Enantioselective One-Photon Excitation of Formic Acid
|
We show experimentally that handedness of excited formic acid molecules is controlled by the direction of propagation of the exciting photons without the need for chiral light. This is shown by exciting K-shell electrons from carbon or oxygene to the unoccupied pi state. We image the handedness of the molecule by coulomb explosion imaging using a COLTRIMS reaction microscope. As we show by calculations, the effect is caused by the dependence of the excitation dipole matrix element on the out-of-plane bending of the molecule within its ground state vibronic wavefunction (Fig 1).
Figure 1: (a) We find that the handedness is controlled by the angles θ and Φ from which the photon hits the molecule. (b) Explanation of the effect: Sketch of the potential energy surfaces involved in the excitation. Horizontal axis: generalized out of plane bending coordinate of formic acid, Q = 0 corresponds to the planar configuration, Q > 0 to the R geometry, and Q < 0 to the S geometry. The lowest potential energy surface shows the electronic ground state and the upper double-well surface shows the π* state with S or R equilibrium configuration. The dashed arrows indicate the electronic excitation. The excitation matrix element depends for each configuration Q differently on the light propagation direction in the molecular frame of reference as defined by the angles ϑ and φ. (c) The π* orbital for a chiral geometry in which the H2 atom is bent by Q = +20° out of plane. The positive (negative) phase of the orbital is encoded the in the orange (cyan) color.
Figure 2: Measured handedness of the excited molecule as function of the photon impact angles θ and Φ as shown in Fig1a. White corresponds to a linear molecule, red color to an excess of the R-enantiomer and blue to an excess of S-enantiomers. Publication: |
