There is currently great interest in the dynamics of electrons and phonons
in low-dimensional systems, where the effects of quantum confinement cause
a dramatic difference in their behaviour as compared with bulk systems. In
this paper we consider a localized electronic impurity state (an electronic
two-level system) linearly coupled to the vibrational modes of an isolated
nanometre-scale insulating crystal, and study the phonon emission rate at
frequencies less than that of the lowest internal vibrational mode, i.e., i
n the acoustic 'gap'. We show that, at finite temperature, electronic energ
y relaxation below the acoustic gap can occur as a result of anharmonic bro
adening of vibrational modes, and we calculate the frequency and temperatur
e dependence of the relaxation provided by this mechanism.