Quantum control of photoion circular dichroism using orthogonal laser beams

The circular dichroism manifested in the photoionization yields of the chiral molecule fenchone has been investigated using two laser beams of different wavelengths intersecting at 90 degrees. This allowed different two-colour ionization schemes to be investigated so that the circular dichroism contributions of each step in the multiphoton ionization process could be deduced. The results showed that the circular dichroism generated by a beam at a wavelength of 260 nm could be strongly influenced by the polarization state of a 520 nm beam propagating in a perpendicular direction. The asymmetry in the ion yields was found to change depending on whether the 520 nm beam was left or right circularly polarized, and if it was linear polarized in a direction parallel or perpendicular to the propagation direction of the 260 nm beam. The control exhibited by the direction of the linearly polarized light is attributed to an orientation dependence of the circular dichroism due to selective excitation of the isotropic ensemble by the 520 nm beam. By contrast, when both beams were circularly polarized, the dependence on the polarization direction of the 520 nm pulse is ascribed to either the interference between different ionization pathways or excitation of a chiral vibronic wavepacket in the molecule. These results are the first demonstration of chiroptical quantum control of total ion yields, showing in principle that all-optical enantio-sensitive chemical processing may be possible in the future.