Circularly Polarized X Rays: Another Probe of Ultrafast Molecular Decay Dynamics

Dissociative nuclear motion in core-excited molecular states leads to a splitting of the fragment Auger lines: the Auger-Doppler effect. We present here for the first time experimental evidence for an Auger-Doppler effect following $\mathrm{F}1s\to a_{1g}^{*}$ inner-shell excitation by circularly polarized x rays in ${\mathrm{SF}}_6$. In spite of a uniform distribution of the dissociating S-F bonds near the polarization plane of the light, the intersection between the subpopulation of molecules selected by the core excitation with the cone of dissociation induces a strong anisotropy in the distribution of the S-F bonds that contributes to the scattering profile measured in the polarization plane.

Figure 1

Comparison of the resonant Auger decay spectra recorded at the $\mathrm{F}1s\to a_{1g}^{*}$ resonance in ${\mathrm{SF}}_6$ molecule with right-handed CPL (CR, red curve), and the average of the RAS spectra recorded with linear vertical [LV (0°), $\mathbf{e}\parallel \mathbf{p}$] and horizontal [LH (90°), $\mathbf{e}\perp \mathbf{p}$] polarizations (black curve). The inset shows a blowup of the kinetic energy region of the F-atomic peak for independent measurements with LV (green), LH (purple), and CR (red) polarizations.

Figure 2

Schematics of the experiment showing possible orientations of the Auger electron momentum $\mathbf{p}$ and dissociation direction $\mathbf{n}$ with respect to the polarization plane $({\mathbf{e}}_{\parallel },{\mathbf{e}}_{\perp })$. (a) Auger electron ejection in the polarization plane. Here ${\mathbf{n}}_{\parallel },{\mathbf{n}}_{\perp }$ are the projections of the dissociation direction $\mathbf{n}$ on the plane of polarization. (b) Auger electron ejection perpendicular to the polarization plane. (c) The intersection of the excitation torus with the cone of dissociation selects the molecules active in the RAS process. (d) The excitation function is isotropic when $\mathbf{n}$ is lying in the $({\mathbf{e}}_{\parallel },{\mathbf{e}}_{\perp })$ plane ($\phi =0°$).

Figure 3

Doppler splitting of the atomic Auger peak following core excitation of the ${\mathrm{SF}}_6$ molecule. The theoretical cross sections (black curves) are shifted in energy to match the experimental spectra; linear polarization LH ($\mathbf{e}\perp \mathbf{p}$), linear polarization LV ($\mathbf{e}\parallel \mathbf{p}$), right-handed circular polarization CR. Baselines are subtracted.