Inter-atomic/molecular Decay in Doped Helium Nanodroplets Induced by Extreme-ultraviolet Radiation

Abstract

This thesis concerns with experimental investigations of inter-atomic/molecular decays in nanometer-sized van der Waals clusters of helium (He), or in He nanodroplets, doped with a few foreign atoms and molecules. Acetylene (C2H2) molecules which are embedded inside, and heavy alkali atoms, rubidium (Rb) and cesium (Cs), which attach to the surface of the host droplet, are chosen as dopants. The doped He droplet is perturbed by tunable extreme ultraviolet (EUV) synchrotron radiation, wherein the host is resonantly photoexcited to different droplet excitation bands derived from He* (1s np, n≥2) (>21 eV), as well as photoionized above the autoionization threshold (~23 eV). Insights into different dopant ionization processes, induced by both the excited and ionized host, are gained by measuring kinetic energies of the ejected electrons and product-ions in coincidence using a co-axial time of flight and velocity map imaging spectrometer.

In case of C2H2 doping, upon resonant photoexcitation to the first droplet excitation band (n=2) at 21.6 eV, C2H2 oligomers are found to be singly ionized via inter-atomic/molecular Coulombic decay (ICD), conventionally known as Penning ionization, from 1s2p ^{1}P, as well as from internally relaxed metastable 1s2s ^{3,1}S, He* states. The loosely-bound structures of embedded neutral C2H2 oligomers, and the formation dynamics of oligomer ions are revealed from the dopant-ion correlated Penning ionization electron spectroscopy (PIES). This establishes PIES as effective experimental technique for investigating the structures of dopant oligomers which can be easily created inside He nanodroplets. Moreover, upon photoexcitation to n = 4 droplet excitation band at 23.9 eV, Penning ionization channels leading to highly excited C2H2+ (C^{2}\Sigma_{g}^{+}, 23.33 eV$ and D^{2}\Sigma_{g}^{+}, 23.53 eV) due to relaxation of 1s4p ^{1}P He* state are identified. These newly discovered Penning channels compete with the known dominant dopant charge transfer (CT) ionization channel following autoionization of the host droplet. This observation opens up possibilities to probe higher lying excited states of various dopant-ions through Penning ionization following the photoexcitation to n = 4 droplet excitation band, which are not accessible upon photoexcitation to n = 2 droplet excitation band, in similar system.

In case of Rb and Cs dopants, both single and double ionization are observed in the dopants following photoexcitation to the first droplet excitation band and direct photoionization of the host. For double ionization of the alkali dimers, attached to the surface of the host cluster, novel inter-atomic/molecular decays through intermediate inner valence ionization of the alkali atoms are reported. The inner valence p-shell of one of the alkali dimer constituents is ionized through resonant ICD from He* (1s2s ^{3,1}S, and 1s2p ^{1}P), and via CT ionization from He_{2}^{+}, following excitation and ionization of the host by the incident photon, respectively. This leads to an excited alkali dimer cation which decays via a subsequent ICD resulting in a doubly ionized dimer, that eventually undergoes Coulomb explosion. These two-step double ionization processes, cascaded-ICD and CT followed by ICD, compete with the known one-step ionization mechanisms, double-ICD and electron transfer mediated decay (ETMD) leading to the same product-ions, respectively. Thus, in this thesis, inter-atomic/molecular ionization processes in two different types dopants following photoexcitation and ionization of the host He nanodroplets are identified, wherein novel single and double ionization channels are presented, while PIES is demonstrated to be a useful tool to probe the structure of embedded dopant oligomer in the host cluster.