## 3 Results  
### 806 nm Pump  
The upper panel of Figure 2 shows the raw image of the spectral distribution of plasma emission during ablation of the tin bulk target, using a 370 ps HP at a fluence of F = 3.2 J cm\({}^{-2}\). This spectrum was collected without the propagation of a 64 fs DP through the plasma plume and is shown for the demonstration of various emission lines from highly-ionized tin LIP. The application of such plasma for HHG has two drawbacks: the first disadvantage is associated with strong incoherent radiation in the range of harmonic generation, which can entirely overlap later radiation, and the second disadvantage is related to a deterioration of the optimal phase-matching conditions between driving and harmonic waves. The latter disadvantage leads to a reduction or entire disappearance of harmonic emissions due to a phase mismatch of the interacting waves. In addition, it also affects the conditions for the resonant amplification of harmonics. The propagation of femtosecond pulses through this plasma did not result in harmonic generation or allow for the observation of the weak lower-order harmonics. To achieve the efficient generation of harmonics, we used a twice smaller fluence of 370 ps pulses on the target surface (1.6 J cm\({}^{-2}\)) when no lines attributed to the ionic transitions in this spectral range were observed (bottom panel of Figure 2). The propagation of the DP through such LIP resulted in the generation of an XUV spectrum, comprising the harmonics of 806 nm radiation up to the 21 st order (H21). One can see a strong emission attributed to the resonance-enhanced H17 (\(\lambda\) = 47.4 nm), which notably exceeded the nearby harmonics. These studies were performed at a 150 ns delay between the HP and the DP. One can see that the strong H17 almost coincides with the positions of some weak emission lines of the plasma (upper panel of Figure 2). This harmonic was 4 to 10 times stronger than the nearest harmonic orders. The raw images of the harmonic spectra provide a better comparative view of the plasma and harmonic emissions. The bottom image of Figure 2 shows the common feature of the gradually decaying plateau-like harmonic distribution along the whole studied XUV spectrum except for the single-enhanced harmonic (H17). These raw images were taken at the saturation conditions of the CCD camera for better viewing of the peculiarities of the harmonic distribution. Notice that all line-outs of the harmonic spectra shown in the following figures were taken using the images collected at the unsaturated conditions of registration. We used two methods to analyze the dynamics of tin plasma spreading out from the target surface. Apart from the plasma emission spectra, we first analyzed the images of the plasma at different moments from the beginning of ablation. Useful information about ablated species can be retrieved by recording the spatial dynamics of the spreading of the plasma plume using a gated intensified charge-coupled device (ICCD) camera, which was used for imaging of the spreading tin LIP within a time interval ranging from 0 to 2000 ns with a variable time gate. In this case, we used 1064 nm, 10 ns pulses as the HP. The used HP energy (E\({}_{\rm HP}\) = 20 mJ) was higher than that used for the harmonic generation, for better  
Figure 2: Raw images of plasma (upper panel) and harmonic (bottom panel) emission spectra. Upper and bottom lines show the calibrated wavelength (in nm) and harmonic axes, respectively. visibility of the plasma plume, while the overall dynamics of the plasma formation and spreading were similar, thus allowing for the determination of the main regularities. At the initial moments from ablation (i.e., up to 50 ns), the LIP was spatially limited inside the dense and very bright small volume. As it was shown by analysis of the time-resolved spectra of the ns-induced LIP in the visible region [44], a strong continuum emission can be seen at the early periods of plasma dynamics.