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

Abstract

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.

Chemiluminescence from the Ba(P3)+N2O→BaO(A Σ1+)+N2 reaction: Collision energy effects on the product rotational alignment and energy release

“…8 Briefly, a 5 Hz pulsed Ba atom beam was generated by focusing the 1064 nm output of a Q-switched neodymium:yttrium aluminum garnet laser ͑Laseroptics LND 532, 10 ns͒ onto a rotating Ba disk at normal incidence in high vacuum conditions. The ablation laser fluence was set to the three specific values of 5.3, 7.6, and 10.8 J / cm 2 with an approximately 0.25 mm diameter laser spot.…”

“…A detailed characterization of the resulting beam at the prevailing laser ablation conditions and in the absence of N 2 O scattering gas has been reported elsewhere. [8][9][10] The time-of-flight ͑TOF͒ technique through pulsed laser-induced fluorescence and time-resolved fluorescence detection was used to this goal. At 3.3 cm from the Ba disk surface where the chemiluminescence is probed, the barium beam is composed mainly of neutral atoms in the 6s 2 1 S 0 , 6s5d 3 D J , and 6s6p 3 P J states in an overall abundance Ͼ98% of that of the total Ba atoms.…”

“…Related time-scale limitations 2 have been considered in a recent study of the chemiluminescent Ba͑ 3 P͒ +N 2 O reaction under beam-gas conditions at hyperthermal collision energies from this laboratory, 8 to account for the observed negative correlation between the rotational alignment of the BaO͑A 1 ⌺ + ͒ product and its vibrational level. Such behavior has been rationalized by assuming that the reaction involves a short-range, partial electron transfer from Ba͑ 3 P͒ to N 2 O which is constrained by the duration of the collision, so that the reaction has a larger probability when the N 2 O has time enough to bend during the collision.…”

“…Advantage was taken here of the ability to modify significantly the Ba͑ 3 P͒ velocity distribution by changing the laser ablation conditions of the atomic beam source used previously. [8][9][10] Average values for both the initial relative velocity and collision energy that have been reported in Ref. 8, where pure thermal conditions were erroneously assumed are recalculated here by applying a different procedure, which is appropriate to the present hyperthermal atomic beam-Maxwellian gas experiments.…”

“…[8][9][10] Average values for both the initial relative velocity and collision energy that have been reported in Ref. 8, where pure thermal conditions were erroneously assumed are recalculated here by applying a different procedure, which is appropriate to the present hyperthermal atomic beam-Maxwellian gas experiments. The analysis of both unpolarized and polarized BaO chemiluminescence spectra and their average collision energy dependence supports the previous assignment of the emission to the A 1 ⌺ + → X 1 ⌺ + band system.…”