Photofragmentation spectra and structures of Sr+Arn, n=2–8 clusters: Experiment and theory

Abstract

The total photofragmentation cross sections of size selected Sr ϩ Ar n ,nϭ2-8, clusters have been recorded in a time of flight ͑TOF͒ mass spectrometer. In the energy range of 21 000-27 000 cm Ϫ1 three absorption bands are observed that are attributed to transitions from the ground to the three excited states that correlate to the 5p orbitals of strontium. No vibrational structure in the spectral bands is observed, even in regions where the spectrum is recorded with a resolution of 1 cm Ϫ1. The absorption spectra are calculated within a semiclassical approximation. Accurate ab initio calculations for the ground X 2 ͚ and the A 2 ͟ and B 2 ͚ excited states of Sr ϩ Ar are combined with a perturbative one-electron model, which includes the spin-orbit coupling, to construct potential energy surfaces for the excited states of all clusters. The theoretical spectra obtained without using any adjustable parameters reproduce the patterns and trends of the experimental spectra, but they are shifted to higher frequencies. A better agreement between theoretical and experimental spectra is obtained by adjusting the depth of the potential wells of the ground X 2 ͚ and excited A 2 ͟ states of the Sr ϩ Ar dimer to the experimentally estimated values. From both calculations we conclude that Sr ϩ Ar 3 is a trigonal pyramid of C 3v symmetry and Sr ϩ Ar 6 has a C 5v symmetry with five argon atoms forming a regular pentagon, one argon atom below and the cation above the pentagon plane. For these clusters, theory reproduces the characteristic blue shifts found in the absorption spectra and the splittings of the doubly degenerate states encountered in these symmetric complexes.

Ionization of Methane Clusters in Helium Nanodroplets

“…Examples include Sr + Ar n and Na(H 2 O) n + clusters. [47,48] In particular, molecular dynamics calculations for Na(H 2 O) n + indicate that a delicate balance of factors may be at work in dictating the most prominent (magic) features in a mass spectrum. It is therefore by no means certain that the cluster charge should be restricted either to the central location or on the surface of the cluster.…”

Hydrated metal ions in the gas phase

“…Whereas this expectation is fulfilled for Li þ Ne 6 and Li þ Ar 6 (Froudakis, Farantos, & Velegrakis, 2000), there are examples which are in marked contrast to this simplified picture. Fanourgakis et al (1998) report a combined spectroscopic and theoretical investigation of Sr þ Ar n , n ¼ 2-8. Employing a classical molecular dynamics simulation with high-level ab initio derived pair additive potentials, they show that the argon atoms solvate the strontium ion asymmetrically, with the Sr þ sitting on top of a regular argon cluster as illustrated in Figure 1, clearly a situation of surface solvation.…”

Many-body effects on the structures and stability of Ba2+Xen (n = 1–39, 54) clusters

“…21 Beyond mass spectrometry, the photodynamics could be addressed in dedicated photofragmentation experiments, as in the case of Sr + Ar n (n ≤ 8). 15 Optical spectroscopy has been used by the Mestdagh group to a) E-mail: kawtherabdessalem@yahoo.fr address barium-coated clusters, 36 and by Lebedev et al 38 to study fluorescence relaxation of barium cations in condensed helium.…”