Maximiliano Rossa

Internal state populations and velocity distributions of monatomic species ejected after the 1064 nm laser irradiation of barium

The plumes accompanying 1064 nm nanosecond pulsed laser ablation of barium in vacuum at three moderate incident laser fluences in the range of 5.3–10.8 J/cm2 have been studied using both wavelength and time resolved optical emission spectroscopy and time-of-flight laser-induced fluorescence. Neutral atoms and both singly and doubly charged monatomic cations in excited states up to near the corresponding ionization limits are identified in the optical emission spectra. The population distributions of low-lying (≤1.41 eV) “dark” states of Ba atoms measured by laser-induced fluorescence revel that the metastable D3J and D12 abundances in the plume are higher than predictions based on assuming a Boltzmann distribution. The D3J and D12 populations are seen, respectively, to decrease slightly and nearly no vary with raising fluence, which contrasts with the increasing trend that is observed in the ground-state Ba(S10) population. At all fluences, the time-of-flight distributions of the whole dark states and of ...

Velocity distributions of Ba (S01, DJ3, D21, P13, and P11) and Ba+(P3∕22) produced by 1064nm pulsed laser ablation of barium in vacuum

Various electronic states of Ba, from ground state up to 2.24eV (S01, DJ3, D21, P13, and P11) together with Ba+(P3∕22), were produced by 1064nm high-irradiance pulsed nanosecond laser ablation of Ba in vacuum. The velocity distribution for every species was obtained from time-of-flight measurements, using pulsed laser induced fluorescence or time-resolved optical emission spectroscopy, as applicable to each species. The distributions are bimodal, Maxwell-Boltzmann functions for S01, DJ3, and D21 and shifted Maxwell-Boltzmann for the rest of the states, with different peak velocities and average, hyperthermal translational temperatures. Possible mechanisms for the production of these velocity distributions are discussed.

Hydration of barium monohydroxide in (H2O)(1-3) clusters: theory and experiment.

The ionization energies (IEes) of small BaOH(H2O)m clusters (m = 1-3), as generated in a laser vaporization-supersonic expansion source have been determined by laser photoionization experiments over the 3.65-4.55 eV energy range. Complementary ab initio studies show that the IEes are in good agreement with computed adiabatic ionization energies and that BaOH(H2O)m structures with a direct coordination of the Ba atom to water molecules are favored over those that are characterized by H-bonded networks involving H2O molecules and the OH group of BaOH. Additional calculations have been performed on the hydration energies for the most stable isomers of the relevant BaOH(H2O)1-3 clusters. A comparison is made between the closed-shell title system and the results of related theoretical studies on the open-shell alkali monohydroxides, which allows for an interpretation of the opposite trends that are found in the cluster size dependence of the vertical ionization energies for both series of systems, and highlights the role of the BaOH unpaired electron in its ionization process. Altogether, the present evidence suggests for the initial steps of the BaOH hydration process to be dominated by electrostatic and polarization interactions between the Ba(+) and OH(-) ion cores, which become both increasingly solvated upon sequential addition of water molecules.

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

The adiabatic ionization potential of the BaOH radical, as generated in a laser vaporization-supersonic expansion source has been determined by laser photoionization experiments to be (4.55 ± 0.03) eV. This value supports the three lowest out of seven previous experimental estimates, the former ranging from 4.35 to 4.62 eV. The present result is compared to 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. The most satisfactory agreement is obtained for either the adiabatic or vertical ionization potentials that derive from post-Hartree-Fock [MP2 and CCSD(T)] treatments of electron correlation, along with consideration of relativistic effects and extensive basis sets for Ba, in both BaOH and BaOH(+). Such conclusions extend to the results of related calculations on the Ba-OH dissociation energies of BaOH and BaOH(+), which were performed to help in calibrating the present computational study. Bonding in BaOH/BaOH(+), as well as possible sources of discrepancy with previous experimental determinations of the BaOH adiabatic ionization potential are discussed.

Chemiluminescent reaction of Ba(P3) with N2O at hyperthermal collision energies: Rotational alignment of the BaO(AΣ+1) product

The chemiluminescent reaction Ba(6s6p (3)P)+N(2)O was studied at an average collision energy of 1.56 eV in a beam-gas arrangement. Ba((3)P) was produced by laser ablation of barium, which resulted in a broad collision energy distribution extending up to approximately 5.7 eV. A series of experiments was made to extract the Ba((3)P) contribution to chemiluminescence from that corresponding to Ba 6s(2) (1)S0 and 6s5d (3)D, which are the other two most populated states in the atomic beam. The fully dispersed polarized chemiluminescence spectra at 400-600 nm from the title reaction were recorded and assigned to a BaO molecule excited in the A (1)Sigma+ level. In addition, the average and wavelength-resolved degrees of polarization associated to the parallel BaO(A (1)Sigma+-->X (1)Sigma+) emission are reported. The analysis of the average polarization degree show that the BaO(A (1)Sigma+) product is significantly aligned, suggesting that the reaction mechanism is predominantly direct. The product rotational alignment was found to depend markedly on the emission wavelength, which revealed a negative correlation with the BaO(A (1)Sigma+) product vibrational state. On the basis of experimental and theoretical investigations on the reactions of N(2)O with both the (1)S0, (3)D, and (1)P1 states of Ba and the lighter group 2 atoms, it is suggested that the Ba((3)P) reaction involves a charge transfer at relatively short reagent separations and that restricted collision geometries at the highest velocity components of the broad distribution are necessary to rationalize the data.

Solvation of barium atoms and singly charged cations in acetonitrile clusters

The size distributions of neutral and cationic Ba x (CH3CN) n (x = 0, +1; n ≤ 7) clusters, as produced by a standard laser vaporization-supersonic expansion pick-up source, were determined from molecular beam experiments. The size distribution for cations is in the range of n = 1-7, whereas only the n = 1 complex is observed for neutral clusters, and these two features are unaffected by the variables controlling the performance of the cluster source. The distinct behavior is compatible with the expected charge-dipole interactions in the ionic species, which are stronger than the dipole induced-dipole interactions at play in neutral clusters, and it is corroborated by the relative magnitude of the theoretical successive binding energies (SBEs) for the lowest-lying isomers of cationic and neutral clusters with n = 1-5, as computed at the density functional theory level. The theoretical results also allow for the rationalization of the bimodal Ba+(CH3CN)1-7 size distribution, featuring an apparent minimum at n = 3, in terms of chiefly 6s-5d σ hybridization of the Ba+ ions, which ultimately leads to a relatively small third SBE for the Ba+(CH3CN)3 complex, as compared to those for n = 1, 2, and 4. Additional Born-Oppenheimer molecular dynamics simulations on the Ba+(CH3CN)2-4 clusters suggest that all of the ligands are coordinated to the Ba+ ion and prevent considering completion of the first solvent shell as responsible for the bimodal size distribution.