Contributed
Echo spectroscopy in multi-level quantum-mechanical rotors

Echo spectroscopy is a central technique in magnetic resonance, electronic and vibrational spectroscopy, enabling researchers to distinguish dynamical dephasing from decoherence phenomena. The interest in echo responses for gas phase rotational spectroscopy is gradually increasing in recent years, with theoretical and experimental results utilizing resonant terahertz fields and non-resonant optical fields to induce a variety of echo responses in multi-level rotational systems. In the flash talk I will present new studies we have performed on rotational echoes, demonstrating the rephasing of centrifugal distortion and new properties we have unraveled that enable echo spectroscopy in a multi-level system.

Different from two-level systems associated with a single transition frequency, multilevel rotational systems manifest quantum rotational revivals due to the unique harmonic rotational level spacing. Thus, following their excitation by an ultrashort optical pulse, the molecules demonstrate periodic rotational dynamics with a long series of alignment events persisting under field-free conditions. In the first work we have demonstrated the rephasing of centrifugal distortion effect in the echo signal and its dependency on pulses’ intensities. While working on this study, we have encountered challenges in finding an optimal echo signal. We came to the realization that in our multi-level system, the echo signal is not only dependent on the two intensities of the pulses, but also on the delay between them, . After a more thorough study, we have found that while the echo grows linearly with the first pulse intensity, there is an optimal intensity of the second pulse for each . This conclusion has opened the possibility of performing echo spectroscopy by judicious control of the rephasing pulse intensity at a certain delay, allowing extraction of the pure decoherence rate of the molecular ensemble. When working with dense gases, the signal decays in short timescales and the extraction of decoherence scales from the echo signals is of high relevance in this regime.