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

Intense femtosecond laser excitation can produce transient states of matter that would otherwise be inaccessible to laboratory investigation. At high excitation densities, the interatomic forces that bind solids and determine many of their properties can be substantially altered. Here, we present the detailed mapping of the carrier density–dependent interatomic potential of bismuth approaching a solid-solid phase transition. Our experiments combine stroboscopic techniques that use a high-brightness linear electron accelerator–based x-ray source with pulse-by-pulse timing reconstruction for femtosecond resolution, allowing quantitative characterization of the interatomic potential energy surface of the highly excited solid.

Sequential multiple photoionization of the prototypical molecule N 2 is studied with femtosecond time resolution using the Linac Coherent Light Source (LCLS). A detailed picture of intense x-ray induced ionization and dissociation dynamics is revealed, including a molecular mechanism of frustrated absorption that suppresses the formation of high charge states at short pulse durations. The inverse scaling of the average target charge state with x-ray peak brightness has possible implications for singlepulse imaging applications.

Abstract:The first time-resolved x-ray/optical pump-probe experiments at the SLAC Linac Coherent Light Source (LCLS) used a combination of feedback methods and post-analysis binning techniques to synchronize an ultrafast optical laser to the linac-based x-ray laser. Transient molecular nitrogen alignment revival features were resolved in time-dependent x-rayinduced fragmentation spectra. These alignment features were used to find the temporal overlap of the pump and probe pulses. The strong-field dissociation of x-ray generated quasi-bound molecular dications was used to establish the residual timing jitter. This analysis shows that the relative arrival time of the Ti:Sapphire laser and the x-ray pulses had a distribution with a standard deviation of approximately 120 fs. The largest contribution to the jitter noise spectrum was the locking of the laser oscillator to the reference RF of the accelerator, which suggests that simple technical improvements could reduce the jitter to better than 50 fs. ©2010 Optical Society of America