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The nondirect product discrete variable representation (npDVR) is developed for the time-dependent Schrödinger equation with nonseparable angular variables and is applied to a hydrogen atom in elliptically polarized strong laser fields. The 2D npDVR is constructed on spherical harmonics orthogonalized on the 2D angular grids of the Popov and Lebedev 2D cubatures for the unit sphere. With this approach we have investigated the dynamics of a hydrogen atom initially in its ground state in elliptically polarized laser fields with the intensity up to I = 10 14 W cm−2 and a wavelength of λ = 800 nm. For these parameters of the laser field and the entire range of ellipticity variation, we have calculated the total excitation and ionization yields of the atom. The performed analysis of the method convergence shows that the achieved accuracy of our calculations significantly exceeds the accuracy of recent works of other authors relevant to the problem, due to the high efficiency of the 2D npDVR in approximating the angular part of the 3D time-dependent Schrödinger equation. We also propose a new simple procedure for infinite summation of the transition probabilities to the bound states of the hydrogen atom when calculating the total excitation yield and prove its accuracy by comparison with conventional methods. The obtained results show the potential prospects of the 2D npDVR for investigating atomic dynamics in stronger laser fields. [ABSTRACT FROM AUTHOR]

Photoionization time delays have been investigated in many streaking experiments in which an extreme ultraviolet (XUV) attosecond pulse is used to ionize the target in the presence of a dressing infrared laser field. The discrepancies between the photoionization time delays thus experiment measured and those from many sophisticated theoretical simulations have generated a great deal of controversy in recent years. The difficulty of achieving an accuracy of the retrieved time delays comes from two facts: a so-called wavepacket approximation is introduced to construct the photoionization electron wavepacket, this approximation is invalid if atto-chirp of XUV is non-zero; the other one is that the lower sensitivity of the streaking spectra to the phase of the photoionization transition dipole. Here we present a time delay retrieval method born from our recently proposed ‘phase retrieval of broadband pulses auto correlation’ (PROBP-AC) technology to overcome above limitations. We carefully exam the validity of our method and make a few compare with some other common used retrieval codes, and the simulations demonstrate that more accurate results can be retrieved using PROBP AC. Based on the present method, the angular dependent photoionization time delays can also be retrieved. Our investigation casts doubts on the measured group time delays in previous streaking experiments. We also mention here that a single photoionization group time delay at the XUV peak energy is not enough to represent a complete photoemission process; instead, a fully characterization of the photoionization group time delay over the whole bandwidth of the wave packet is required.

We report on strong-field ionization of dense water gas in a short infrared laser pulse. By employing a unique combination of photoelectron spectroscopy with a liquid micro-jet technique, we observe how the character of electron emission at high kinetic energies changes with the increase of the medium density. This change is associated with the process of laser-assisted electron scattering (LAES) on neighboring particles, which becomes a dominant mechanism of hot electron emission at higher medium densities. The manifestation of this mechanism is found to require densities that are orders of magnitude lower than those considered for heating the laser-generated plasmas via the LAES process. The experimental results are supported by simulations of the LAES yield with the use of the Kroll–Watson theory.