Photodissociation kinetics of aluminum cluster ions: Determination of cluster dissociation energies

Abstract

The photodissociation of aluminum clusters, Al+n (n=7–17), has been studied over a broad energy range (1.88–6.99 eV). Measurements of the lifetimes of the photoexcited clusters are described. Dissociation energies have been determined by comparing the measured lifetimes with the predictions of a simple RRKM model. The dissociation energies show an overall increase with cluster size, but there are substantial oscillations around n=7–8 and n=13–15. Cluster cohesive energies are derived from these results and from previous measurements of the dissociation energies of the smaller clusters. The cohesive energies of the larger clusters (n>6) are in good agreement with the predictions of a simple model based on the bulk cohesive energy and the cluster surface energy. However, the cohesive energies are substantially larger than the results of recent ab initio calculations. The photodissociation spectrum of Al+8 has been measured and shows a broad absorption feature with a maximum ∼470 nm.

Structures and stabilities of Aln+, Aln, and Aln− (n=13–34) clusters

“…We provide here a brief summary of the bibliography which is most relevant to the present work ͑for example, we do not mention many papers dealing with aluminum clusters with 2-12 atoms͒. Jarrold et al 11 performed collision-induced dissociation experiments on Al n + ͑n =3-26͒ and obtained enhanced cluster stabilities for Al 13 , Al 13 + , Al 14 + , and Al 23 + . Taylor et al 12 measured ultraviolet photoelectron spectra of Al n − with n =3-32 and found evidences of electron shell closings for sizes n = 13, 19, and 23.…”

“…Taylor et al 12 measured ultraviolet photoelectron spectra of Al n − with n =3-32 and found evidences of electron shell closings for sizes n = 13, 19, and 23. Both Ray et al 13 and King and Ross 14 concluded that Al 15 + has a very low stability and/or dissociation energy. Saito et al 15 reported mass spectra of bare Al n + cluster ions and identified enhanced abundances for n = 14, 17, 20, 23, and 28.…”

“…Jarrold and Bower 18 determined the mobilities of Al n + cluster ions, providing direct experimental information about the average shape of the clusters. Cha et al 19 measured photoelectron spectra of Al n − ͑n =1-15͒ clusters and found the spectrum of Al 13 − to be consistent with icosahedral symmetry. Li et al 20 employed photoelectron spectroscopy of Al n − with n up to 162 atoms to corroborate the electron shell closings at 13,19, and 23 atoms.…”

“…Cha et al 19 measured photoelectron spectra of Al n − ͑n =1-15͒ clusters and found the spectrum of Al 13 − to be consistent with icosahedral symmetry. Li et al 20 employed photoelectron spectroscopy of Al n − with n up to 162 atoms to corroborate the electron shell closings at 13,19, and 23 atoms. More recently, Neal et al 21 measured the heat capacity curves for Al n + cations with more than 16 atoms and found that the smallest cluster to show a well-defined melting transition is Al 28 + , suggesting that some well-defined structural order might emerge at that cluster size.…”

“…A controversy has arisen regarding the correct global minimum ͑GM͒ for Al 13 : the optimizations based on parametrized potential models 32,34,35,41,42,46,52 invariably predict an icosahedral structure, but these methods do not contain an explicit description of the electronic degrees of freedom and their accuracy is questionable. A majority of the ab initio calculations ͓mostly based on density functional theory ͑DFT͒ and differing in the election of exchangecorrelation functional, aluminum pseudopotential, and basis set͔ predict an icosahedron as the most stable structure, [22][23][24][25]27,28,30,33,37,39,45 but a small number of calculations 26,31,38,48,49 predict a decahedron to be more stable.…”