In this paper of the series, which gives the full account of an earlier communication [Liu et al. J. Chem. Phys. 1996, 105, 5294], the reaction wave packet coherence of iodine in rare-gas fluids is studied, both experimentally and theoretically, in the gas-to-liquid transition region. The phase coherence dynamics, which exhibit striking density behavior, are resolved in real time and across a broad density range (0-35 nm(-3)) for three different rare gases : helium, neon, and argon. In the solvent, the wave packet reaction coherence is observed on time scales longer than that of the solute vibrational motion, and the loss of coherence is due to solvent collision-induced predissociation and collision-induced dephasing. By means of molecular dynamic simulations, the experimental results are reproduced by classical and semiclassical treatments. The centrifugal force, by vibration-rotation coupling, is found robe dominant at low and intermediate densities. A motion narrowing effect causes the contribution of vibration-rotation coupling to dephasing to decrease at high densities. Direct contributions from the solute-solvent fords increase with the solvent density throughout the whole dynamics The combining influence of the two forces gives rise to the drastically different behaviors observed in three different density regions.