A study of the photoexcited triplet state of C-60 (C-3(60)) in liquid
solution (toluene, methylcyclohexane) was made using pulsed-EPR techni
ques. The spin-lattice relaxation time (T-I), phase-memory time (T-m),
and line width (Delta v(1/2)) were measured as function of temperatur
e (170-360 K). The results were used to analyze some aspects of the sp
in dynamics of C-3(60). The data confirm earlier findings that the rel
axation toward Boltzmann equilibrium is responsible for the initial gr
owth of the signal amplitude when FT-EPR spectra are measured as a fun
ction of delay time between laser excitation and pi/2 microwave pulse.
Also, it is found that Delta v(1/2)approximate to 1/pi T-m in both so
lvents over the whole temperature range. This establishes that there i
s a negligible contribution to the line width from unresolved C-13 hyp
erfine structure. The contribution of the electron spin-spin interacti
on modulated by rotational motion to the relaxation rates is analyzed.
The analysis shows that the value of the zero-field splitting paramet
er D, which determines the magnitude of this contribution, is about an
order of magnitude less than that found in rigid matrices at low temp
erature. A significant residual, temperature-independent, relaxation c
ontribution is found which may stem from the rapid interconversion bet
ween triplet states that causes the reduction in D upon going from sol
id state to liquid solution.