Imaginary echoes

Phys. Rev. X 6, 041056 (2016)

Credit: HTTPS://CREATIVECOMMONS.ORG/LICENSES/BY/3.0/

Echoes are observed in many nonlinear physical situations, such as spin echoes and optical echoes. Now, Kang Lin and co-workers from China, France and Israel have experimentally observed new types of molecular alignment echoes in CO₂ and N₂O. The team detected two types of echo — alignment echoes whose angular orientation is controlled by the polarization of two ultrashort pulses, and imaginary alignment echoes that appear at negative time delays. To probe the spatiotemporal molecular dynamics, the cold-target recoil-ion-momentum spectroscopy technique was employed. A supersonic gas jet of CO₂ or N₂O molecules was subject to a pair of polarized-skewed and time-delayed femtosecond laser pulses. By scanning the delay of the circularly polarized probe pulse, snapshots of the angular distribution of the molecular axis at various times were taken via Coulomb explosion.