Photoionization of hydrogen atoms by coherent intense high-frequency short laser pulses: Direct propagation of electron wave packets on large spatial grids

Abstract

The time-dependent Schrödinger equation for the hydrogen atom and its interaction with coherent intense high-frequency short laser pulses is solved numerically exactly by propagating the singleelectron wave packets. Thereby, the wavefunction is followed in space and time for times longer than the pulse duration. Results are explicitly shown for 3 and 10 fs pulses. Particular attention is paid to identifying the effect of dynamic interference of photoelectrons emitted with the same kinetic energy at different times during the rising and falling sides of the pulse predicted in [Ph.V. Demekhin and L.S. Cederbaum, Phys. Rev. Lett. 108, 253001 (2012)]. In order to be able to see the dynamic interference pattern in the computed electron spectra, the photoelectron wave packet has to be propagated over long distances. Clearly, complex absorption potentials often employed to compute spectra of emitted particles cannot be used to detect dynamic interference. For the considered high-frequency pulses of 3 and 10 fs durations, this requires enormously large spatial grids. The presently computed photoionization and above-threshold ionization spectra are found to exhibit pronounced dynamic interference patterns. Where available, the patterns are in very good agreement with previously published results on the photoionization spectra which have been computed using a completely different method, thus supporting the previously made assumption that the above-threshold ionization processes are very weak for the considered pulse intensities and high carrier frequency. The quiver motion in space and time of a free electron in strong laser pulses is also investigated numerically. Finally, a discussion is presented of how fast the atom is ionized by an intense pulse.

Dissociative ionization ofH2+using intense femtosecond XUV laser pulses

“…2(a) to 11.1 fs in Fig. 2(b) leads to more interference peaks emerging, similar to the case of dynamic interference in hydrogen [14][15][16][17], but now visible along the diagonal in the JES. In addition, the JES and NES for the 5.6 fs pulse in Fig.…”

“…This indicates that multiphoton processes at these intensities cannot be neglected and will contribute to differences in the JESs calculated using the present model and the JESs obtained from the full TDSE calculations. Such differences were observed in the atomic case [16] in terms of differences in the relative amplitudes of the peaks in the PES.…”

“…[16], it was suggested that the combination of using small simulation volumes and CAPs placed at the box boundaries would make it impossible to produce dynamic interference in the PES, and enormously large simulation volumes (radial coordinate up to r max = 10 000) were used in their calculations to obtain the PESs for hydrogen. However, as shown in Fig.…”

“…There has recently been theoretical interest in the ionization of simple atomic systems using such intense XUV laser pulses [12][13][14][15][16][17]. Intensity modulations in the photoelectron spectra (PESs) were observed and explained as follows.…”

“…Standard methods of obtaining PESs from numerical calculations include projection on plane waves [16,26], projection on scattering states [22,27], and the usage of flux methods [28][29][30][31]. In the first method, huge simulation volumes are required, as it must be ensured that after the end of the pulse, the scattered parts of the wave packet are well separated from its bound part and not reflected from the box boundaries.…”