Science & tech · 2022
Can changing the of light waves control how helium atoms become excited?
Yes—varying the relative phase of first- and third-harmonic extreme ultraviolet pulses controlled the population of helium’s 2s² state through between two-photon excitation paths. The electron yield was highest when the light’s relative phase matched the phase difference of the atomic amplitudes governing those paths.
Details
The study varied the relative phase of first-harmonic (ω₁) and third-harmonic (ω₃) extreme ultraviolet light pulses to control the population of the 2s² state in helium. This control used interference between two two-photon excitation paths: ω₁ + ω₁ and ω₃ − ω₁.
The researchers monitored the state population by measuring the total electron yield from the autoionization decay of the 2s² state. Maximum yield occurred when the relative phase of the two harmonics matched the phase difference of the complex atomic amplitudes governing the two excitation paths.
In the spectral region of the resonance, the calculated trend of atomic phase differences agreed well with the measured data, provided that the time-reversed −ω₁ + ω₃ path was also included.
The results open a route to accessing phase differences between two-photon ionization paths involving intermediate states that are far apart in energy. They also open a route to interferometry in the extreme ultraviolet range by monitoring final-state populations.
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