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 $y{}^7{P}_{2,3,4}^o\leftarrow a{}^7{S}_3$ excitation. Due to relaxation during ejection we obtain bare, excited Cr${}^{*}$ atoms in the $z{}^5{P}_{1,2,3}^o$ states. The Cr${}^{*}$ atoms are photoionized into a region close above the threshold of the $a{}^{6}S$ continuum of Cr ii. Discrete autoionizing states ($g{}^5{D}_{2,3,4}$ and $e{}^3{D}_{1,2,3}$) 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 $g{}^5{D}_{2,3,4}\leftarrow z{}^5{P}_{1,2,3}^o$ line strengths provide insight into the relaxation process, and we find that energetically lower $J$ components of $z{}^5{P}_{1,2,3}^o$ are significantly more strongly populated.

Figure 1

Energy-level diagram of gas-phase Cr atoms [17] depicting the laser excitations (solid arrow) and nonradiative relaxation (dashed arrow). Electron configurations are also given. IP indicates the ionization potential.

Figure 2

One-color two-photon PI spectrum of Cr-doped He${}_{\mathrm{N}}$, detected at the Cr mass (52 u). The bare Cr atom transitions $y{}^7{P}_{2,3,4}^o\leftarrow a{}^7{S}_3$ are indicated as vertical lines at 27 728.87, 27 820.23, and 27 935.26 cm${}^{-1}$, respectively [17], and the droplet-broadened excitation is represented by the broad feature. The lower graphs show detailed scans of the AI transitions from $z{}^5{P}_{1,2,3}^o$ to $g{}^5{D}_{2,3,4}$ (red) and to $e{}^3{D}_{1,2,3}$ (green). A fit of the Fano line profile convoluted with a Gaussian function (see text) is shown as the dashed line. The spectrum has been normalized according to the laser pulse energy.