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The CNT sheet fabricated by the floating catalyst chemical vapor deposition (CVD) method has attracted great attention due to its easy fabrication process and promising mass production at low cost. However, the randomly oriented CNT sheet with a loose stacking density shows relatively poor mechanical properties. In this work, a highly aligned dense CNT sheet was successfully fabricated by a simple process of two-time stretching and pressing of a multilayered CVD-grown CNT sheet. Drastic nanotube rearrangements occurred during stretching and pressing processes. A polymer-like tensile necking behavior was observed during the stretching process, accompanied by inter-tube junction breakage due to long-distance slippage. Simultaneously the CNT sheet was thickened after the stretching process due to the increase of the inter-layer space, which could be effectively eliminated by the following pressing treatment. After two-time stretching and pressing, a highly aligned dense CNT sheet was fabricated with the volume density increasing to 0.98 g cm(-3) (by 109%) and the tensile strength increasing to 598 MPa (by 221%) compared to the as-prepared CNT sheet.

In this and the accompanying paper we give a full account of results in earlier reports [Liu, Q.; e? al. Nature 1993, 364, 427. Potter, E. D.; et al. Chem. Phys. Lett. 1992, 200, 6051 on the experimental and theoreticalstudies of the femtosecond to picosecond dynamics of dissociation, recombination, and coherence of iodine in large argon clusters. Using different size distributions of I2*Ar, (E -8-40), in a molecular beam, and tuning the wavelength of the pump and probe laser beams, the reaction dynamics over a wide range of energies, states, and reaction coordinates are examined. Both A-state direct dissociation and B-state predissociation are studied, covering a pump wavelength range of 460-700 nm. The probe wavelength was tuned from 280 to 350 nm to resolve the motion and relaxation of iodine at different internuclear separations on different states of recombination. From these systematic studies of the ultrafast dynamics, the microscopic picture of solvation is established with the following concepts: First, the initial occurrence of caging in large clusters involves the coherent recombination of the atoms; the bond re-forms in the solvent structure as the wave packet bounces back from the solvent shell. This femtosecond caging, which has not been observed previously, reveals the role of the solvent at early times. Second, incoherent, diffusive caging occurs on a longer time scale, and it is this caging process that involves physical movement of the solvent. Third, the entire process of caging depends on the time scale of bond breakage, as evidenced by the contrast between the dynamics following excitation onto the A (femtosecond dissociation) and B (picosecond predissociation) states. The key here is the relative time scales for bond breakage and solvent reorganization. Fourth, product state (MA') vibrational relaxation occurs on a much longer time scale than that for caging or dephasing of the wave packet motion. As shown in the accompanying paper, molecular dynamics (MD) calculations support this microscopic picture. The MD simulations reproduce the experimental observations and detail the microscopic influence of solvent temperature, rigidity, and structure. The simulations cover the femtosecond to picosecond time scales which are essential for characterizing the evolution of solvation and its equilibration.

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 collisioninduced predissociation and collision-induced dephasing. By means of molecular dynamics simulations, the experimental results are reproduced by classical and semiclassical treatments. The centrifugal force, by vibration-rotation coupling, is found to be 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 forces increase with the solvent density throughout the whole dynamics range. The combining influence of the two forces gives rise to the drastically different behaviors observed in three different density regions.

In this paper, the molecular dynamics of dissociation and recombination of iodine in argon clusters are reported and compared with the experimental results of the preceding paper et al. J. Phys. Chem. 1995, 99, 11309). The microscopic processes of bond breakage and bond reformation are visualized with the help of simple classical models. The coherent recombination observed experimentally is shown to be closely related to the coherent nature of bond breakage and the homogeneity of local structures in large clusters. Caging, which is found to be efficient in the large clusters, is ultrafast: The caging time scale ranges from femtoseconds to picoseconds depending on the bond breaking time as well as the size and temperature of the cluster. With the inclusion of the known solute (iodine) femtosecond dynamics and the solute-solvent LennardJones interaction potentials, the molecular dynamics simulations reproduce the corresponding experimental results and link the microscopic picture to the observables. We examine the effect of the local structure, cluster size and temperature, bond distance, and the state excited on the wave packet dynamics of the solute in the solvent. The new mechanism of coherent, concerted caging in the solvent structure is described. From the experimental and theoretical studies we provide some concepts regarding the molecular basis of the dissociation and recombination phenomena.