Ground-state energy of H-: a critical test of triple basis sets

We report an improved variational upper bound for the ground state energy of H- using Hylleraaslike wave functions in the form of a triple basis set having three distinct distance scales. The extended precision DQFUN of Bailey, allowing for 70 decimal digit arithmetic, is implemented to retain sufficient precision. Our result exceeds the previous record [A. M. Frolov, Euro. J. Phys. D 69, 132 (2015)], indicating that the Hylleraas triple basis set exhibits comparable convergence to the widely used pseudorandom all-exponential basis sets, but the numerical stability against roundoff error is much better. It is argued that the three distance scales have a clear physical interpretation. The new variational bound is -0.527 751 016 544 377 196 590 814 469 a.u

Ellipticity dependence of excitation and ionization of argon atoms by short-pulse infrared radiation

© 2020 American Physical Society. When atoms or molecules are exposed to strong short-pulse infrared radiation, ionization as well as frustrated tunneling ionization can occur, in which some of the nearly freed electrons recombine into the initial ground or an excited bound state. We analyze the ellipticity dependence of the relative signals that are predicted in a single-active-electron (SAE) approximation, the validity of which is checked against a parameter-free multielectron R-matrix (close-coupling) with time-dependence approach. We find good agreement between the results from both models, thereby providing confidence in the SAE model potential to treat the process of interest. Comparison of the relative excitation probabilities found in our numerical calculations with the predictions of Landsman et al. [A. S. Landsman, New J. Phys. 15, 013001 (2013)NJOPFM1367-263010.1088/1367-2630/15/1/013001] and Zhao et al. [Y. Zhao, Opt. Express 27, 21689 (2019)OPEXFF1094-408710.1364/OE.27.021689] reveals good agreement with the former for short pulses. For longer pulses, the ellipticity dependence becomes wider than that obtained from the Landsman et al. formula, but we do not obtain the increase compared to linearly polarized radiation predicted by Zhao et al

Linear dichroism in few-photon ionization of laser-dressed helium

Abstract: Ionization of laser-dressed atomic helium is investigated with focus on photoelectron angular distributions stemming from two-color multi-photon excited states. The experiment combines extreme ultraviolet (XUV) with infrared (IR) radiation, while the relative polarization and the temporal delay between the pulses can be varied. By means of an XUV photon energy scan over several electronvolts, we get access to excited states in the dressed atom exhibiting various binding energies, angular momenta, and magnetic quantum numbers. Furthermore, varying the relative polarization is employed as a handle to switch on and off the population of certain states that are only accessible by two-photon excitation. In this way, photoemission can be suppressed for specific XUV photon energies. Additionally, we investigate the dependence of the photoelectron angular distributions on the IR laser intensity. At our higher IR intensities, we start leaving the simple multi-photon ionization regime. The interpretation of the experimental results is supported by numerically solving the time-dependent Schrödinger equation in a single-active-electron approximation. Graphic abstract: [Figure not available: see fulltext.

Photoelectron spectroscopy of laser-dressed atomic helium

© 2020 authors. Photoelectron emission from excited states of laser-dressed atomic helium is analyzed with respect to laser intensity-dependent excitation energy shifts and angular distributions. In the two-color exteme ultraviolet (XUV)-infrared (IR) measurement, the XUV photon energy is scanned between 20.4 eV and the ionization threshold at 24.6 eV, revealing electric dipole-forbidden transitions for a temporally overlapping IR pulse (≈1012Wcm-2). The interpretation of the experimental results is supported by numerically solving the time-dependent Schrödinger equation in a single-active-electron approximation