 650 as, due to competing temporal and spectral constraints. In addition, we validate a simple heuristic expression for the sequential double-ionization contribution in comparison with ab initio calculations.
We quantify sequential and nonsequential contributions in two-photon double ionization of helium atoms by intense ultrashort extreme-ultraviolet pulses with central photon energies (h) over bar omega(ctr) near the sequential double-ionization threshold. If the spectrum of such pulses overlaps both the sequential ((h) over bar omega > 54.4 eV) and nonsequential ((h) over bar omega < 54.4 eV) double-ionization regimes, the sequential and nonsequential double-ionization mechanisms are difficult to distinguish. By tracking the double-ionization asymmetry in joint photoelectron angular distributions, we introduce the two-electron forward-backward-emission asymmetry as a measure that allows the distinction of sequential and nonsequential contributions. Specifically, for <($)over bar>omega(ctr) = 50 eV pulses with a sine-squared temporal profile, we find that the sequential double-ionization contribution is the largest at a pulse length of 650 as, due to competing temporal and spectral constraints. In addition, we validate a simple heuristic expression for the sequential double-ionization contribution in comparison with ab initio calculations.
Keywords
Differential Cross-Sections, Double-Photoionization, Gases, Physics
