Enhancement of Above Threshold Ionization in Resonantly Excited Helium Nanodroplets

Clusters and nanodroplets hold the promise of enhancing high-order nonlinear optical effects due to their high local density. However, only moderate enhancement has been demonstrated to date. Here, we report the observation of energetic electrons generated by above-threshold ionization (ATI) of helium (He) nanodroplets which are resonantly excited by ultrashort extreme ultraviolet (XUV) free-electron laser pulses and subsequently ionized by near-infrared (NIR) or near-ultraviolet (UV) pulses. The electron emission due to high-order ATI is enhanced by several orders of magnitude compared with He atoms. The crucial dependence of the ATI intensities with the number of excitations in the droplets suggests a local collective enhancement effect.

Figure 1

Photoelectron spectra of He droplets of different mean size $⟨N⟩$ (colored lines) compared with He atoms (black lines, $N=1$). Both droplets and atoms are resonantly excited to the $1s4p$ state at a photon energy of 23.74 eV and ionized by NIR pulses (a) and UV pulses (b). The intensity of the probe pulses is $I_{\mathrm{NIR},\mathrm{UV}}\approx {10}^{13}{\mathrm{W}\mathrm{cm}}^{-2}$. The two pulses were delayed by $\tau =4\mathrm{ps}$ in (a) and temporally overlapped ($\tau =0$) in (b). The vertical gray dashed lines indicate the energy 10 $U_p$. See text for discussion of the simulation curves (solid orange line for $N=1$ and dashed colored lines for different $N$).

Figure 2

Logarithmic intensity plot of the electron spectra (vertical axis) as a function of the delay between the XUV pump and the UV probe pulses (horizontal axis). Red regions indicate high signal intensities, whereas the blue area shows the low-signal background. The short-time dynamics around zero delay is displayed in (a) on a linear scale, and the long-time dynamics is shown on a logarithmic delay scale in (b). The mean droplet size is $⟨N⟩=41000$.

Figure 3

Energy level diagram illustrating the competing relaxation pathways in an excited He droplet. Following resonant one-photon excitation by an XUV pump pulse, the droplet can autoionize (AI), vibronically relax into the $1s2s{}^{1}S$ state, or decay by ICD if a second He atom is excited nearby. Relaxation is quenched if a UV probe pulse induces multiphoton ionization shortly after the excitation.

Figure 4

Electron spectra for UV probe pulses at pump-probe delays 0, 0.2, 9.5 ps (black, red, blue lines, respectively). The spectrum recorded with only the pump pulse was subtracted to better visualize the contribution of ATI induced by the probe pulse. The inset shows on a logarithmic scale the area of the ATI peaks including linear and exponential fits. The mean droplet size is $⟨N⟩=41000$.

Figure 5

Mean electron kinetic energy as a function of the mean number of excitations per He droplet for $I_{\mathrm{UV}}=2.5\times {10}^{11}{\mathrm{W}\mathrm{cm}}^{-2}$, see text. The dashed line is the result of the semiempirical model fitted to the TDSE simulation for He atoms at that value of $I_{\mathrm{UV}}$. The open circle indicates the measured value for atomic He ($I_{\mathrm{UV}}={10}^{13}{\mathrm{W}\mathrm{cm}}^{-2}$).