Fragmentation dynamics of the vibrationally excited ammonia-argon van der Waals complex

Zusammenfassung

Quantum mechanical calculations have been performed to study the fragmentation dynamics of the Ar-NH3 van der Waals complex excited in the ν2 ("umbrella") vibrational level with υ2=1. Vibrational predissociation has been investigated for different initial quasibound states, corresponding to excitation in the stretching or bending van der Waals levels or in the tunneling motion of the NH3 umbrella inversion, for both ortho and para varieties of NH 3, and for three values of the total angular momentum J and its projection Ω onto the intermolecular axis, (J,Ω)=(0,0), (1,0), and (1,±1). The calculations were performed for two different intermolecular potential energy surfaces, one determined ab initio by Bulski et al. [J. Chem. Phys. 94, 491 (1991)], and the other where the ab initio potential has been scaled to fit spectroscopic data of Ar-NH3, proposed by van Bladel et al [J. Phys. Chem. 95, 5414 (1991)]. The lifetime obtained from the ab initio potential lies within the limits set by experiment for the lowest ortho Π (i.e., |Ω|=1) level with J=1, while the scaled potential gives too short a lifetime. The results also show that excitation of the van der Waals stretching mode accelerates the predissociation but does not affect the NH3 final state rotational distribution very much. On the other hand, bending or tunneling excitation does lead to a very different final rotational state distribution. Ortho states have very similar final state distributions for Σ(|Ω|=0) and Π(|Ω|=1) states, but this is not true for para states. Finally, comparing the ab initio and scaled potentials, no systematic trend emerges for the amount of even (+) and odd (-) symmetry of the final states, but the scaling of the potential shortens the lifetimes, gives a wider final state rotational distribution, and provides more excitation in the k quantum number (the projection of the NH3 rotational angular momentum on its C3 axis). © 1995 American Institute of Physics.