Photoionization of few electron systems: a hybrid coupled channels approach

Abstract

We present the hybrid anti-symmetrized coupled channels method for the calculation of fully differential photo-electron spectra of multi-electron atoms and small molecules interacting with strong laser fields. The method unites quantum chemical few-body electronic structure with strongfield dynamics by solving the time dependent Schrödinger equation in a fully anti-symmetrized basis composed of multi-electron states from quantum chemistry and a one-electron numerical basis. Photoelectron spectra are obtained via the time dependent surface flux (tSURFF) method. Performance and accuracy of the approach are demonstrated for spectra from the helium and beryllium atoms and the hydrogen molecule in linearly polarized laser fields at wavelengths from 21 to 400 nm. At long wavelengths, helium and the hydrogen molecule at equilibrium inter-nuclear distance can be approximated as single channel systems whereas beryllium needs a multi-channel description.wavelengths has remained out of computational reach. The particular difficultly arises from the fact that, in order to compute photoelectron spectra the asymptotic part of the wavefunction is required. This needs large simulation box volumes and access to exact single continuum states to project the wavefunction onto at the end of time propagation. Having large simulation boxes and computing single continuum states of a multi-electron system are expensive tasks, making these kind of computations costly or outright impossible.In this respect, a recently developed method called the time dependent surface flux (tSURFF) method [19,20] has turned out to be an attractive solution. In the tSURFF approach, the wavefunction outside a certain simulation box is absorbed, and the electron flux through the box surface is used to obtain photoelectron spectra. This way photoelectron spectra can be computed with minimal box sizes.We deal with the difficulties of the few body problem and computation of photoelectron spectra by combining quantum chemical structure with tSURFF for single electron systems through a coupled channels approach. The ansatz is similar in spirit to the one presented in [4]. However, unlike in [4], we deal with antisymmetrization exactly. We discretize our multi-electron wavefunctions with the neutral ground state of the system and with anti-symmetrized products of the system's single ionic states and a numerical one-electron basis. This ansatz is suitable to study single ionization problems. The ionic and neutral states are computed by the COLUMBUS code [21] giving us the flexibility to treat the ionic states at various levels of quantum chemistry. While the fully flexible active electron basis describes the ionizing electron, the ionic basis describes the core polarization and the exact anti-symmetrization ensures indistinguishability of the electrons. The inclusion of the field-free neutral helps us to get the right ionization potential and start with the correct initial state correlation without much effort. We call our method hybrid fully anti-symmetrized couple...

A First-Principles Time-Dependent Density Functional Theory Framework for Spin and Time-Resolved Angular-Resolved Photoelectron Spectroscopy in Periodic Systems

“…The t-SURFF method is a well established technique that has been successfully employed to study ionization of atoms and small molecules under strong laser fields [24][25][26][27][28][29] . This method has only recently been exported to TDDFT by some of the authors 30 .…”

Role of Multi-Electron Effects in the Asymmetry of Strong-Field Ionization and Fragmentation of Polar Molecules: The Methyl Halide Series

“…These essential developments have been implemented in the weak-field asymptotic theory (WFAT). [17][18][19][20] Other methods that have proven successful in describing strong-field ionization (SFI) of polar molecules include time-dependent multi-electron methods [21][22][23][24][25] but the predictive power of all theories largely remains to be established.…”

Perfect absorption in Schrödinger-like problems using non-equidistant complex grids

“…Conditions on the derivatives can be omitted for the same reasons and with the same precaution about using explicitly symmetric forms of the operators as discussed above, Eqs. (14) and (15).…”

“…[2] and have since be used to solve several challenging problems in the strong laser-matter interactions [14][15][16][17][18]. All these calculations were performed using a FEM basis.…”

“…Remarkably, the ground state energy of the system is exactly at -1/2, as in the three-dimensional Hydrogen atom. It has been demonstrated that the mathematical behavior of absorption in this simple system generalizes to the analogous Schrödinger equations for one-and two-electrons systems in up to 6 spatial dimensions [15,20]. For all studies below use a vector potential of the form…”