Vibrational relaxation in neat crystals of naphthalene by picosecond cars

Picosecond delayed CARS experiments on totally symmetric modes in naphthalene at 1.5 K are reported. The Raman lineshape of the vibrational excitons is lorentzian and vibrational relaxation can be surprisingly slow. The Raman lineshape of the A_g exciton level of the 766 cm^?1 vibrational mode reveals that the low-temperature lorentzian lineshape occurs by motional narrowing At higher temperature the exciton is trapped by interaction with lattice phonons.