Photon-energy dependence of single-photon simultaneous core ionization and core excitation in ${\mathrm{CO}}_2$

We have studied the $K^{-2}V$ process corresponding to simultaneous $K$-shell ionization and $K$-shell excitation in the $\mathrm{C}{\mathrm{O}}_2$ molecule. We define these $K^{-2}V$ states as super shake-up, at variance with the “conventional” $K^{-1}{v}^{-1}V$ shake-up states. While the nature and evolution with photon energy of the conventional shake-up satellites has been the object of many studies, no such data on a large photon-energy range were previously reported on super shake-up. The $\mathrm{C}{\mathrm{O}}_2$ molecule is a textbook example because it exhibits two well-isolated $K^{-2}V$ resonances (with V being $2{\pi }_u^{*}$ and $5{\sigma }_g^{*}$) with different symmetries resulting from shake-up processes of different origin populated in comparable proportions. The variation of the excitation cross section of these two resonances with photon energy is reported, using two different experimental approaches, which sheds light on the excitation mechanisms. Furthermore, double-core-hole spectroscopy is shown to be able to integrate and even expand information provided by conventional single-core-hole X-ray Photoelectron Spectroscopy (XPS) and Near-Edge X-ray Absorption Fine Structure (NEXAFS) techniques, revealing, for instance, $g-g$ dipole forbidden transitions which are only excited in NEXAFS spectra through vibronic coupling.

Figure 1

At $h\nu =760\mathrm{eV}$, experimental $K^{–2}V$ and $K^{–2}$ spectra at the C $K^{–2}$ edge in $\mathrm{C}{\mathrm{O}}_2$ obtained by filtering three- and four-electron coincident events. The photoelectron(s) are coincident with hypersatellite Auger (260–320 eV) and second Auger (225–260 eV) electrons. Electrons below 5 eV have been rejected from the $K^{–2}$ signal to reduce the noise. The $K^{–2}V$ theoretical spectra show direct (red line and bars) and conjugate (green line and bars) contributions. Vertical bars give the integrated theoretical cross sections for each peak. Experimental absolute cross sections ($\square$) include an additional 30% contribution from second Auger electrons (190–225 eV) falling in the same energy range than O($1s$) photoelectrons.

Figure 2

Photoelectron spectra at $h\nu =2300\mathrm{eV}$. The spectrum is dominated by the oxygen $1s$ and satellite lines. By accumulating 1250 times more signal the $C\left(K^{–2}V\right)$ lines become visible in the enlarged inset.

Figure 3

Comparison of the experimental and theoretical spectra at 2300 eV. The vertical bars correspond to the theoretical cross section and the black squares to the experimental values. To take into account the angular correction factor due to different β parameters for direct $(\beta =2)$ and conjugate $(\beta =0)$ channels the scales differ by a factor of 2.8.