# Frequency-resolved photon-electronic spectroscopy for excited state population detection

## TL;DR

This paper introduces a novel frequency-resolved photon-electronic spectroscopy method that uses a second delayed laser pulse and Fourier transformation to accurately detect and reconstruct excited state populations in atoms, advancing understanding of atomic excitation in strong laser fields.

## Contribution

The paper presents a new spectroscopy technique that separates ionization signals from different excited states, enabling precise population reconstruction, which was previously challenging.

## Key findings

- Successfully reconstructed excited state populations in helium and hydrogen.
- Demonstrated the method's ability to distinguish between different excited states.
- Validated the technique's accuracy with high-precision measurements.

## Abstract

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.