XUV frequency comb spectroscopy

We demonstrate high resolution frequency comb spectroscopy in the extreme ultraviolet for the first time by exciting helium from the ground state using 51.56 nm pulses. Precision spectroscopy in the extreme ultraviolet on helium atoms and helium
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ions is very interesting as it could provide more stringent tests of quantum electrodynamics than currently possible with hydrogen. One problem is that spectroscopy in the extreme ultraviolet typically requires amplification and harmonic upconversion of visible light sources, which can introduce difficult to control systematic errors as a result of so called frequency -chirping effects.