Observation of fractional revivals of a molecular wave packet

The study and control of molecular dynamics through the use of femtosecond laser techniques require an understanding of various orders of molecular wave-packet phenomena such as revivals. We present the observation of fractional revivals in a vibrational wave packet: the $B$ state of the ${\mathrm{Br}}_2$ molecule. Fractional revivals occur in anharmonic systems at characteristic times in the nonclassical regime when, due to dephasing between vibrational eigenstates, the wave packet partially localizes into sub-wave-packets. Half and quarter revivals were clearly observed, as well as indications of one-sixth revivals.