Unexpected size distribution of Ba(H2O)n clusters: why is the intensity of the Ba(H2O)1 cluster anomalously low?

Abstract

An experimental and theoretical study on the reactivity of neutral Ba atoms with water clusters has been conducted to unravel the origin of the irregular intensity pattern observed in one-photon ionization mass spectra of a Ba(H(2)O)(n)/BaOH(H(2)O)(n-1) (n = 1-4) cluster distribution, which was generated in a laser vaporization-supersonic expansion source. The most remarkable irregular feature is the finding for n = 1 of a lower intensity for the Ba(+)(H(2)O)(n) peak with respect to that of BaOH(+)(H(2)O)(n-1), which is opposite to the trend for n = 2-4. Rationalization of the data required consideration of a distinct behavior of ground-state and electronically excited state Ba atoms in inelastic and reactive Ba + (H(2)O)(n) encounters that can occur in the cluster source. Within this picture, the generation of Ba(H(2)O)(n) (n > 1) association products results from stabilizing collisions with atoms of the carrier gas, which are favored by intramolecular vibrational redistribution that operates on the corresponding collision intermediates prior to stabilization; the latter is unlikely to occur for Ba + (H(2)O) encounters. Overall, this interpretation is consistent with additional in-source laser excitation and quenching experiments, which aimed to explore qualitatively the effect of perturbing the Ba atom electronic state population distribution on the observed intensity pattern, as well as with the energetics of various possible reactions for the Ba + H(2)O system that derive from high level ab initio calculations.

New determination of the adiabatic ionization potential of the BaOH radical from laser photoionization-molecular beam experiments and ab initio calculations

“…The BaOH radicals were generated in a homemade pickup, laser-vaporization source. 23 Barium vaporization was accomplished by mildly focusing the 1064 nm output of a pulsed Nd:YAG laser [Big Sky Laser-Quantel CFR 400, 5.5 ns full width at half maximum (FWHM), (1.4 ± 0.7) J/cm 2 laser fluence] onto the surface of a rotating pure Ba disk, which was placed 4 mm off-axis and 2 mm downstream from the nozzle of a pulsed solenoid valve (General Valve series 9, 400 μm diameter). The laser-vaporized Ba species were perpendicularly entrained in a supersonic jet, as produced by expanding 2 bars of a He : H 2 O (0.988 : 0.012) gas mixture through the pulsed valve.…”

“…The rotational temperature of BaOH radicals in the molecular beam was estimated from separate laser-induced fluorescence (LIF) measurements on BaO molecules (IP a = 6.91 eV) (Ref. 21), which are also present in the supersonic expansion presumably 23 as products of the reaction between water and ground-state Ba atoms. BaO molecules were probed 2 cm downstream from the faceplate of the pulsed valve within the source vacuum chamber by the fundamental output of the above-mentioned dye laser operating on F-548.…”

“…photoionization experiments on the electronic ground-state BaOH radical, as generated in a laser vaporization-supersonic expansion source. 23 The use of a laser ionization source along with the rotational cooling of BaOH species attained in the supersonic expansion results in a lower uncertainty, by about one order of magnitude in IP a (BaOH) with respect to previous estimates of this quantity. The present and previous experimental values for IP a (BaOH) are compared to the results of ab initio calculations, as performed using both quantum chemistry at different levels of theory and density functional theory, and trying several effective core potentials (and their accompanying basis sets) for Ba.…”

Hydration of Barium Monohydroxide in (H2O)1–3 Clusters: Theory and Experiment

“…Recent results on the hydration and reactions of neutral barium atoms in water clusters from this laboratory, provide a case in point. 6 A series of experiments and ab initio calculations has been conducted to show that, as a result of distinct cluster-growing processes involving ground-state and electronically excited state Ba atoms, Ba(H 2 O) n (n = 14) and BaOH(H 2 O) m (m = 13) clusters are produced with similar abundances in a laser vaporization-supersonic expansion source. This was taken in the present study as an advantage point to investigate the stepwise hydration in water clusters of the BaOH radical.…”

“…[1][2][3][4] In particular, a considerable interest on group 2 of metals have aroused due to their divalent nature that allows only one intermediate charge state, M  , between the stable neutral metal, M, and the doublycharged M 2 ions, which are present in solutions. 3 Many aspects of the structure, reported, as a result of reactions that occur either during the cluster formation process 6,[8][9][10][11] or upon their subsequent photoionization, 5 photodissociation, [8][9][10][11] and collisioninduced dissociation. 12 For the former case, the investigation of the hydration of a metal atom/ion or a MOH/MOH  radical in (H 2 O) n clusters through cluster size-resolved experiments is feasible.…”