UC Berkeley

Upon formation, helium nanodroplets evaporatively cool to 0.37 K and thus are in a superfluid state. The ultracold droplets have a very low binding energy and are optically transparent. In contrast, when exposed to extreme ultraviolet (XUV) and x-ray radiation, a variety of complex relaxation and disintegration dynamics may ensue. This dissertation explores dynamics induced in droplets via multiphoton absorption in the XUV and x-ray regimes. In the XUV regime, helium droplets become electronically excited, with two broad absorption features originating from atomic helium states. The lower absorption feature at 21.6 eV originates from n=2 atomic helium states, while the upper feature centered at 23.7 eV arises from higher-lying atomic Rydberg states. After single photon absorption, a variety of relaxation mechanisms have been observed, such as ejection of Rydberg atoms, interband relaxation and Hen* formation. Multiphoton absorption leads to additional deexcitation pathways as a result of interactions between excited helium atoms. At higher photon energies, in the soft x-ray regime, individual atoms in the droplet are ionized via single photon ionization. With high intensity x-ray free electron laser (X-FEL) light sources, droplets can become highly ionized. The ionized droplet may disintegrate from the Coulomb repulsion between ions. Alternatively, if the droplet is sufficiently ionized, the freed electrons are trapped by the collective Coulomb potential of the parent ions, resulting in nanoplasma formation. The quasineutral nanoplasma can then disintegrate via hydrodynamic expansion. The simple electronic structure of atomic helium and uniform density of liquid helium make helium droplets an excellent system for studying complex energy transfer, relaxation, and charging dynamics common to condensed phase media.

Energy transfer and relaxation following multiphoton absorption into the lower, n=2 helium droplet absorption feature is studied by femtosecond time-resolved photoelectron spectroscopy in combination with XUV intensity-dependent ion yield measurements. With many photoexcited helium atoms in the droplet, resonant interatomic Coulombic decay (ICD) emerges as a possible deexcitation mechanism. In ICD, an excited atom relaxes by transferring its energy to a neighboring excited atom, resulting in ionization with freed electron carrying away the excess energy. This process is common in van der Waals clusters and condensed phase media, such as biological systems. Previous experiments have revealed that beyond ICD between two excited atoms, with high excitation densities, ICD can occur between many excited atoms leading to a host of inelastic processes as well. Here, measurements are performed in a lower XUV intensity regime than previous studies, such that ICD is limited to interactions between two excited atoms. A high-order harmonic pulse at 21.6 eV in the XUV, resonant with the lower droplet absorption feature, is used to electronically excite the droplet. Relaxation dynamics are then measured using a 3.1 eV UV probe pulse at various XUV-UV pump-probe delays. Ion yield measurements reveal a quadratic dependence on the XUV intensity in smaller droplets (~104 atoms/droplet) and a linear relationship in larger droplets (~106 atoms/droplet). The ICD lifetime is measured to be ~1 ps and found to be a competitive mechanism by which the droplet relaxes, even at low excitation densities.

The charging and disintegration dynamics of helium droplets exposed to intense (~1016 W/cm2), soft x-ray pulses at 838 eV photon energy are explored via single shot coincidence measurements of ion time of flight spectra and small angle x-ray scattering patterns. Experimental conditions encompass an extended range of ionization conditions in droplets, from the pure Coulomb explosion regime to the formation of nanoplasmas. Interpretation of these ionization dynamics is important for better understanding of a host of complex processes initiated by intense x-ray pulse light—matter interactions, both intentionally and as unavoidable byproducts of X-FEL based experiments. Ion time-of-flight spectra are used to determine the maximum ion kinetic energy resulting from the x-ray—droplet interaction, while scattering images encode the droplet size and absolute photon fluence. In correlating the droplet size, x-ray fluence, and maximum ion kinetic energy, a continuous relationship between the degree of ionization and ion kinetic energy is observed across the transition from weakly to strongly ionized droplets. Across all experimental conditions, results indicate that the maximum ion kinetic energy is governed by Coulomb repulsion from unscreened cations. Additionally, the results are consistent with the emergence of a spherical shell of unscreened ions around a quasineutral plasma core with the onset of frustrated ionization by electron trapping. The thickness of this shell is reduced to less than 2% of the droplet radius at the highest degrees of ionization frustration.