Compositional control of rovibrational wave packets in the E( 1 Σ g + ) “shelf” state of Li 2 via quantum-state-resolved intermediate state selection

Compositional control in the preparation of rovibrational wave packets is demonstrated in the E( 1 Σ g + ) state of gas-phase Li 2 molecules using ultrafast pump–probe laser spectroscopy combined with quantum-state-resolved intermediate state selection. The intermediate state, from which subsequent ultrafast excitation occurs, is a stationary rovibrational level in the A( 1 Σ u + ) state of Li 2 , produced by cw laser excitation from the ground X( 1 Σ g + ) state. The effect that the intermediate state has on the final composition of the wave packet is investigated by comparing the transients resulting from ultrafast pump–probe excitation of two different intermediate states ( v A =14, J A =18 versus v A =13, J A =18 ). In these experiments the pump wavelength is compensated so that in each case the same E -state eigenstates ( v E =13–18, J E =J A ±1 ) make up the wave packet, but with different amplitudes. Theory predicts, and experiments confirm, that the relative amplitudes of the rovibrational eigenstates are strongly dependent upon the intermediate state and determine the spatial and temporal evolution of the wave packet. Evidence for this includes differences in the observed pump–probe transients and dramatically different amplitudes of the beat frequencies in the Fourier analysis of the time-domain transients. Theoretical three-dimensional wave packet simulations highlight how the composition of the wave packet is used to vary its spatial and temporal evolution.