Criterion for Distinguishing Sequential from Nonsequential Contributions to the Double Ionization of Helium in Ultrashort Extreme-Ultraviolet Pulses - Liu, A. H.; Thumm, Uwe P. E. (My University, 2015)
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.
Keywords: Differential Cross-Sections; Double-Photoionization; Gases; Physics