  Atomic excitation to excited states in strong laser field is the key to
high-order harmonic generation below ionization threshold, yet remains unclear
mainly due to the lack of proper detection methods. We propose a
frequency-resolved photon-electron spectroscopy technique to reconstruct
population of excited states with the second delayed laser pulse. The technique
utilizes Fourier transformation to separate ionization from different excited
states to different positions on the spectrum. With the advantage of
separation, we provide a scheme to reconstruct populations on different excited
states after the first pulse. The scheme is validated by high-precision
population reconstruction of helium and hydrogen atoms.
