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 T 1 and T M (the phase memory time) were measured by saturation recovery and electron spin echo spectroscopies. For both salts T 1 decreased from a few milliseconds at 8 K to less than 1 μs at 40 K. T M was 1 to 3 μm between 8 and 25 K and decreased rapidly with increasing temperature to become equal to T 1 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 T M (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 C 60; in C − 60 this vibration may couple to the triply degenerate electronic ground state to produce a dynamic Jahn—Teller distortion.