Photoexcitation and photoionization dynamics of doped liquid helium-4 nanodroplets

The photoexcitation and photoionization dynamics of sodium atoms deposited on the surface of helium nanodroplets and aromatic molecules (aniline, phenol and toluene) embedded in the interior of droplets have been investigated by a variety of spectroscopic techniques. The mean droplet sizes varied in the range of ≈ 2 000 - 20 000 atoms. For the first time, the excitation spectra of Na-doped helium droplets corresponding to Rydberg states of Na atoms have been measured from the lowest excited 3p state up to the ionization threshold. All lines in the excitation spectra are shifted and broadened with respect to atomic lines. The desorption of bare excited Na atoms and NaHen, n=1-4, exciplexes was observed upon excitation. The experiments revealed that the relative abundance of desorbed species, their internal energy states, their speed and angular distributions are specific to the state of sodium atom to which it was excited to on the droplet's surface. In the lowest excited states (3p, 4s, 3d and 4p), we observed rather regular dynamics. The photoelectron spectroscopy of products revealed the desorption of excited sodium atoms in the initially excited state and in lower lying states which were populated by radiative decay of the higher ones. The velocity distributions showed interesting characteristics: the mean kinetic energy of desorbed sodium increased linearly with excitation frequency, while angular anisotropy varied monotonically. In contrast, the recorded velocity distributions of exciplexes did not manifest systematically such regular properties. It was found that the excitation spectra and relative abundances of exciplexes depended on the mean droplet size, while the velocity distributions of desorbed species were, in general, not dependent on size. The tentative explanation of the observed features is based on the approximation that "Na-helium nanodroplet" interaction potentials can be described by a sum of Na-He pair potentials over the helium atoms constituting the nanodroplet. The shapes and shifts of the transitions to low excited states of Na obtained within this model are in a good agreement with experimental observations. The velocity distributions of desorbed Na atoms can be qualitatively interpreted by the overall interaction potentials of sodium with a helium nanodroplet where they act as counterparts in a diatomic system. In contrast, the NaHen exciplex formation on the surface of nanodroplets appears to be mainly governed by the Na-He pair potentials. The excitation to higher than 4p states of Na on the droplet's surface revealed complex situations. The experimental spectra correlated poorly to those calculated within the pair-wise interaction model. The velocity distributions of desorbed sodium and exciplexes often consisted of two components compared to one in the lower states. The photoelectron spectra disclosed the presence of species in the states that could not be populated by radiative decay of the higher ones, indicatin