High-repetition-rate and high-photon-flux 70 eV high-harmonic source for coincidence ion imaging of gas-phase molecules

Unraveling and controlling chemical dynamics requires techniques to image structural changes of molecules with femtosecond temporal and picometer spatial resolution. Ultrashort-pulse x-ray free-electron lasers have significantly advanced the field by enabling advanced pump-probe schemes. There is an increasing interest in using table-top photon sources enabled by high-harmonic generation of ultrashort-pulse lasers for such studies. We present a novel high-harmonic source driven by a 100 kHz fiber laser system, which delivers 10$^{11}$ photons/s in a single 1.3 eV bandwidth harmonic at 68.6 eV. The combination of record-high photon flux and high repetition rate paves the way for time-resolved studies of the dissociation dynamics of inner-shell ionized molecules in a coincidence detection scheme. First coincidence measurements on CH$_3$I are shown and it is outlined how the anticipated advancement of fiber laser technology and improved sample delivery will, in the next step, allow pump-probe studies of ultrafast molecular dynamics with table-top XUV-photon sources. These table-top sources can provide significantly higher repetition rates than the currently operating free-electron lasers and they offer very high temporal resolution due to the intrinsically small timing jitter between pump and probe pulses

Femtosecond crystallographic experiment in wide-bandgap LiF crystal

Dachraoui H, Oberer C, Heinzmann U. Femtosecond crystallographic experiment in wide-bandgap LiF crystal. Optics Express. 2011;19(3):2797-2804.We report a femtosecond crystallographic study of the dependence of the free-carries generation to the alignment of a crystalline sample to the laser polarization. The probe pulse transmission exhibits a pi/2 modulation that is shown to be correlated with the direction dependence of the effective electron mass. This observation suggests that nonlinear ionization is the first channel for free electron generation during the laser pulse. Moreover, the temporal evolution of the probe pulse transmission indicates the dominance of the avalanche ionization and that nonlinear ionization provides the initial seed electrons for avalanche." (C)2011 Optical Society of Americ

Radiationless S-1 -> S-0 phenyl deactivation pathway: an investigation of iodine-marked bi-phenyl on a silicon surface by means of time resolved core-level photoelectron spectroscopy

Michelswirth M, Dachraoui H, Mattay J, Heinzmann U. Radiationless S-1 -> S-0 phenyl deactivation pathway: an investigation of iodine-marked bi-phenyl on a silicon surface by means of time resolved core-level photoelectron spectroscopy. Molecular Physics. 2012;110(4):207-216.The S-1 -> S-0 radiationless deactivation of iodine terminated bi-phenyl immobilized on a silicon surface was probed by analysing the I4d signature (BE: 45.6 eV, 47.3 eV) by means of High Harmonic Generation (HHG) based photoelectron spectroscopy. Modifications of the 4d(5/2,3/2) spectroscopic contents spanning about 0.2 ps after UV activation (266 nm) were verified as showing a transient molecular response character. A localization to the terminated phenyl substructure in the complex structural environment on the surface was ensured according to the core-level nature of the recorded I4d. The activation of the bi-phenyl achieved by UV irradiation, corresponding to the UV absorption band-edge, was verified as being dominated by a B pi -> B pi* phenyl excitation. Time-Dependent Density Functional Theory (TD-DFT) modellings were therefore performed. They were matched to Configuration Interaction semi-empirical calculations (CI-MNDO) verifying the Rustagi-Ducuing relation. The simulated singlet-singlet excitation spectrum was referenced to the spectra of an iodine terminated monomer and a linear oligophenyl chain (N=8). Thus the deactivation response studied was assigned to a conical intersection promoted S-1(pi*)-> S-0(pi) reaction pathway

Direct time-domain observation of laser pulse filaments in transparent media

Dachraoui H, Oberer C, Michelswirth M, Heinzmann U. Direct time-domain observation of laser pulse filaments in transparent media. Physical Review A. 2010;82(4): 043820.The interplay among self-focusing, energy depletion, and plasma formation is fundamental to the understanding of laser-matter interaction. In this article, we present the first direct time-resolved observations of the propagation of intense femtosecond laser pulses experiencing conical emission, self-focusing, self-guiding, beam filamentation, plasma defocusing, and continuum generation in wide-band-gap dielectrics. We demonstrate that continuum generation involves different mechanisms as a function of deposited energy

Interplay between Electronic Correlations and Coherent Structural Dynamics during the Monoclinic Insulator to Rutile Metal Phase Transition in VO2

Dachraoui H, Müller N, Obermeier G, Oberer C, Horn S, Heinzmann U. Interplay between Electronic Correlations and Coherent Structural Dynamics during the Monoclinic Insulator to Rutile Metal Phase Transition in VO2. Journal of Physics. Condensed Matter. 2011;23(43): 435402

Interplay between absorption, dispersion and refraction in high-order harmonic generation

Dachraoui H, Auguste T, Helmstedt A, et al. Interplay between absorption, dispersion and refraction in high-order harmonic generation. Journal of Physics. B, Atomic, Molecular and Optical Physics. 2009;42(17): 175402.We report a detailed experimental and theoretical study on high-order harmonic generation of a femtosecond Ti-sapphire laser focused at an intensity of around 10(15) W cm(-2) onto a high-pressure (50-210 mbar) neon gas cell of variable length (1-3 mm). Using thorough three-dimensional simulations, we discuss the interplay between the different factors influencing the harmonic-generation efficiency, i.e. phase matching determined by the electronic and atomic dispersions, re-absorption of the harmonics by the medium and refraction of the generating laser beam. Generically, we find that, in our generation conditions, the emission yield of harmonics from the plateau region of the spectrum is absorption limited, whereas the emission from harmonics in the cut-off is strongly reduced due to both electron dispersion and ionization-induced refraction of the laser beam. A good agreement between the numerical results and the experimental data is obtained for the harmonic yield dependence on the various generation parameters (gas pressure, medium length and laser intensity)

Photoinduced reconfiguration cycle in a molecular adsorbate layer studied by femtosecond inner-shell photoelectron spectroscopy

Dachraoui H, Michelswirth M, Siffalovic P, et al. Photoinduced reconfiguration cycle in a molecular adsorbate layer studied by femtosecond inner-shell photoelectron spectroscopy. Physics Review Letters. 2011;106(10): 107401

Transient fs-time-resolved ESCA photoemission of iodophenylphenol molecules adsorbed as a monolayer on a silicon (100) surface

Heinzmann U, Michelswirth M, Dachraoui H, et al. Transient fs-time-resolved ESCA photoemission of iodophenylphenol molecules adsorbed as a monolayer on a silicon (100) surface. In: Photon Impact (Strong Field) [Abstracts] . Journal of Physics: Conference Series. Vol 194. IOP Publishing; 2009