Breakdown of the Two-Step Model in $K$-Shell Photoemission and Subsequent Decay Probed by the Molecular-Frame Photoelectron Angular Distributions of ${\mathrm{CO}}_2$

We report results of measurements and of Hartree-Fock level calculations of molecular-frame photoelectron angular distributions (MFPADs) for C $1s$ photoemission from ${\mathrm{CO}}_2$. The agreement between the measured and calculated MFPADs is on average reasonable. The measured MFPADs display a weak but definite asymmetry with respect to the ${\mathrm{O}}^{+}$ and ${\mathrm{CO}}^{+}$ fragment ions at certain energies, providing evidence for an overlap of gerade and ungerade final ionic states giving rise to a partial breakdown of the two-step model of core-level photoionization and its subsequent Auger decay.

Figure 1

Experimental and theoretical 2D plots of C $1s$ MFPADs of ${\mathrm{CO}}_2$ at photon energies 303.8, 307, 312.2, 320.9, and 329.8 eV. $\theta =0°$ and 180° are the directions of ${\mathrm{O}}^{+}$ and ${\mathrm{CO}}^{+}$, respectively. See text for details.

Figure 2

Polar plots of C $1s$ MFPADs of ${\mathrm{CO}}_2$ with molecular orientation parallel to the $E$ vector at photon energies 307, 311.8, 312.2, 319.4, 320.9, and 329.8 eV. The dots correspond to measurements. Solid and dashed curves are calculations with symmetric and asymmetric geometries, respectively. See text.