We have explored the use of laser driven high-order harmonic generation to probe the electronic structure and symmetry of conjugated polyatomic molecular systems. We have investigated non-adiabatically aligned samples of linear symmetric top, nonlinear symmetric top and asymmetric top molecules, and we have observed signatures of their highest occupied molecular orbitals in the dependence of harmonic yields on the angle between the molecular axis and the polarization of the driving field. A good quantitative agreement between the measured orientation dependence of high harmonic generation and calculations employing the strong field approximation has been found. These measurements support the extension of molecular imaging techniques to larger systems. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

We gratefully acknowledge the support of the staff of the Central Laser Facility at Rutherford Appleton Laboratory and James Lazarus from Imperial College London for technical assistance. R T also acknowledges the Spanish Department of State of Education and Universities, and the European Social Fund for financial support. The experiment time was granted by the CCLRC (experiment A2P2/05) in the Target Area One of the Astra Laser Facility. This work has been supported by EPSRC and RC UK.

17 pags., 9 figs. 1 tab., 1 app.