Peapods present a model system for studying the properties of dimensionally constrained crystal structures, whose dynamical properties are very important. We have recently studied the rotational dynamics of C 60 molecules confined inside single walled carbon nanotube (SWNT) by analyzing the intermediate frequency mode lattice vibrations using near-infrared Raman spectroscopy. The rotation of C 60 was tuned to a known state by applying high pressure, at which condition C60 first forms dimers at low pressure and then forms a single-chain, nonrotating, polymer structure at high pressure. In the latter state the molecules form chains with a 2-fold symmetry. We propose that the C 60 molecules in SWNT exhibit an unusual type of ratcheted rotation due to the interaction between C 60 and SWNT in the ''hexagon orientation,'' and the characteristic vibrations of ratcheted rotation becomes more obvious with decreasing temperature. molecular rotation ͉ peapods B ecause of its high symmetry and relatively weak intermolecular interactions, the fullerene C 60 presents a nearly ideal system for studying different crystal structures possible in onedimensionally constrained nanostructure systems, such as C 60 peapods [C 60 molecules inserted into single walled carbon nanotubes (SWNTs)] (1). Such nanostructures are emerging as a class of nanoscale materials with tunable mechanical, electronic, thermal, and optical properties (2, 3). In contrast to bulk C 60 crystals, C 60 inserted in SWNTs form monomeric linear chains in which each C 60 has only two nearest neighbors, as compared to 12 neighbors in crystalline bulk C 60 (4). As a result the intermolecular interactions in these self-assembled carbon nanostructures and the dynamic behavior of the encapsulated molecules are expected to be different from those in solid fullerenes. It is well known that the solid C 60 crystallizes into a face centered cubic structure at room temperature, and the molecules perform nearly free rotations (5). For the peapods, recent theoretical simulations suggest that the C 60 s have two different orientations with low potential energy when they are trapped into SWNTs, the pentagonal and hexagonal orientations (6-8) with a pentagon and a hexagon perpendicular to the tube axis, respectively. Very recent experimental studies on peapods by inelastic neutron scattering (INS) and nuclear magnetic resonance (NMR) suggest that the linear arrays of C 60 molecules still rotate inside SWNTs at room temperature, either showing quasi-free rotation or being dynamically disordered with high orientational mobility (9, 10). However, how the C 60 molecules rotate inside SWNTs, especially at room temperature, is still an interesting open question. It is not only necessary to search another effective experimental method to study the possible rotation of confined C 60 in detail, but also very important to study the rotation from a different perspective. Considering that C 60 rotates down to very low temperature, it would be helpful to employ other techniques to change the r...