Photoexcitation and photoionization dynamics of doped liquid helium-4 nanodroplets

Abstract

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 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, indicating that relaxation plays an important role during the detachment. The experiments on the photoionization of Na-doped helium droplets revealed that their ionization threshold is red-shifted with respect to a free sodium atom. The time-of-flight mass spectra showed that the directly created Na+ ion was solvated in the droplet. The indirect evidence of creating special Na-droplets in high Rydberg states, when the ionic core remains inside the cluster and the weakly bound electron orbits outside, was obtained. The shape of ionization transition and the droplet size dependence of the ionization threshold are in line with the model of a pair-wise additive interaction of the created sodium ion with helium atoms in the nanocluster. The photoionization of aromatic molecules (aniline, phenol, and toluene) in helium droplets was studied with photoelectron spectroscopy. The photoelectron spectra resemble closely those of gas phase molecules except for the droplet size dependent shift. This shift is caused by the lowering of the ionization threshold upon solvation and can be readily estimated. The individual peaks in photoelectron spectra are broadened, which is thought to partially reflect the rearrangement of helium upon ion solvation. The droplet size and kinetic energy dependences of the peak broadening towards lower energy may be attributed to the relaxation of the photoelectrons as they pass through a helium droplet.