Motional narrowing in the spin relaxation of free excitons?

The signature of motional narrowing in the spin relaxation of excitons is t he inverse proportionality between the spin relaxation time tau(s) and the momentum relaxation time T-2. We find such a relation in ZnSe quantum wells over a range of temperatures reaching from 8 up to 200 K. For low temperat ures T < 60 K where the excitonic T-2 is determined by scattering with acou stic phonons motional narrowing might be the reason for;he increase of spin lifetime. But for higher temperatures where exciton ionization by optical phonons dominates we attribute the effect to a suppression of electron-hole exchange interaction due to the reduced spatial overlap of electron and ho le wave functions.