Stability of two-component alkali clusters formed on helium nanodroplets

Metal clusters have proven to be particularly well suited test objects for studying the transition from molecular quantum dynamics to solid state physics [1,2,3,4]. Clusters formed of monovalent alkali metals have been studied in great detail both experimentally as well as theoretically [1,2,3,5,6,7]. In these simple metal clusters the electronic structure is dominated by the number of valence electrons whereas the ionic cores are of secondary importance. The electrons are delocalized, and the electronic system exhibits a shell structure that is closely related to the well-known nuclear shell structure. The simple model of the free-electron gas inside a spherical potential well of the dimension of the cluster (Jellium model) applies particularly well to alkali clusters [2,3,6,7].

Important information on the electronic structure of metal clusters has been gained from simple cluster abundance spectra, reflecting the stability with respect to fragmentation: Clusters in which the number of valence electrons matches the spherical shell-closing numbers are produced more abundantly. In addition, odd-even alternations reflect the enhanced stability of paired electron configurations [8,2]. These shell effects have also been observed with metal clusters formed in helium nanodroplets [9].

Besides the strongly bound clusters (covalent or metallic) which are usually observed in experiments, alkalis can form van der Waals-type molecules and clusters in which all electrons are spin-oriented and strongly localized [5,10]. Although bonding in these high-spin clusters was predicted to be quite strong for lithium clusters (‘ferromagnetic bonding’), sodium clusters were theoretically found to be much more loosely bound than their metallic counterparts [5], and the same behavior may be expected for the heavier alkali species.

While most atomic species reside inside the helium droplets due to the attractive interaction with the helium surroundings, alkali atoms and molecules are weakly bound to the droplet surface in bubble states. This weak binding energy of the order of 10 K leads to the fact that out of all clusters formed on the droplet surface preferentially the weakly bound ones remain attached to the droplets [11]. This leads to an enrichment of weakly bound high-spin diatomic and triatomic molecules of up to a factor 10 4 [12,13].

The formation of high-spin alkali clusters using the helium nanodroplet technique has been reported by our group [10]. In this previous experiment we observed characteristic differences in the abundance spectra of light and heavy alkali clusters. While alkali clusters of sizes up to 25 atoms were seen in the case of sodium (Na) and potassium (K), cluster sizes exceeding 5 and 3 atoms are strongly suppressed in the case of rubidium (Rb) and cesium (Cs), respectively. This behavior has been interpreted in terms of the reduced stability of high-spin states of Rb and Cs clusters with respect to depolarization, leaving behind hot, unpolarized clusters that may fragment and escape out of the detection volume. Depolarization may be induced by the strong second-order spin-orbit interaction present in the heavy alkali atoms, causing spontaneous spin flipping into the unpolarized state. Alternatively, the localized, spin-oriented electrons may evolve into a delocalized collective state as the cluster size grows larger, which eventually gives rise to spin flipping [10].

Based on these findings the question arises how stable mixed clusters of light and heavy alkali atoms in highspin states are. In other words, how does the binding of one or several heavy alkali atoms (Rb or Cs) to a cluster consisting of light alkalis (Na or K) affect the stability of the compound cluster? In order to shed some light on this issue we report on a series of measurements of abundance spectra of mixed alkali clusters formed in high-spin states on the surface of helium nanodroplets using femtosecond (fs) photo ionization (PI) in combination with mass-selective ion detection.

Besides numerous techniques for producing beams of metal clusters [1], the aggregation of metal atoms inside helium nanodroplets has been established as an alternative route to forming metal clusters of well defined composition [4]. Using this technique, the metal clusters are formed in the ultracold environment of the helium droplets at temperatures in the millikelvin range. Moreover, helium nanodroplets can be efficiently loaded with a variety of different atomic or molecular species.

In the experiment reported here, a beam of helium nanodroplets is consecutively doped with two different species of alkali atoms in two separate pickup cells to form two-component clusters on the droplets. The growth statistics for alkali clusters is found to deviate from the Poissonian distribution [14]. Further downstream the mixed clusters are photo ionized by fs laser pulses from a modelocked Ti:Sa laser. Nonresonant fs PI at high pulse repetition rate (80 MHz) i

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