A measurement of the 1S-2S transition frequency in He+ would enable fundamental physics tests, but the required extreme ultraviolet radiation makes this a challenge. The authors observe such transition using radiation produced by high-harmonic generation of frequency comb pulses, in a manner that is compatible with future precision spectroscopy.
- Elmer L. Gründeman
- Vincent Barbé
- Kjeld S. E. Eikema
