Semiclassical description of vibrational quantum coherence in a three dimensional I(2)Ar(n) (n < or = 6) cluster: a forward-backward initial value representation implementation.

The semiclassical (SC) initial value representation (IVR) has been applied to describe true quantum coherence effects in a complex molecular system in full three dimensional space. The specific quantity considered is the time-dependent probability distribution of the I(2) vibrational coordinate following photoexcitation of I(2) in a rare gas cluster. The "forward-backward" version of the IVR method is shown to be capable of capturing detailed quantum coherence in this quantity, coherence that cannot be described by a classical Wigner model (which is equivalent to a linearized approximation to the more general SC-IVR). Solvent effects on this vibrational quantum coherence have also been investigated for a I(2)Ar(n) (n=1,6) cluster. A solvent cage consisting of six argon atoms reduces the fraction of iodine molecules that dissociate (an example of the "cage effect") and also diminishes, but does not entirely eliminate, quantum coherence in the vibrational motion of the molecules that remain undissociated.