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.
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.
The temperature dependence of the state-to-state vibrational relaxation rate constant (k(nu)(21-Delta nu)) for collisions between I(2)(B,nu(')=21) and He at very low kinetic energies was studied. The fluorescence from I(2)(B,nu(')=21-Delta nu(')) with Delta nu(')=1-5 indicates that in the temperature range of 0.6-8.2 K these states are populated by only one collision with He. The behavior of k(nu)(21-Delta nu) with temperature can be divided into two groups. The group with quantum changes Delta nu(')=1-3 shows scattering resonances in the low temperature region, with a general monotonical decrease of the rate constant with temperature, suggesting the importance of van der Waals interactions. This behavior is supported by the calculation of the probability of tunneling through the centrifugal barriers. For collisions in which 4-5 quanta are lost in a single event, there are no evidences of scattering resonances and the values of the relaxation rate constants could be determined only at the highest temperatures of this study. This suggests that relaxation occurs via impulsive collisions. The branching ratios for each channel are also temperature dependent and this behavior also suggests that the energy transfer mechanism changes with Delta nu(').
A systematic experimental study of the laser ablation of three alkaline-earth metals (Ca, Mg, and Ba) was carried out to understand the ablation processes of the metals. In this work the infrared laser ablation of alkaline-earth metals was studied by mass spectrometry together with the optical emission spectroscopy of the species generated in the ablation process. The analysis and modeling of the results were done using a heuristic equation that includes the Arrhenius, the screening, and the lineal regions that describe the three different regimes usually observed as a function of the laser fluence. We intend to prove that this equation depends on a fit surface parameter that includes the roughness surface, and it allows us to determine a critical fluence that can be related to the physical properties of the metal target.
The reaction of CF, with NO, was studied at 296 2 2 K using two different absolute techniques Absolute rate constants of ( I 6 ? 0.3) X lo-" and ( 2 1-03+07) X lo-" cm3 molecule-' s-' were derived by IR fluorescence and UV absorption spectroscopy, respectivelyThe reaction proceeds via two reaction channels: CF, + NO, -CF,O + FNO. (70 t 12)% and CF, + NO, + CF,O + NO, (30 ? 12)%. An upper limit of 1 1 % for formation of other reaction products was determined. The overall rate constant was within the uncertainty independent of total pressure between 0.4 to 760 torr. Q
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