Chemical analysis: Double core-hole spectroscopy with free-electron lasers

Highlights: • We have explored intriguing non-linear effects such as double core-hole formation. • Multiphoton absorption is key to the observation of double core-holes in molecules. • We have carried out X-ray two-photon photoelectron spectroscopy. • Experimental work validated the theory of two-photon photoelectron spectroscopy. • This work demonstrates that our methodology will be very effective with future FELs. - Abstract: Free-electron lasers with their femtosecond pulse duration, high pulse energy and tunable photon energy in a regime from XUV to hard-X-ray have opened up several research avenues in physics, chemistry and biology. We have exploited the attributes of these new lasers by exploring intriguing non-linear effects such as double core-hole formation via multiphoton absorption. We have carried out X-ray two-photon photoelectron spectroscopy for molecular chemical analysis using the Linac Coherent Light Source. Our experimental work aimed to probe the chemical environment of specific atomic sites in several molecules and validate the theory of two-photon photoelectron spectroscopy. This work demonstrates that our methodology will be very effective with the future intense, high repetition rate as well as laser-seeded or self-seeded photon facilities.