Fano resonances observed in helium nanodroplets

Doubly excited Rydberg states of helium (He) have been studied in nanodroplets using synchrotron radiation. Although qualitatively similar to their atomic counterparts, the Fano resonances in droplets are broader and exhibit blueshifts which increase for the higher excited states. However, varying the droplet size hardly affects the shapes of the resonances. Furthermore, additional dipole-forbidden resonances appear which are not seen in the He atom. We discuss these features in terms of localized atomic states perturbed by the surrounding He atoms.

Figure 1

(a) Electron signals of He atoms at a nozzle temperature of $T=40$ K [black (lower) line] and of He droplets [red (upper) line] consisting of ${10}^9$ He atoms ($T=8$ K) as a function of the photon energy. (b) Fano profile fit (red) to the $2s2p+$ resonance of the atomic resonance signal (black). (c) Fano profile fit (red) to the droplet equivalent of the $2s2p+$ resonance (black). The blue line shows the convolution of the atomic $2s2p+$ resonance with the line profile of the singly excited $1s2p$ droplet line shape from Ref. [8]. (d) ${\mathrm{He}}^{+}$ and ${\mathrm{He}}_2{}^{+}$ ions yield spectra recorded at $T=17$ K ($\langle N\rangle \approx 7000$). The black (lower) line depicts the ${\mathrm{He}}^{+}$ yield from the He background (chopper closed), the blue (middle) line shows the ${\mathrm{He}}^{+}$ yield from the droplet beam with background subtraction (chopper $\mathrm{open}-\mathrm{closed}$), and the red (upper) line shows the ${\mathrm{He}}_2{}^{+}$ yield from the droplet beam with background subtraction (chopper $\text{open}-\text{closed}$).

Figure 2

(a) Electron-yield spectra at various nozzle temperatures corresponding to gas phase atoms and nanodroplets of various sizes. (b) Electron-yield spectra centered at the $2s2p+$ resonance. (c) and (d) Baseline-subtracted electron signals for a droplet size of ${10}^{11}$ atoms (7 K) for selected energy ranges along with the atomic $2s2p+$ resonance (blue dashed line).

Figure 3

Computed excitation energies for doubly excited resonances accessible in dipole transitions of the He dimer (top) along with the nearest neighbor ($\text{nn}$) distribution (bottom) for He droplets (taken from [27]). The excited states correlated with the dipole-allowed transitions in the He atom are plotted as solid lines, and the dipole-forbidden transitions as dashed lines. The first three droplet resonances are marked as thick tick marks on the $y$ axis.

Figure 4

Droplet-size dependence of the shift of the droplet resonance with respect to the atomic line (a) and the Fano $q$ parameter (b) for the first three resonances of the $2snp+$ series. Dashed lines are linear fits to guide the eyes. The respective atomic $q$ parameters are shown as colored tick marks on the $y$ axis.