High-Resolution Infrared Absorption Spectroscopy of C 60 Molecules and Clusters in Parahydrogen Solids

We report the isolation of C 60 molecules in cryogenic parahydrogen (pH 2 ) solids by the rapid vapor deposition method. New theoretical simulations of rovibrational spectra for low-temperature isolated 12 C 60 molecules, including boson-exchange symmetry restrictions on the rotational levels, predict a characteristic “null gap” and unequal rotational line spacings for low- J values. High-resolution IR absorption spectra of the C 60 /pH 2 samples failed to show rotationally resolved features, and in fact suggest that the majority of the C 60 molecules are not rotating. However, spectra of the F 1u (1) vibrational mode near 530 cm -1 show line widths of ≈0.2 cm -1 fwhm, the sharpest IR absorption bands for C 60 reported to date. Visible absorption spectra also show sharp features in the ≈600 nm region, supporting our contention of well-isolated C 60 molecules. The C 60 molecules appear to stabilize the pH 2 solid, inhibiting the fcc to hcp conversion which usually occurs upon annealing of rapid vapor deposited pH 2 solids to T ≈ 5 K. We also report surprisingly strong C 60 -induced IR activity in the pH 2 solid, and propose this phenomenon as a diagnostic for H 2 molecules adsorbed by carbon nanotubes. C 60 /pH 2 samples grown in an enclosed cell by laser ablation of solid C 60 appear to contain predominantly (C 60 ) n clusters; these clusters are too small to exhibit “bulk” vibrational or electronic properties, as determined by IR and UV/visible absorption spectroscopies. Future experiments to disentangle the contributions of 13 C isotopic substitution, pH 2 matrix effects, and the putative hindered rotation of C 60 molecules to the observed C 60 /pH 2 IR line shapes are presently under consideration.