## 3 Results
### From RABBITT Regime to Streaking Regime
If the EUV and IR pulses overlap, there are additional side bands at even harmonic orders (LAPE) [30]. The additional side band (60 th harmonic) located between the 59 th and 61 st harmonic order, oscillates as a function of IR-EUV time delay due to interference between photoelectrons that emit one IR photon after absorption of a 61 st-order EUV photon and those that absorb one IR photon after absorption of a 59 th-order EUV photon. For constructive interference at the side band, there is destructive interference in the odd-order single-photon peaks, and vice versa. Note that, here, a bandwidth-limited EUV pulse train is assumed and, hence, the different harmonics interact with the initial state at the exact same time. If the photoelectron were excited by a 1.7 fs APT (Figure 3, blue line), the spectrogram would show deviations from a pure RABBITT spectrogram (Figure 4b). For IR-EUV delays \(\tau_{\text{IR-EUV}}\) at which a higher yield at the 59 th band is observed, the yield is reduced at the 61 st band, and vice versa. In addition, the 59 th- and 61 st-order bands show IR-induced and IR-EUV delay-dependent broadening. For even shorter duration of the APT envelope (1.1 fs, Figure 3, brown line), where EUV lines overlap (Figure 4c), the resulting spectrogram shows a signature of the vector potential of the IR pulse. However, for certain IR-EUV delays \(\tau_{\text{IR-EUV}}\), the direct photoemission peaks corresponding to the odd harmonics can still be distinguished. This intermediate regime (Figure 4b,c) exhibits features of both RABBITT and streaking spectra. Further reduction of the pulse train envelope leads to excitation with a single-attosecond pulse (Figure 3, green line) and correspondingly broad kineticenergy distribution of the excited electron wave packet (Figure 4d). This results in a typical streaking spectrogram, where the peak shift as function of \(\tau_{\text{IR-EUV}}\) reproduces IR vector potential. This transition illustrates that RABBITT and attosecond-streaking are two extremal cases of laser-assisted photoemission [30]. The manifestation of the LAPE mechanism critically depends on the time structure of the involved EUV and IR fields. In the limit of rather long IR and EUV pulses, photoemission side-band formation is observed. For very short EUV pulses (streaking case) and rapidly amplitude-modulated EUV fields (RABBITT case) the interference of different excitation pathways leads to a streaking and a RABBITT spectrogram, respectively. Note that all spectrograms are calculated for the same IR pulse. Thus, the appearance of either a RABBITT or a streaking type of spectrum is not at all related to IR field properties, but is basically determined by the time structure of the EUV field. Interestingly, continuous transition between both regimes seen in Figure 4 can be understood as an example of the quantum-classical correspondence principle: In the RABBITT regime, we observed discrete photoelectron side-band peaks caused by exchange of IR photons with the electron wave packet. The time structure of the electron wave packet created by the APT and this interaction with the photon field results in final electron momentum distribution that is characteristic for quantized light-matter interaction, i.e., discrete side bands appear. Moreover, the information of photoemission dynamics is imprinted on the RABBITT signal via the phase of the photoelectron wave function, which is a genuine quantum property. RABBITT, as an interferometric measurement, works as a quantum clock [14]. In contrast, the streaking regime exhibits classical behavior. Here, photoelectrons also undergo multiple interactions with the photon field but, due to different accessible interference pathways, resulting in continuously varying momentum distribution, and quantized light-matter interaction is no longer evident.