Multiple rainbow singularities in product rotational distributions of exothermic collisions : a probe for the P4 anisotropy of the system Ar(1S0) + N2(C3Πu)

The effect on product rotational distributions of the energy released during exothermic processes is investigated with the model of collisions with a rigid, anisotropic shell. With increasing exothermicity, rotational-rainbow maxima become apparent in model integral cross sections for rotational excitation, due to the increasing importance of the final-state repulsion with respect to that in the initial state. Such rainbow maxima are expected to occur in product distributions of excitation transfer, chemical reactions, and dissociation processes. Two different rotational-rainbow maxima can appear even for homonuclear product molecules when the interaction potential contains substantial anisotropy described by a fourth-order Legendre polynomial P4. This type of anisotropy can explain the bimodal rotational distribution of the product N2(C) of the exothermic excitation-transfer collision Ar*(3P0,2)+N2(X), with an energy release ΔE=701 and 521 meV for the 3P0 and 3P2 states, respectively, as measured by Nguyen and Sadeghi at a temperature of 90 K (=11 meV). The scaling of the rainbow positions with the exothermicity is reproduced. The best agreement with the experimental data is obtained with a ratio of anisotropy parameters c4/c2=1.8. Such anisotropy may be caused by the promotion of a 2σu electron to a 2πg orbital when going from the X to the C state of N2. .AE