Intense terahertz (THz) pulses with MV/cm peak-field amplitudes have become experimentally feasible with the advents in table-top THz generation methodologies. Such fields provide a new control-handle over the rotations of polar molecules in the gas phase, as they primarily interact with molecular rotors via their permanent dipole moments, differing yet complementing ultrashort optical pulses that interact with molecules via their polarizability tensor. When applied to linear molecules, optical pulses induce molecular ALIGNMENT while THz pulses induce ORIENTATION. In linear molecules both the dipole vector and the most polarizable axis coincide at the inter-atomic axis of the molecule, thus serving to rotate the molecules about the same axis. However, for asymmetric tops like SO2, the dipole and the most polarizable axes lie along different molecular frame axes, turning optical and THz fields as two distinct rotational handles. Well-orchestrated application of these pulses can provide complete three-dimensional control over the molecular angular distribution of such molecules - a long standing goal in molecular physics and chemistry.
In the first part of the talk I will present our recent experimental and theoretical results of optical induced alignment and THz induced orientation in gas phase SO2 molecules that highlight the different rotational dynamics induced by these two distinct rotational handles. On the theoretical front, simulating the rotational dynamics of asymmetric molecules like SO2 at room temperature remains a highly demanding computational task that is effectively impossible by exact methods for dynamics. We overcame this issue by employing the Random Phase Wave Functions method [1,2] that was also verified by experimental results .
In the second part I will present a decay phenomenon that was recently unveiled in by a series of time-resolved measurements of the rotational dynamics induced by an optical pulse and a THz-field. We have found that despite their exact same pressure and temperature, transiently oriented molecules decay at a faster rate than aligned molecules . This is attributed a coherent radiative decay mechanism that we believe is general to all resonantly induced dynamics, however has been discarded previously. I will present our recent experimental results and suggest a theoretical model that incorporates a coherent radiative term into the typical Hamiltonian of the problem.
 D. Gelman and R. Kosloff, Chem. Phys. Lett., 381, 129 (2003).
 S. Kallush, and S. Fleischer, Phys. Rev. A, 91, 063420 (2015).
 R. Damari, S. Kallush, and S. Fleischer, Phys. Rev. Lett. 117, 103001 (2016).
 R. Damari, D. Rosenberg, and S. Fleischer, Phys. Rev. Lett. 119, 033002 (2017).
Rephasing the rotational centrifugal effect by laser-induced alignment echoes
Dina Rosenberg1,2, Ran Damari1,2, Shimshon Kallush3,4 and Sharly Fleischer1,2
1 Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Tel Aviv 6997801, Israel
2 Tel-Aviv University Center for Light-Matter Interaction, Tel Aviv 6997801, Israel
3 Department of Physics and Optical Engineering, ORT Braude College, P.O. Box 78, Karmiel 21982, Israel
4 The Fritz Haber Research Center and The Institute of Chemistry, The Hebrew University, Jerusalem 91904, Israel
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 .
 D. Rosenberg, R. Damari, S. Kallush, and S. Fleischer, J. Phys. Chem. Lett. 8 ,5128 (2017).
 E. L. Hahn, “Spin Echoes,” Phys. Rev. 80, 580 (1950).