Relaxation dynamics of 3^3He and 4^4He clusters and droplets studied using near infrared and visible fluorescence excitation spectroscopy

The relaxation dynamics of electronically excited 3He and 4He clusters and droplets is investigated using time-correlated near-infrared and visible (NIR/VIS) fluorescence excitation spectroscopy. A rich data set spanning a wide range of cluster and droplet sizes is produced. The spectral features broadly follow the vacuum ultraviolet excitation (VUV) spectra. However, when the NIR/VIS spectra are normalised to the VUV fluorescence, regions with distinctly different cluster size and isotope dependence are identified, enabling deeper insight into the relaxation mechanism. Particle density, location of atomic-like states and their principal quantum number, n, are found to play an important role in the relaxation. For states with n = 3 and higher, only energy within the surface region is transferred to excited atoms which are subsequently ejected from the surface and fluoresce in vacuum. For states with n = 2, energy from the entire region within clusters and droplets is transferred to the surface, leading to the ejection of excited atoms and excimers. Here, the energy is transferred by excitation hopping, which competes with radiative and non-radiative decay, making ejection and NIR/VIS fluorescence inefficient in increasingly larger droplets

Observation of Atomiclike Electronic Excitations in Pure 3He and 4He Clusters Studied by Fluorescence Excitation Spectroscopy.

The structure of the electronically excited states of 3He and 4He clusters is investigated using fluorescence excitation spectroscopy. Distinct bands are observed energetically close to atomic 1s-ns, nd, np transitions and attributed to perturbed excited He atomiclike states with different principle and orbital quantum numbers. The line shifts and widths of the bands of 3He and 4He clusters of the same size are different and correlate with the average particle density inside the clusters calculated using the density functional method

Bubble formation and decay in 3He and 4He clusters.

The energy transfer in 3He and 4He clusters electronically excited by monochromatic synchrotron radiation is investigated by luminescence spectroscopy. Depending on the cluster size and the isotopic constitution, either sharp, broadened, or shifted emission bands of single He molecules are observed. The spectral features show that He molecules emit light either within a bubble inside the cluster or in the vacuum after desorption from the cluster. From the luminescence intensity, the cluster diameter, and the radiative lifetime, an average velocity of ~7 m/s of bubbles in 4He clusters could be deduced. In the nonsuperfluid 3He clusters this velocity was observed to be significantly lower

Character of tightly bound excitons in small argon clusters: Insights from size-dependent energy shifts.

The structure of the first electronically excited states of small Ar M clusters (M <=100) embedded in large NeN (N=57500) clusters is investigated using fluorescence excitation spectroscopy. In the energy range of the characteristic absorption of Ar clusters (11.5–12.9 eV), surface excitons of Ar clusters embedded in Ne disappear, while additional absorption bands appear. They are assigned to excitons at the interface between the Ar cluster and the Ne host cluster. The observed energy shift of all absorption bands is proportional to the logarithm of the cluster radius. This can be understood in the Frenkel-exciton model, taking the resonant excitation transfer into account

Evolution of the charge localization process in xenon cluster ions: From tetramer to dimer cores as a function of cluster size.

The charge localization process in Xe+N cluster ions (N=40– 20 000) is investigated with fluorescence spectroscopy methods. New discrete and continuous luminescence bands in the visible and near infrared spectral range are observed and are assigned to radiative transitions of ionic dimers, trimers and tetramers inside Xe clusters. The bands are related to the 5p5 2P1/2 →5p5 2P3/2 transition of electronically excited atomic Xe ions and the 6p→6s transitions of electronically excited Xe neutrals. The dependence of the size of the ionic centers on the Xe cluster size is discussed. In large clusters discrete lines are due to embedded dimer emission and they are identified as 2(1/2)u→1(3/2)g transitions between different vibrational levels. Line positions are blue-shifted by 30 (→±1) meV with respect to free molecular dimer ions. The energy shift is due to the interaction of the surrounding neutral Xe cluster atoms with the embedded ionic Xe molecules

Size and Isotope Effects of Helium Clusters and Droplets: Identification of Surface and Bulk-Volume Excitations

We report a comprehensive investigation of the electronically excited states of helium clusters and droplets of sizes ranging from a few to several 10(7) atoms using time-resolved fluorescence excitation spectroscopy and quantum chemical ab initio calculations. We employ various approaches for our analysis considering the lifetime-dependence of the fluorescence intensity, spectral shifts, intensity scaling with cluster size, isotopic dependence, and density-dependence of the calculated electron wave function radii. A unique feature of helium clusters and droplets is their radially varying particle density. Our results show that short-lived fluorescence is sensitive to regions of increased density and probes excitations located in the bulk volume, whereas long-lived fluorescence is sensitive to regions of reduced density such as for small clusters or for the surface of large droplets. Spectra of (3)He droplets serve as a reference for low density, but are free from contributions of small clusters. This allows us to distinguish regions of reduced density as these can be due to both surface states or small clusters. Our analysis reveals a picture where spectral features are related to regions of different density due to isotopic composition, cluster size, and surface or bulk volume location of the excitations. The 2s and 2p related excitations appear as blue-shifted wings for small clusters or for excited atoms within the surface layer, whereas in the bulk-volume of large droplets, they appear as distinct bands with large intensities, dominating the entire spectrum. Excitations at energies higher than 23 eV are unambiguously assigned to regions of low and medium density location within the deeper parts of the surface layer but show no relation to the bulk volume. Our findings support the idea that in liquid helium high-lying states and, in particular, Rydberg states are quenched in favor of the 2s and 2p excitations