Ionization Energies and Ground-State Structures of Neutral La_n (n = 2–14) Clusters: A Combined Experimental and Theoretical Investigation

Abstract

Neutral lanthanum clusters are studied by photoionization time-of-flight mass spectroscopy, laser threshold photoionization spectroscopy, and density functional theory (DFT). Mass abundance spectra (MS) registered at multiple photoionization wavelengths in the range of 195–230 nm by single photon ionization reveal the production of all sizes, La n (n ≥ 50), in good abundance, nullifying previously predicted low abundances for certain sizes in the 3–14 size range. Also, the MS do not reveal the extraordinary stability of any specific size, as one would expect, from previous theoretical predictions of 7- and 13-atom clusters as magic. Ionization energies (IEs) are measured for La n (n = 2–14) clusters. DFT has been used to determine the stable geometric isomers for 2- to 10-atom clusters and to calculate their IEs. The theoretical IEs of 2–7 atom clusters are in decent agreement with their experimental values; however, the theoretical IEs are somewhat lower by ∼0.4 eV for n ≥ 8 than their experimental IEs.

Threshold Photoionization and Density Functional Theory Investigations of Small Lanthanum Monoxide Clusters, LanO (n = 2–10)

“…No dramatic increase of abundance corresponding to any lanthanum monoxide clusters or odd-even alterations in the abundance, with respect to the number of metal atoms, was observed in the mass abundance spectrum. Similar observations were made for pure lanthanum clusters in a previous study . It was possible to produce all lanthanum monoxides along with the bare La clusters in a size range of 2 to 50 atoms, as was discussed in our previous paper .…”

“…Similar observations were made for pure lanthanum clusters in a previous study . It was possible to produce all lanthanum monoxides along with the bare La clusters in a size range of 2 to 50 atoms, as was discussed in our previous paper . The PI spectra of La 2 O and La 8 O are presented in Figure , and the PI spectra for other sizes are provided in the Supporting Information (Figures S2–S6).…”

“…The IE of La n O clusters as a function of cluster size is presented in Figure . For comparison, previously published ionization onset values for the La n and Y n /Y n O clusters are also shown. The trend of variation of IE with cluster size for the La n O clusters is different from the bare La n clusters.…”

“…Solid squares and triangles represent experimental values for the La n O and Y n O clusters, while open squares and triangles represent the same for the bare metal La n and Y n clusters, respectively. The data for La n , Y n O, and Y n were obtained from previously published papers and are presented here for comparison. , …”

“…The laser vaporization/reaction metal cluster setup that was built recently in our laboratory was used in this study to produce small lanthanum oxide clusters in the gas phase and to obtain a laser threshold PI mass spectrum. The experimental setup was discussed in detail in our previous paper . Lanthanum foil (Alpha Aesar) target of length × breadth × thickness = 25 × 7 × 1 mm, purity 99.9%, was laser vaporized by a Q-switched Nd:YAG laser (Litron Lasers, Nano-SG 120-20) at the second harmonic wavelength of 532 nm with a pulse energy of ∼12 mJ and a pulse duration of 7 ns, focused to a ∼1 mm diameter spot on the target.…”