Electron spin relaxation times of C^-₆₀ in solution

Abstract

Potassium and calcium salts of C-60 have been studied in solid and liquid solutions by ESR and optical spectroscopies. Over the temperature interval from 8 to 45 K the electron spin relaxation times T1 and TM (the phase memory time) were measured by saturation recovery and electron spin echo spectroscopies. For both salts T1 decreased from a few milliseconds at 8 K to less than 1 μs at 40 K. TM was 1 to 3 μm between 8 and 25 K and decreased rapidly with increasing temperature to become equal to T1 near 40 or 50 K. Below 40 K the linewidth of the anion signal was about 5.5 G, which is much larger than the linewidth calculated from TM (0.02 G at 10 K). Above about 50 K the linewidth was relaxation time determined and increased rapidly with increasing temperature until reaching a plateau near the melting point of the solution. Analysis of the relaxation rates for C-60 between 35 K and the melting (softening) point of the solutions gives an Arrhenius activation energy between 2.2 and 2.9 kJ/mol (180-240 cm-1) depending on the solvent. This measured range of activation energies for relaxation is close to and somewhat less than the lowest vibrational mode (≈ 270 cm-1 of C60; in C-60 this vibration may couple to the triply degenerate electronic ground state to produce a dynamic Jahn-Teller distortion. © 1992.