We suggest a method for intrinsic qunatification of the molecular degree of alignment, induced by an ultrashort laser pulse.

Coherent control of molecular rotations has been thoroughly studied and vastly utilized in the last three decades. Motivated by obtaining spectroscopic signatures from the molecular frame, researchers have invented a large and sophisticated toolbox for controlling the angular distribution of gas phase molecules via their interaction with strong laser pulses.

Recently, vast efforts are made toward rotational echo spectroscopy of molecules in the gas in order to characterize their rates of decay and decoherence. However, different from the typical two-level systems, where echo spectroscopy is well understood and broadly utilized (such as Nuclear magnetic resonanace, electronic and vibrational dynamics), the multi-level character of molecular rotations manifest in much richer and more convoluted dynamics  [1,2]. In my talk I will discuss our recently developed technique for rotational echo spectroscopy and focus on a specific application of rotational echoes as an intrinsic calibration method for the alignment factor induced in the gas. The latter, calculated as the value of cos^2(theta) (where theta is the angle between the internuclear molecular axis and the laser polarization axis) averaged over the entire molecular ensemble (hence <<cos^2(theta)>>) and is the key observable of interest to researchers as it quantifies the degree to which the molecular ensemble is aligned along a chosen lab-frame axis.   

Quantification of <<cos^2(theta)>> is quite challenging and requires accurate assessment of various experimental parameters that are hardly  accessible in many cases (such as the exact length of interaction, the gas density and even the anisotropic polarizability of the gas). I will present recent experimental and theoretical results that enable direct quantification of the alignment factor based on the most basic features of the rotational echo response. The developed and validated all-optical technique does not require knowledge of the most elusive experimental parameters mentioned above. A paper about this new method is currently under review.  

References:

[1]     D. Rosenberg, R. Damari, S. Kallush, and S. Fleischer, J. Phys. Chem. Lett. 8, 5128 (2017).

[2]     D. Rosenberg, R. Damari, and S. Fleischer, Phys. Rev. Lett. 121, 234101 (2018).