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 show 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 the sub-femtosecond/sub-angstrom electronic motion in laser excited molecules.Tomographic imaging of molecules using high harmonic generation (HHG) has attracted wide interest [1]. The method can be easily described in the framework of a strong-field three-step model [2,3].In this model, a portion of the electron wave function corresponding to the highest occupied molecular orbital (HOMO) tunnels into the continuum and is accelerated in a strong oscillating optical field. This continuum part of the wave function is treated as a free electron wave packet, which interferes coherently with the bound part of the HOMO when it returns to the molecule. Recombination dipole radiation is emitted on every half-cycle of the driving field and the coherent superposition of this radiation over multiple cycles forms a discrete spectrum of odd-order high harmonics. The spectrum contains information about the HOMO structure. Tomographic reconstruction achieves sub-angstrom spatial resolution de-1

Molecules can efficiently and selectively convert light energy into other degrees of freedom. Disentangling the underlying ultrafast motion of electrons and nuclei of the photoexcited molecule presents a challenge to current spectroscopic approaches. Here we explore the photoexcited dynamics of molecules by an interaction with an ultrafast X-ray pulse creating a highly localized core hole that decays via Auger emission. We discover that the Auger spectrum as a function of photoexcitation-X-ray-probe delay contains valuable information about the nuclear and electronic degrees of freedom from an element-specific point of view. For the nucleobase thymine, the oxygen Auger spectrum shifts towards high kinetic energies, resulting from a particular C-O bond stretch in the pp* photoexcited state. A subsequent shift of the Auger spectrum towards lower kinetic energies displays the electronic relaxation of the initial photoexcited state within 200 fs. Ab-initio simulations reinforce our interpretation and indicate an electronic decay to the np* state.

High harmonic spectra show that laser-induced strong field ionization of water has a significant contribution from an inner-valence orbital. Our experiment uses the ratio of H2O and D2O high harmonic yields to isolate the characteristic nuclear motion of the molecular ionic states. The nuclear motion initiated via ionization of the highest occupied molecular orbital (HOMO) is small and is expected to lead to similar harmonic yields for the two isotopes. In contrast, ionization of the second least bound orbital (HOMO-1) exhibits itself via a strong bending motion which creates a significant isotope effect. We elaborate on this interpretation by simulating strong field ionization and high harmonic generation from the water isotopes using the time-dependent Schrödinger equation. We expect that this isotope marking scheme for probing excited ionic states in strong field processes can be generalized to other molecules.