## 3 Results
### From RABBITT Regime to Streaking Regime  
To demonstrate the transition from the conditions of a RABBITT to a streaking experiment in a simplified solid-surface system, EUV attosecond pulse duration is successively reduced. A reduction of attosecond EUV pulse duration in the time domain implies a broadening of the corresponding EUV excitation spectrum. The attosecond excitation pulses corresponding to the EUV spectra applied in the following are shown in Figure 3.  
Figure 3: Normalized intensity of EUV excitation pulses (solid line) in the time domain corresponding to the EUV spectra shown in Figure 4a–d. Pulse train envelopes are also shown (dotted lines). The envelope full width at half maximum (FWHM) pulse durations are (**a**) 6.7 fs, (**b**) 1.7 fs, (**c**) 1.1 fs, and (**d**) 474 as. Figure 2: Schematic high harmonic spectrum of a few-cycle IR pulse. In the intermediate energy regime, separated harmonic lines of equal height form a plateau. Spectral filtering in this regime leads to EUV APTs. In the high-energy regime, a cutoff continuum is formed. To obtain SAPs, this regime needs to be spectrally filtered. In a RABBITT experiment, an attosecond pulse train is applied as the excitation pulse. Here, its envelope full width at half maximum (FWHM) pulse duration is 6.7 fs (Figure 3, red line). The corresponding excitation spectrum in the RABBITT regime consists of two well-separated narrow spectral EUV lines located at the 59 th and 61 st harmonic order (Figure 4a) corresponding to 91.3 and 94.4 eV, respectively. Now, we move from a pure RABBITT regime to a pure streaking regime by reducing the duration of the APT envelope. As fewer and fewer subpulses of the EUV pulse train interfere with each other, the resulting spectral features of the harmonics become broader. For a 1.7 fs (FWHM) long attosecond pulse train envelope, three EUV pulses interact with the system (Figure 3, blue line), resulting in two broad but still fully separated lines in the EUV spectrum (Figure 4b). Further reduction of the APT envelope duration to 1.1 fs leads to very weak side pulses and a pronounced main attosecond pulse in the APT (Figure 3, brown line). Spectral lines in the corresponding EUV spectrum overlap (Figure 4c). A single-attosecond pulse (Figure 3, green line) of 474 as duration was obtained for broad spectral distribution (Figure 4d). Time-resolved photoelectron spectra are calculated by solving the full one-particle TDSE while the excitation spectrum is varied. The initial wave function of the electron is localized at 5 A below the surface. This initial depth inside the material was chosen since it closely fits the effective mean free path of the photoelectrons considered here. Note that the kinetic energies of the photoelectrons are close to the minimum of the universal curve [38]. The time-independent potential consists of jellium potential and screened Yukawa potential. The electron is excited by the EUV pulse (train) and propagates toward the surface, where it is emitted into the vacuum. For all EUV excitation pulses,  
Figure 4: Simulated photoelectron spectra (color plots) corresponding to four different EUV excitation spectra (black lines). All spectra were individually normalized. EUV pulse (train) was given by the Fourier transform of the EUV spectra. IR pulse was treated independently, with a center wavelength of 800 nm, pulse duration of 5 fs, and maximum electric field amplitude \(E_{0}=~{}1~{}\mathrm{V/nm}\). Effective propagation distance was 5 Å, and initial-state binding energy was \(\varepsilon_{0}=-52.9~{}\mathrm{eV}\). the electron wave packet starts to interact with the same 5 fs 800 nm IR cosine pulse with electric field amplitude \(E_{0}=1\) V/nm. For excitation by an attosecond pulse train (Figure 3, red line), the resulting time-resolved photoelectron spectrum is a typical RABBITT spectrogram, as shown in Figure 4a. For delays between the IR and EUV pulse beyond the temporal overlap of both pulses, there are two photoelectron peaks located at energies corresponding to odd harmonics (not shown).