Two-photon double ionization of the helium atom by ultrashort pulses

Abstract

Abstract. Two-photon double ionization of the helium atom was the subject of early experiments at FLASH and will be the subject of future benchmark measurements of the associated electron angular and energy distributions. As the photon energy of a single femtosecond pulse is raised from the threshold for two-photon double ionization at 39.5 eV to beyond the sequential ionization threshold at 54.4 eV, the electron ejection dynamics change from the highly correlated motion associated with nonsequential absorption to the much less correlated sequential ionization process. The signatures of both processes have been predicted in accurate ab initio calculations of the joint angular and energy distributions of the electrons, and those predictions contain some surprises. The dominant terms that contribute to sequential ionization make their presence apparent several eV below that threshold. In two-color pump probe experiments with short pulses whose central frequencies require that the sequential ionization process necessarily dominates, a two-electron interference pattern emerges that depends on the pulse delay and the spin state of the atom.

Multiphoton ionization and stabilization of helium in superintense xuv fields

“…The figure clearly shows that the distribution has a backward-forward asymmetry even at the highest intensity considered, demonstrating the breakdown of the single-particle picture, wherein a symmetric double-lobe (dipole) distribution would be found. The results are in accordance with recent results obtained at weaker fields [56,57,63], and shows that the back-to-back ejection mechanism is largely preserved even at very strong fields.…”

“…With the ultrashort pulse considered here, the second photon is absorbed before the residual ion has had time to relax to the ground state, but if the duration is increased to beyond 20 cycles, relaxation may occur and a double-peak structure appears (not shown here). The fact that the two peaks, corresponding to sequential two-photon double ionization in the long-pulse limit, shift toward each other in the short-pulse regime and eventually merge into one single peak (located at equal energy sharing) is well known and has been studied in a series of papers in the weak-field (perturbative) limit [33,[56][57][58][59][60][61][62][63][64][65]. The results in Fig.…”

Two-photon double ionization of helium by attosecond laser pulses: Evidence of highly correlated electron motion

“…Provided one of the electrons is emitted perpendicular to the laser polarization direction, it is found that the angular distribution of the other electron is characterized by three lobes. The results are similar to those recently reported for the corresponding process in the hydrogen negative ion [R. The problem of direct (nonsequential) two-photon double ionization of helium has been studied extensively in recent years, as exemplified by numerous theoretical [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] and experimental [19][20][21][22][23][24] works. This breakup process is fundamental in the sense that it is one of the simplest processes in nature where electron correlations are exhibited, manifested by a rather complex interplay between the electrons.…”

“…The problem of direct (nonsequential) two-photon double ionization of helium has been studied extensively in recent years, as exemplified by numerous theoretical [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] and experimental [19][20][21][22][23][24] works. This breakup process is fundamental in the sense that it is one of the simplest processes in nature where electron correlations are exhibited, manifested by a rather complex interplay between the electrons.…”

Laser-assisted XUV double ionization of helium: Energy-sharing dependence of joint angular distributions

“…Comprehensive numerical studies on the DI of helium following the absorption of a few XUV photons were carried out by Parker et al [16] starting 15 years ago. Following the 2005 experiment of Hasegawa et al [7], two-photon DI of helium has been the subject of several theoretical studies [15,[17][18][19][20][21][22][23][24][25][26][27][28]. In particular, Zhang et al [26] calculated joint angular distributions (JADs) for two-photon DI by XUV pulses in both the nonsequential (39.5 eV < ω XUV < 54.4 eV) and the sequential ( ω XUV > 54.4 eV) regimes for different energy sharings of the emitted electrons.…”