Chemiluminescence from the Ba(3P) +N2O-- BaO(A1S+) +N2 reaction: Collision energy effects on the product rotational aligment and energy release

M. ROSSA, C.A. RINALDI , J.C. FERRERO

JOURNAL OF CHEMICAL PHYSICS

AMER INST PHYSICS

Año: 2010 p. 340304 - 340310

0021-9606

Both fully dispersed unpolarized and polarized chemiluminescence spectra from theBa
3P+N2O reaction have been recorded under hyperthermal laser-ablated atomicbeam-Maxwellian gas conditions at three specific average collision energies Ec in the range of4.82–7.47 eV. A comprehensive analysis of the whole data series suggests that the A 1
+→X 1
+band system dominates the chemiluminescence. The polarization results revealed that theBaO
A 1
+ product rotational alignment is insensitive to its vibrational state 
at Ec=4.82 eVbut develops into an strong negative correlation between product rotational alignment and 
at 7.47eV. The results are interpreted in terms of a direct mechanism involving a short-range, partialelectron transfer from Ba
3P to N2O which is constrained by the duration of the collision, so thatthe reaction has a larger probability to occur when the collision time is larger than the time neededfor N2O bending. The latter in turn determines that, at any given Ec, collinear reactiveintermediates are preferentially involved when the highest velocity components of thecorresponding collision energy distributions are sampled. Moreover, the data at 4.82 eV suggest thata potential barrier to reaction which favors charge transfer to bent N2O at chiefly coplanargeometries is operative for most of the reactive trajectories that sample the lowest velocitycomponents. Such a barrier would arise from the relevant ionic-covalent curve crossings occurringin the repulsive region of the covalent potential Ba
3P¯N2O
1
+; from this crossing theBaO
A 1
+ product may be reached through mixings in the exit channel with potential energysurfaces leading most likely to the spin-allowed b 3 and a 3
+ products. The variation withincreasing Ec of both the magnitude of the average BaO
A 1
+ rotational alignment and theBaO
A 1
+ rovibrational excitation, as obtained from spectral simulations of the unpolarizedchemiluminescence spectra, consistently points to additional dynamic factors, most likely thedevelopment of induced repulsive energy release as the major responsible for the angularmomentum and energy disposal at the two higher Ec studied. The results of a simplified versionof the direct interaction with product repulsion-distributed as in photodissociation model do notagree with the observed average product rotational alignments, showing that a more realisticpotential energy surface model will be necessary to explain the present results. © 2010 American