Photoionization and ab initio study of Ba(H2O)n (n = 1–4) clusters

Abstract

An experimental and theoretical study of the photoionization energies (IE's) of Ba(H(2)O)(n) clusters containing up to n = 4 water molecules has been performed. The clusters were generated by a pick-up source combining laser vaporization with pulsed supersonic expansion, and then photoionized by radiation of 272.5-340 nm. The experimentally determined IE(e)'s for n = 1 to 4 are 4.56 ± 0.05, 4.26 ± 0.05, 3.90 ± 0.05 and 3.71 ± 0.05 eV. This cluster size dependence of IE is reproduced within ±0.06 eV employing the mPW1PW91 density-functional and CCSD(T, Full) quantum-chemical methods combined with the 6-311++G(d,p) basis set for the H and O atoms and three different relativistic effective core potentials for Ba atoms. The calculations indicate that the lowest energy hydration structures represent the most relevant contributions to both the vertical and adiabatic ionization energies. Experimental and theoretical evidence correlates with the progressive surface-delocalization of the electron from the hydration cavity around the Ba atom and suggests that the intra-cluster electron transfer is possible even for small aggregates.

Solvation of barium atoms and singly charged cations in acetonitrile clusters

“…Ba x (CH 3 CN) n clusters (x = 0 for neutrals and x = +1 for cations) were generated in a pick-up source combining 1064 nm laser vaporization of a rotating Ba disk, 3 with a supersonic expansion of a gaseous He:CH 3 CN (0.944:0.056) mixture through a pulsed solenoid valve (200 µm diameter) at a stagnation pressure of 2 bars. The cluster beam was collimated with a 1.0 mm slit skimmer placed 10 cm downstream from the nozzle before entering the extraction/acceleration region of a Wiley-McLaren time-of-flight mass spectrometer (TOF-MS) (80 cm flight length).…”

“…The cluster beam was collimated with a 1.0 mm slit skimmer placed 10 cm downstream from the nozzle before entering the extraction/acceleration region of a Wiley-McLaren time-of-flight mass spectrometer (TOF-MS) (80 cm flight length). 3 The nascent neutral clusters were ionized in the extraction region by the second, third, or fourth harmonics (532, 355, or 266 nm, respectively) of a pulsed Nd:YAG laser, and the resulting ions were extracted by applying a continuous high voltage to the extraction plates. 3 Additional experiments were performed in a wider range of ionization laser fluences to get estimates on the laser power dependence of the various species observed in the mass spectra.…”

“…3 The nascent neutral clusters were ionized in the extraction region by the second, third, or fourth harmonics (532, 355, or 266 nm, respectively) of a pulsed Nd:YAG laser, and the resulting ions were extracted by applying a continuous high voltage to the extraction plates. 3 Additional experiments were performed in a wider range of ionization laser fluences to get estimates on the laser power dependence of the various species observed in the mass spectra. The detection of the nascent cationic clusters was instead carried out in the absence of any ionization laser.…”

“…The DFT approach was chosen on the basis of a previous study from this laboratory addressing to the photoionization of Ba(H 2 O) n clusters, 3 where in addition to mPW1PW91, three hybrid functionals had been evaluated, i.e., B3LYP, M06-2X, and the long range-corrected version of the functional wPBE, i.e, LC-wPBE, as defined in the Gaussian 09 package. As well, for Ba atoms, a number of pseudo-potentials (and their corresponding valence basis sets) were tried, such as SDD, CEP-121G, and LanL2DZ, as defined in the Gaussian 09 software and the RECP's developed by Lim et al, 22 and by Bauschlicher et al 23 25) added support for this theoretical approach to suitably describe systems, which are characterized by covalent and non-covalent interactions, including van der Waals interactions.…”

“…2 For heavier group 2 metals like calcium and barium, the interactions with H 2 O molecules are instead predicted to favor interior clusters even for the smallest sizes attained (n ≤ 4). 2,3 The distinct behavior of Mg with respect to Ca and Ba is likely due to the larger polarizabilities of the latter 2 since the electron density distributions (associated with the M valence electron pair) are constrained to the cluster surface region and delocalized over the cluster in surface and interior structures, respectively. 4 Studies on the microsolvation of group 2 singly charged M + ions consistently suggest that their ground-state open-shell ns 1 configurations cause a strong interaction with molecules of the first solvent shell, leading to most stable structures where the cations are embedded in interior M + (solvent) n clusters.…”