Ultrafast electronic dynamics in helium nanodroplets

Helium nanodroplets have emerged as a test bed for the study of isolated quantum liquids and as an ideal matrix for trapping atoms and molecules in a weakly interacting, cryogenic environment. Their high transparency at visible and infrared wavelengths facilitates the study of dissolved species with traditional spectroscopy techniques. At photon energies above ~21 eV, however, the droplets themselves begin to absorb to form complex excited states that have proven a challenge for both experiment and theory. A variety of frequency- and time-domain methods have been used to characterise electronically excited droplet states and their relaxation channels. This review focuses on a recent series of time-domain experimental studies that have revealed several phenomena such as interband relaxation dynamics within the droplet environment, and provided deeper insight into previously detected relaxation channels, including the ejection of Rydberg atoms (He*) and molecules (), the dynamics of highly excited droplet states, and photoassociation to produce strongly-bound excimer species (such as ). A brief outline of corresponding ab initio efforts for the theoretical description of electronically excited He droplet states and their relaxation dynamics will also be given.