Rotational Echoes: Rephasing of Centrifugal Distortion in Laser-Induced Molecular Alignment

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 talk I will focus on the rephasing property of rotational echoes induced by two ultrashort optical pulses in a gas phase ensemble of molecules.

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. However, due to the finite rigidity of the molecules (centrifugal distortion) the molecules experience significant dephasing and distortion of the alignment signal with time and the alignment events become gradually longer in duration and with increasing number of oscillations.

When applying the second optical pulse after a delay of a few rotational revivals, the accumulated dephasing in the signal is significant and therefore also visible in the echo signal. The first echo signal appears with the same dephasing, only in the reversed phase-time direction. Since the echo manifests the same periodicity as the fundamental revivals, the rephasing its signal occurs gradually in every appearance. When reaching the zero echo signal, at twice the delay between the pulses, the rephasing is complete. Afterwards, the echo signal continues to diphase jointly with the already non-rephasing fundamental revivals.

In my talk I will also discuss the dependence of the echo signal on the intensities of the driving pulses and present a quantum-mechanical version of Hahn`s famous track-runners analogy to spin echoes.