Interactions between (8s-6d) and (7s-5d) wavepackets in Rb examined by parametric four-wave mixing

Quantum beating resulting from the coherent superposition of states in the Rb atom with pairs of femtosecond pulses has allowed for weak interaction between the 7S-5D$_{5/2}$ and 8S-6d$_{5/2}$ wavepackets to be examined. The dominant 7S-5D$_{5/2}$ beating was identified at 18.225 THz with an accuracy on par with NIST data and was demonstrated to persist in excess of 1 ns after excitation. A weaker beating of the 8s-6d$_{5/2}$ oscillation is measured at 10.732 $\pm$ 0.002 THz (where the uncertainty represents the frequency resolution when no zero padding is used in the discrete Fourier transform), which is 0.5 cm$^{-1}$, or 10 times the frequency resolution, above that given by NIST data. This measured impact of the 7S-5D$_{5/2}$ wavepacket on the 8S-6d$_{5/2}$ superposition is consistent with the classical model of coupled oscillators.