Vibrationally Resolved $K$-shell Photoionization of CO with Circularly Polarized Light

Diffraction of a low energy ($<4\mathrm{e}\mathrm{V}$) carbon-$K$-photoelectron wave that is created inside a CO molecule by absorption of a circularly polarized photon is investigated. The measurements resolve the vibrational states of the $K$-shell ionized $\mathrm{C}{\mathrm{O}}^{\mathrm{+}}$ molecule and display the photoelectron diffraction patterns in the molecular frame. These show significant variation for the different vibrational states. This effect is stronger than predicted by state-of-the-art theory. As this study is performed close to C-$K$-threshold and, therefore, far below the molecule’s $\sigma$-shape resonance, this surprisingly strong effect is not related to that resonance phenomenon.

Figure 1

Ground vibrational state of CO and the first vibrational states of carbon-$K$-ionized $\mathrm{C}{\mathrm{O}}^{\mathrm{+}}$. The equilibrium internuclear distances are 1.128 Å and 1.079 Å [7], and the vibrational spacings are 269 and 301 meV [14] for CO and $\mathrm{C}{\mathrm{O}}^{\mathrm{+}}$, respectively. Top: products of the CO ground state and $\mathrm{C}{\mathrm{O}}^{\mathrm{+}}$ vibrational wave functions.

Figure 2

C-$K$-photoelectron (carbon-$K$-shell) energy distribution as created by photons of 298.3 eV energy. The solid line is a sum of the photo lines of the contributing vibrational $\mathrm{C}{\mathrm{O}}^{\mathrm{+}}$ states (dotted lines), represented by distorted Lorentz shapes according to 13.

Figure 3

Molecular frame photoelectron distributions for (a)  297.3 eV, (b) 298.3 eV, (c) 299.3 eV, and (d) 300.7 eV photon energy. Column (e) shows the circular dichroism (CD, the normalized difference between the angular distribution obtained for left and right circularly polarized light) at a photon energy of 298.3 eV. Top to bottom: angular distribution for the sum of all vibrational states, those for ${\nu }^{\prime }=0$, ${\nu }^{\prime }=1$, ${\nu }^{\prime }=2$. The molecule is oriented horizontally within the plane of the figure with the O atom to the left. Furthermore, the paper plane corresponds to the polarization plane of the left hand circularly polarized photons propagating into the plane. Grey line: relaxed core HF calculations normalized to the maximum of the experimental distribution. The black line shows a fit of spherical harmonics up to $l=5$.