High Harmonic Generation from Multiple Orbitals in N2

Molecular electronic states energetically below the highest occupied molecular orbital ( HOMO) should contribute to laser- driven high harmonic generation (HHG), but this behavior has not been observed previously. Our measurements of the HHG spectrum of N-2 molecules aligned perpendicular to the laser polarization showed a maximum at the rotational half- revival. This feature indicates the influence of electrons occupying the orbital just below the N-2 HOMO, referred to as the HOMO-1. Such observations of lower- lying orbitals are essential to understanding subfemtosecond/subangstrom electronic motion in laser- excited molecules.