Vibrational predissociation of the ND3-Ar Van der Waals complex: Comparison with NH3-Ar

We study the vibrational predissociation of ND3-Ar through excitation of the nu(2) ''umbrella'' mode of ND3, with the same (scaled or unsealed) ab initio potential and the same quantum mechanical method as used earlier for NH3-Ar [J. Millan et al., J. Chem. Phys. 103, 4138 (1995)]. Just as in that case, we find that a simultaneous excitation of one of the Van der Waals modes (stretch, Sigma or Pi bend, of symmetry A(1), A(2), or E) has a strong effect on the lifetime of the complex and on the product state distribution. In particular, the excitation of the Van der Waals stretch mode leads to shorter lifetimes and colder rotational state distributions of the emerging ND3 fragment. With the original (unsealed) ab initio potential, which produces the most realistic lifetimes (line widths in the infrared spectrum) for NH3-Ar, the spread of the lifetimes for different initial quasi-bound states of ND3-Ar is much larger than for NH3-Ar. This may be related to the more localized nature of the (quasi-)bound states of ND3-Ar and the shorter average distance [R]. The rotation-inversion (j(k)(+/-)) state distributions of the emerging ND3 fragment are highly non-statistical,as in ND3-Ar, but this fragment contains (absolutely and relatively) less rotational energy than the NH3 fragment in the photo-dissociation of NH3-Ar. Most of the (quasi-)bound levels of the ND3-Ar complex show a clear preference for an ND3 inversion-tunneling state of definite + or - parity, but the distribution of the emerging ND3 fragment over its +/- states does not reflect this preference. (C) 1997 American Institute of Physics.