Helium atoms generate X-rays nearly twice the expected energy limit

Helium atoms generate X-rays nearly twice the expected energy limit

Helium atoms produced soft X-rays far beyond the energy limit expected from the usual one-electron model of high-harmonic generation. The extra X-ray energy appears to come from two strongly correlated electrons returning to the helium atom together and releasing their energy in a single photon. The weak secondary signal could give scientists a new way to track how electrons interact over extraordinarily short femtosecond and attosecond timescal…