Superfluid He nanodroplets are ideal model systems for studying the photodynamics of weakly-bound nanostructures, both experimentally and theoretically; in most cases, superfluidity results in slow relaxation of energy and angular momentum. Using ultrashort tunable XUV pulses, it is now possible to follow the relaxation dynamics of excited helium nanodroplets in great detail.
The relaxation of photoexcited nanosystems is a fundamental process of light-matter interaction. Depending on the couplings of the internal degrees of freedom, relaxation can be ultrafast, converting electronic energy into atomic motion within a few fs, or slow, if the energy is trapped in a metastable state that decouples from its environment. An international research team from Germany, Spain, Italy, the USA, and the local team at the FERMI free-electron laser (FEL), studied helium nanodroplets resonantly excited by femtosecond extreme-ultraviolet (XUV) pulses from FERMI. The researchers found that, despite their superfluid nature, helium nanodroplets in their lower electronically excited states undergo ultrafast relaxation by forming a void bubble, which eventually bursts at the droplet surface thereby ejecting a single metastable helium atom. These results help understanding how nanoparticles interact with energetic radiation, as happens when single nanoparticles are directly imaged at hard-x-ray FEL facilities.
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Image: Figure left: Simulated density distribution of a helium nanodroplet shorty after it is excited by an XUV laser pulse (Courtesy by M. Barranco). Figure right: Measured photoelectron spectra showing ultrafast energy relaxation within less than a picosecond.