Photoionization using the xchem approach: Total and partial cross sections of Ne and resonance parameters above the 2s22p5 threshold

Abstract

The XCHEM approach interfaces well established quantum chemistry packages with scattering numerical methods in order to describe single-ionization processes in atoms and molecules. This should allow one to describe electron correlation in the continuum at the same level of accuracy as quantum chemistry methods do for bound states. Here we have applied this method to study multichannel photoionization of Ne in the vicinity of the autoionizing states lying between the 2s 2 2p 5 and 2s2p 6 ionization thresholds. The calculated total photoionization cross sections are in very good agreement with the absolute measurement of Samson et al.[J. Electron Spectrosc. Relat. Phenom. 123, 265 (2002)], and with independent benchmark calculations performed at the same level of theory. From these cross sections, we have extracted resonance positions, total autoionization widths, Fano profile parameters, and correlation parameters for the lowest three autoionizing states. The values of these parameters are in good agreement with those reported in earlier theoretical and experimental work. We have also evaluated β asymmetry parameter and partial photoionization cross sections and, from the latter, partial autoionization widths and Starace parameters for the same resonances, not yet available in the literature. Resonant features in the calculated β parameter are in good agreement with the experimental observations. We have found that the three lowest resonances preferentially decay into the 2p −1 d continuum rather than into the 2p −1 s one [Phys. Rev. A 89, 043415 (2014)], in agreement with previous expectations, and that in the vicinity of the resonances the partial 2p −1 s cross section can be larger than the 2p −1 d one, in contrast with the accepted idea that the latter should amply dominate in the whole energy range. These results show the potential of the XCHEM approach to describe highly correlated process in the ionization continuum of many-electron systems, in particular molecules, for which the XCHEM code has been specifically designed.

Attosecond photoionization dynamics in neon

“…The propagation of the single-electron orbitals is de-termined by the Qand P-space equations, Eqs. (7) and (8), respectively. By setting η i j (t) either to be 0 or h i j (t), the only contribution in Q-space which may mix the magnetic quantum number m in the orbital |φ i (t) is n (ρ −1 (t)) n i jkl Q(t)ρ jl nk (t)W k l (t)|φ j (t) .…”

“…To this end we need a theory where we can control the level of approximation, and we need to investigate a system which is possible to describe at such different levels. Thus, we describe the photoionization of the Ne ground-state [5][6][7] mediated by an ultrashort XUV pulse using the time-dependent restricted-active-space self-consistentfield (TD-RASSCF) approach [8][9][10][11]. This method permits the introduction of restrictions on the number of excitations in the active orbital space and is a generalization of the multiconfigurational time-dependent Hartree-Fock method [12].…”

Disentangling Spectral Phases of Interfering Autoionizing States from Attosecond Interferometric Measurements

“…To describe this physical scenario, we have solved the timedependent Schrödinger equation (TDSE) in the velocity gauge by expanding the time dependent wave function in a basis of Ne eigenstates that includes 1 S e , 1 P o , 1 D e , and 1 F o symmetries, and consists of both bound and continuum states. All Ne eigenstates have been evaluated by using XCHEM [28,32]. Briefly, continuum states have been evaluated by including four parent ions corresponding to the configurations 1s 2 2s 1 2p 6 and 1s 2 2s 2 2p 5 , from which neutral states were constructed by augmenting with an additional electron described by both active space orbitals and a hybrid basis of monocentric Gaussian and B-spline functions (GABS basis [33]).…”

Partial cross sections and interfering resonances in photoionization of molecular nitrogen

“…Here we summarize how QCPs, CCE and GABS combine to achieve an accurate description of photoionization processes in molecular systems. For more information on the theoretical details of this method, as well as on its application to the photoionization of He, H 2 and Ne, which confirmed the viability of the XCHEM approach and motivated this work, we refer the reader to the past publications [14,15].…”

“…It relies on a hybrid description of the wave function in terms of Gaussian functions and B-splines, allowing to overcome the problems associated with either type of function. The applicability of the XCHEM method in atomic and small diatomic benchmark systems has been well established [14,15]. The next step is to apply the method to a poly-electronic molecular system.…”