Theoretical study of double ionization/excitation (K-2V) processes induced by one or two photon absorption

Projet de thèse commencé le 1er octobre 2018.

Laboratoire porteur
Description of the project

The interest of the X-ray community for double core hole (DCH) ionization/excitation has grown in the last decade. One of the main reasons is that DCH spectroscopy exhibits a stronger sensitivity to the chemical environment than conventional single-core-hole spectroscopy. It has benefitted from the new developments in the X-ray techniques. In one hand, the advent of intense X-ray Free Electron Laser (X-FEL) sources allows two-photon processes in the X-ray domain [1]. On a second hand, improvements in coincident electron spectroscopy (magnetic bottle) [2a] and photoelectron spectroscopy at high energy (HAXPES) [2b] implemented at synchrotron make possible performing high- resolution single-photon DCH spectroscopy.

Single-site (K-2) [3] and two-site (K-1 K-1) [4] DCH states have thus been recently recorded and identified on synchrotron line sources, although they represent only a tiny fraction (10-3–10-5) of the K-1 dominant ionization process.

For the first time in 2013 [5] a new class of states (K-2V) corresponding to simultaneous K -shell ionization and K-shell excitation by a single photon absorption have been evidenced and theoretically interpretated [5-10] by our teams at LCPMR. Formally, K-2V states can be considered as a “super” shake-up process accompanying the K-shell ionization, where the “shaken” electron is also a K-shell electron. Such events present a specific interest for spectroscopists because two excitation pathways, with comparable intensities, called “direct” and “conjugate”, contribute to the signal (see figure below). Briefly, the direct path is described by a dipolar K-shell ionization accompanied by monopolar excitation (shake-up) of the remaining K-shell electron to vacant V orbital, while the conjugate path implies a dipolar excitation of a K-shell electron to a vacant V' orbital accompanied by monopolar ionization (shake-off) K-shell ionization. Fig.

Fig.1: Schematic picture of the single photon K-2V direct (left panel) and conjugate (right panel) channels

Thanks to a theoretical model taking into account those two pathways and numerical
calculations performed with a home-made ab-initio package, we have successfully described
K?2V processes for a series of small molecular systems (C2H2, C2H4, C2H6, CS2, SF6, CH3-
CN). We provided absolute cross sections in excellent agreement with experimental data

From these recent works, it appears that K?2V (this name was proposed by our group)
represents « a real new » photoelectron spectroscopy technique displaying many
advantages compared to usual XPS - X-ray Photoelectron Spectroscopy- (K?1) or NEXAFS -
Near-Edge X-ray Absorption Fine Structure Spectroscopy - (K?1V) spectroscopies. The K?2V
spectroscopy has unique properties, which are listed below:

  1. Dipole transition 1s-> ?* transition usually observed in NEXAFS are also present in K?2V photoelectron spectra via the conjugate path.
  2. Forbidden g->g or 1s->ns transitions in NEXAFS are allowed in K?2V direct path.
  3. The energy window for observation of K?2V below the K?2 threshold is larger (20 eV) than, for K?1V below the K?1 threshold (5 eV) in NEXAFS. This helps to distinguish the associated transitions of different atoms in different chemical environment (see Fig. 2).
  4. In the satellite spectra (K?1v'-1V) of XPS, the direct shake-up process is generally much stronger than the conjugate channel. In K?2V, conjugate and direct paths have similar, amplitude making this spectroscopy much richer but more involved, i.e. more theory is needed.
Fig.2: Evidence of larger energy window in K-2V (20 eV)/NEXAFS(5 eV)
Fig.2: Evidence of larger energy window in K-2V (20 eV)/NEXAFS(5 eV)

Our original model based on a Post Hartree-Fock Configuration Interaction picture (presentation @ Journée SOLEIL@UPMC in Nov/6/2017 : Simulations de processus complexes en couche interne : voies vers de nouvelles spectroscopies ? par Stéphane Carniato, LCPMR) explains why the direct and conjugate contributions are comparable for these « super » shake-up processes : the direct pathway relies only on core-excited configurations of the initial molecular state and are thus unusually weak.

The interest in this DCH spectroscopy also grew substantially few years ago with the technological development of the XFEL, which provides intense and femtosecond x-ray laser pulses. Efficient formation of double core states became thus possible by two consecutive absorption of one photon.

Most of the theoretical work done so far was focused on the electronic structure of double core hole states. Those who took an interest in dynamical effects used rate-equation models within the fixed nuclei approximation.

Following K-2V formation by sequential absorption of photons instead of one single photon absorption, an opportunity offered by theis kind of experiments, could however be very challenging as, through this process, direct channel can be avoided (dipolar absorption in presence of a core hole only).

The PhD project will consist in studying in more details the outputs for this new techniques from two complementary points of view, i.e. a time dependent and time independent approach.

Our ultimate goal is to develop, during this work, a comprehensive and versatile model likely to describe DCH formation in molecular systems of, for example, astmospherical interest and to provide predictive informations to guide experiments.