Fig. 3.

Far-infrared spectra of H₂O @ C₆₀ at 3.5 K. The times indicate the elapsed intervals after cooling from 77 K. The circled numbers refer to the transition assignments in Fig. 1B. The time-dependence of the peaks indicates slow ortho-to-para conversion. Peaks 4 and 5 are initially saturated due to the strong absorption and thick sample.