309 / 2019-03-14 11:01:02

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

Excited state population,Frequency-resolved photon-electron spectroscopy,Fourier transforms

全体主题 > Fundamental Physics at Extremes

Excited states play an essential role in a wide range of phenomena in strong field physics, including multiphoton ionization, Freeman resonance, Rabi resonance, below-threshold harmonic generation, and the generation of the Rydberg state. In the early days, De Boer and Muller used a long pulse (nanosecond probe pulse) to ionize the excited atoms and guarantee different energy peaks in energy distributions derived from different excited states. Then, the population can be obtained by calculating the area probability of each peak. Additionally, the metastable state, due to its long lived property, can be detected at the microchannel plate detector (MCP) after a long flight. However, these measurements require that the lifetime of the excited state be longer than several hundred nanoseconds (10-7 s), which is far longer than the lifetime (10−9s) of most of the excited states in atoms and molecules.
We propose a frequency-resolved photon-electron spectroscopy technique to reconstruct a 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 a high-precision population reconstruction of helium and hydrogen atoms. Besides, note that the time scale (10-13s) in our scheme is much smaller than the lifetime of excited states in atoms and molecules. This technique is helpful to measure the population of bound states and to further understand the phenomena involving the excited states.