Fano resonances in chromium photoionization spectra after photoinduced ejection from a superfluid helium nanodroplet

The photoinduced ejection of atoms from superfluid helium nanodroplets is exploited to populate metastable atomic states which cannot easily be reached from the ground state. We demonstrate this method with chromium (Cr) atoms which are located inside the droplet. The ejection is triggered by the 𝑦⁢7𝑃𝑜2,3,4←𝑎⁢7𝑆3 excitation. Due to relaxation during ejection we obtain bare, excited Cr* atoms in the 𝑧⁢5𝑃𝑜 1,2,3 states. The Cr* atoms are photoionized into a region close above the threshold of the 𝑎⁢6𝑆 continuum of Cr ii. Discrete autoionizing states (𝑔⁢5𝐷2,3,4 and 𝑒⁢3𝐷1,2,3) embedded into the continuum are observed as sharp lines with characteristic, asymmetric line profiles. These Fano profiles provide information about the coupling of the autoionizing states to the continuum (autoionization rates) and cross sections for photoionization and resonant excitation. The 𝑔⁢5𝐷2,3,4←𝑧⁢5𝑃𝑜 1,2,3 line strengths provide insight into the relaxation process, and we find that energetically lower 𝐽 components of 𝑧⁢5𝑃𝑜 1,2,3 are significantly more strongly populated.