Molecular Frame Photoelectron Emission in the Presence of Autoionizing Resonances

We have measured the angular distribution of valence-shell photoelectrons excited by circularly polarized light from fixed-in-space ${\mathrm{N}}_2\mathrm{O}$ molecules, near to and on top of resonances due to Rydberg states embedded in the ionization continuum. The sign of the circular dichroism for ionization into the ${\mathrm{N}}_2{\mathrm{O}}^{+}\mathbf{(}B{}^2\Pi ,(1\pi {)}^{-1}\mathbf{)}$ state is reversed on top of the lowest dominant resonances. Measured angular distributions are well predicted by state-of-the-art multichannel configuration interaction calculations. The change in sign of the circular dichroism at the peak of the resonance is the result of a rapid change in the phases of resonant dipole matrix elements by a factor of $2\pi$ as the energy is scanned across the resonance.

Figure 1

The $F_{11}$ function for DI Eq. (1) (a) and the related cut of the MFPAD induced by LHC light (b) (see text) at $h\nu =19.5\mathrm{eV}$. Dots: experiment; full line: calculations; dashed line: calculations corrected for bending. Theory and experiment are normalized such that the total DI cross sections are identical.

Figure 2

The $F_{LN}$ functions (a)–(e) and the related cut of the MFPAD (f) induced by LHC light measured at 18.55 eV for Eq. (1). Dots: experiments; thin line (red): calculations; dashed line: calculations corrected for bending; thick line (blue): calculations corrected for bending assuming a $w=25\mathrm{meV}$ width. These data are normalized to the same total cross section equal to 1: the absolute values are obtained by scaling the $y$ axis by 33.798 Mb ($w=0$) and 23.011 Mb ($w=25\mathrm{meV}$), respectively.

Figure 3

Evolution of the computed MFPAD for the ${\mathrm{N}}_2\mathrm{O}$ molecule oriented perpendicular to the light propagation axis as shown, nearby and on top of the lowest $\sigma$ and $\pi$ resonances. The inset displays the computed photoionization cross section for reaction (1), where the dark arrows indicate the experimentally studied energies.