Energy dependence of rotational and vibrational distributions of diatomic nitrogen(C) for the argon(3P0,2) + diatomic nitrogen(X) excitation transfer reaction

The Ar* + N2(X) ¿ N2(C, v', N') + Ar excitation transfer reaction has been investigated experimentally in two different atomic beam experiments. The inelastic cross sections Qv' = 0(E) and Qv' = 1(E) to the v' vibrational level have been measured in the energy range 0.06 E(eV) 6, using a crossed beam machine. Both cross sections show a behaviour typical for a curve crossing mechanism, with maximum values Q0 = 8.0 Å2 and Q1 = 1.2 Å2 at E = 0.16 eV and E = 0.13 eV, respectively. The oscillatory behaviour of the ratio Q1(E)/Q0(E), as first observed by Cutshall and Muschlitz, is also present in our data. Within the model of Gislason et al. the results indicate a decreasing bond stretching with increasing energy. As an alternative we discuss the possibility that the oscillation is due to a different energy dependence of the cross sections for the Ar*(3P0) and Ar*(3P2) fine structure states in the mixed beam of metastable Ar*. The vibrational and rotational distributions have also been measured at E = 0.065 eV in a small scale atomic beam-scattering cell experiment, which can be considered as an intermediate between a bulk experiment and a crossed beam experiment. The relative vibrational populations are nv' = 100, 16.0, 3.03 and 0.31 for v' = 0 through 3, with rotational "temperatures" of Trot,v' = 1960, 1010, 370 and 130 K. Pronounced deviations ("hump") of the Boltzmann rotational distributions occur at N' ˜ 27 for v' = 0, 1 and 2, with a fractional population of 1, 3 and 11%. For v' = 0 the "hump" is largely obscured by overlap with the v' = 1 bandhead. These bimodal distributions are in qualitative agreement with the results of Nguyen and Sadeghi for v' = 0. The results are discussed within the framework of a curve crossing mechanism with the Ar+-N-2 diabatic potential as an intermediate. By assuming equal charges on both N atoms the Coulomb potential of the collinear orientation lies lower (0.45 eV at R = 2.5 Å) than the perpendicular orientation, with the consequence of different transfer probabilities for both orientations. Within a classical model or rotational excitation the final N' values can be calculated for both orientations, resulting in much higher N' values for the perpendicular orientation. This mechanism supplies a qualitative explanation for the observed bimodal rotational distributions.