We use the strong field approximation to study the single-photon ionization of the hydrogen atom in the time domain. During the interaction of the laser pulse and the atom, the electron is pumped to the Volkov state and has a widely energy distribution at the beginning of the laser pulse. With the time evolution, only the photon-electron with the energy ?ω -I_p is survived, where ω is the laser angular frequency, and I_p is the ionization potential of the atom. Therefore we observed how the energy spectrum of the ionized electron builds in the time domain.
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