Abstract
In the strong-field approximation, we calculate the electron momentum distribution following strong-field ionization of atomic hydrogen by an $N$-cycle circularly polarized laser pulse. For $N\ensuremath{\lesssim}5$, our results show a pronounced interference structure in addition to the well-known carrier-envelope phase difference (CEPD) sensitivity. With increasing $N$ we observe a gradual change to a momentum distribution which is free of interference and CEPD effects and which is characterized by well-resolved above-threshold ionization peaks, consistent with the quantized photon picture. The changes in the interference structures are explained by the spectral properties of the pulse. We compare the results of the strong-field approximation with those obtained by the saddle-point method and show that while the saddle-point method is able to reproduce the results for $N\ensuremath{\gtrsim}5$, it fails to explain the interference structure and the asymmetry of the distribution for smaller $N$.
Published Version
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