Visualizing ultrafast weak-field-induced rotational revivals of air molecules at room temperature

The ability to observe quantum coherence and interference is crucial for understanding quantum effects in nonlinear optical spectroscopy and is of fundamental interest in quantum mechanics. Here, we present an experimental study combined with theoretical analysis and numerical simulations to identify the underlying process behind the rotational revivals induced by a pair of time-delayed ultrafast femtosecond laser pulses for air molecules under ambient conditions. Our time-resolved two-dimensional alignment measurements confirm that one-step non-resonant Raman transitions from initial states of mixed molecules play a dominant role, showing a signature of weak-field-induced rotational revivals. Furthermore, we demonstrate that such rotational revival spectra can simultaneously measure the entire pure rotational Raman spectra and observe the quantum interference between two transition pathways from a given initial state. This work provides a powerful tool to observe, control, and identify the rotational dynamics of mixed molecular samples under weak-field excitations.