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Updated: Dec 14, 2025

Measurement of Ultrafast Vibrational Coherences in Polyatomic Radical Cations with Strong-Field Adiabatic Ionization
Published on: August 6, 2018
Direct Measurement of the Forbidden 2^{3}S_{1}→3^{3}S_{1} Atomic Transition in Helium
K F Thomas1, J A Ross1, B M Henson1
1Laser Physics Centre, Research School of Physics, The Australian National University, Canberra, ACT 2601, Australia.
Abstract:
We present the detection of the highly forbidden 2^{3}S_{1}→3^{3}S_{1} atomic transition in helium, the weakest transition observed in any neutral atom. Our measurements of the transition frequency, upper state lifetime, and transition strength agree well with published theoretical values and can lead to tests of both QED contributions and different QED frameworks. To measure such a weak transition, we develop two methods using ultracold metastable (2^{3}S_{1}) helium atoms: low background direct detection of excited then decayed atoms for sensitive measurement of the transition frequency and lifetime, and a pulsed atom laser heating measurement for determining the transition strength. These methods could possibly be applied to other atoms, providing new tools in the search for ultraweak transitions and precision metrology.
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