Generation of fractional and multiple imaginary rotational alignment echoes

We present a theoretical investigation to understand the connection between the formation of different rotational alignment echoes and the underlying excitation pathways induced by temporally delayed laser pulses. The fractional and multiple imaginary alignment echoes are predicted and demonstrated in the linear polar molecule carbonyl sulfide. We also use a two-dimensional spectrum obtained from higher-order alignment echo signals to identify the underlying quantum coherence. This work deepens our understanding of the alignment echo phenomenon. It provides a way to gain insight into the relationship between the echo signal and the underlying pathway with potential applications in rotational echo spectroscopy.