Femtosecond x-ray fluorescence

This paper reports the observation of K-shell vacancies in light elements via x-ray fluorescence using few-cycle-driven laser harmonics as the excitation source. The peak of the fs x-ray fluorescence spectrum can already be determined, in spite of the extremely low fluorescence yield of low-atomic-number atoms, with a precision of the order of 1 eV with a present-day sub-10-fs high harmonic source. The results indicate that x-ray probing of molecular dynamics with a few is resolution may become feasible up to photon energies in the water window via time-resolved x-ray fluorescence, x-ray absorption, or x-ray photoelectron spectroscopy.