Tracking hole localization in $K$-shell and core-valence-excited acetylene photoionization via body-frame photoelectron angular distributions

Asymmetry in the molecular-frame photoelectron angular distributions from core-hole- or core-valence-excited polyatomic targets with symmetry-equivalent atoms can provide direct evidence for core-hole localization. Using acetylene as an example, we contrast the small asymmetry that can be seen in direct core-level ionization, due to the competition between two competing pathways to the continuum, with ionization from core-valence-excited HCCH, which offers the prospect of observing markedly greater asymmetry.

Figure 1

HCCH carbon $1s^{-1}$ MFPADs at 4 eV photoelectron energy averaged over photon polarization direction. Top: $1{\sigma }^{-1}$ and $2{\sigma }^{-1}$ components at rms asymmetric stretch geometry; asterisks label the carbon atom with the core hole. Middle: total MFPAD at asymmetric stretch geometry. Bottom: total MFPAD at equilibrium geometry. The MFPADs were rescaled to the same maximum magnitude.

Figure 2

Potential-energy curves of the C $1s\text{−}\pi$* states of HCCH in linear geometry with the C–C distance fixed at its ground-state equilibrium value of 2.273 bohrs. Results are shown for four-state-averaged MCSCF and multireference CISD calculations.

Figure 3

MFPADs for the lower (left) and upper (right) $\Pi$* states of HCCH at a C–C displacement of 0.06 bohr. MFPADs are averaged over all photon polarization directions and summed over degenerate components for each state. Results are for photoelectron energies of 2 eV (top), 4 eV (middle), and 6 eV (bottom).

Figure 4

Diabatization of computed CISD potential-energy curves, following the treatment of Ref. [15].

Figure 5

Vibrational $\nu =0$ probability densities from two-state diabatic model.