No abstract

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.

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.

No abstract

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 / cm 2 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 ͑Յ1.41 eV͒ "dark" states of Ba atoms measured by laser-induced fluorescence revel that the metastable 3 D J and 1 D 2 abundances in the plume are higher than predictions based on assuming a Boltzmann distribution. The 3 D J and 1 D 2 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 Ba͑ 1 S 0 ͒ population. At all fluences, the time-of-flight distributions of the whole dark states and of various of the emitting levels are bimodal and well described by Maxwell-Boltzmann and shifted Maxwell-Boltzmann velocity functions, respectively, with different average translational temperatures ͗T͘ for each state. The ͗T͘ values for the dark states are insensitive to the fluence, while for all emitting species marked variations of ͗T͘ with fluence are found. These observations have been rationalized in terms of material ejection from the target being dominated by a phase explosion mechanism, which is the main contributor to the Ba͑ 1 S 0 ͒ population. Thermionic emission from the target surface can also release initial densities of free electrons and cations which, at the prevailing irradiances, will arguably interact with the incident laser radiation by inverse bremsstrahlung, leading to further excitation and ionization of the various plume species. Such a heating mechanism ensures that the energy injected to the plume will alter the propagation velocities of the primary inverse bremsstrahlung absorbers, i.e., cations, to a major extent than those of neutral atoms with increasing fluence. Electron-ion recombination occurring early in the plume expansion can lead to the generation of both neutral and ionic species in a manifold of long-lived Rydberg states, from which a radiative cascade will likely ensue. The distinct fluence dependences of the Ba͑ 3 D J ͒ and Ba͑ 1 D 2 ͒ populations and velocity distributions show up the major complexity that distinguishes their populating mechanisms with respect to the remaining species.