Field-free molecular alignment detection by 4f coherent imaging

Time-resolved detection of field-free molecular alignment is investigated by phase contrast. The technique based on a 4f imager operating as an inverted Zernike spatial filter makes it possible to discriminate between positive and negative molecular alignment revivals produced in a linear molecule. The measurements are performed in a way that minimizes the contamination of the signal by the plasma generated during the aligning pulse. The observations are supported by a semi-analytical model, from which the degree of alignment produced at the beam focus is estimated.