Experimental and theoretical study of neutral AlmCn and AlmCnHx clusters

Abstract

Neutral Al(m)C(n) and Al(m)C(n)H(x) clusters are investigated both experimentally and theoretically for the first time. Single photon ionization through 193, 118, and 46.9 nm lasers is used to detect neutral cluster distributions through time of flight mass spectrometry (TOFMS). Al(m)C(n) clusters are generated through laser ablation of a mixture of Al and C powders pressed into a disk. An oscillation of the vertical ionization energies (VIEs) of Al(m)C(n) clusters is observed in the experiments. The VIEs of Al(m)C(n) clusters change as a function of the numbers of Al and C atoms in the clusters. Al(m)C(n)H(x) clusters are generated through an Al ablation plasma-hydrocarbon reaction, an Al-C ablation plasma reacting with H(2) gas, or through cold Al(m)C(n) clusters reacting with H(2) gas in a fast flow reactor. The VIEs of Al(m)C(n)H(x) clusters are observed to vary as a function of the number of H atoms in the clusters. Density functional theory and ab initio calculations are carried out to explore the structures, ionization energies, and electronic structures of the Al(m)C(n) and Al(m)C(n)H(x) clusters. C=C bonds are favored for the lowest energy structures for Al(m)C(n) clusters. H atoms can be bonded to either Al or C atoms in forming Al(m)C(n)H(x) clusters, with little difference in energy. Electron density plots of the highest occupied molecular orbitals (HOMOs) for closed shell species and the singly occupied molecular orbitals (SOMOs) for open shell species of Al(m)C(n) and Al(m)C(n)H(x) clusters are presented and described to help understand the physical and chemical properties of the observed species. VIEs do not simply depend on open or closed shell valence electron configurations, but also depend on the electronic structure details of the clusters. The calculational results provide a good and consistent explanation for the experimental observations, and are in general agreement with them. All calculated clusters are found to have a number of low lying isomeric structures.