We present a new technique for measuring the relative delay between a soft x-ray FEL pulse and an optical laser that indicates a sub 25 fs RMS measurement error. An ultra-short x-ray pulse photo-ionizes a semiconductor (Si(3)N(4)) membrane and changes the optical transmission. An optical continuum pulse with a temporally chirped bandwidth spanning 630 nm-710 nm interacts with the membrane such that the timing of the x-ray pulse can be determined from the onset of the spectral modulation of the transmitted optical pulse. This experiment demonstrates a nearly in situ single-shot measurement of the x-ray pulse arrival time relative to the ultra-short optical pulse.

Ultrafast electron transfer in dissociating iodomethane and fluoromethane molecules was studied at the Linac Coherent Light Source free-electron laser using an ultraviolet-pump, X-ray-probe scheme. The results for both molecules are discussed with respect to the nature of their UV excitation and different chemical properties. Signatures of long-distance intramolecular charge transfer are observed for both species, and a quantitative analysis of its distance dependence in iodomethane is carried out for charge states up to I21+. The reconstructed critical distances for electron transfer are in good agreement with a classical over-the-barrier model and with an earlier experiment employing a near-infrared pump pulse.

X-ray–optical pump–probe experiments at the Linac Coherent Light Source (LCLS) have so far been limited to a time resolution of 280 fs fwhm due to timing jitter between the accelerator-based free-electron laser (FEL) and optical lasers. We have implemented a single-shot cross-correlator for femtosecond x-ray and infrared pulses. A reference experiment relying only on the pulse arrival time information from the cross-correlator shows a time resolution better than 50 fs fwhm (22 fs rms) and also yields a direct measurement of the maximal x-ray pulse length. The improved time resolution enables ultrafast pump–probe experiments with x-ray pulses from LCLS and other FEL sources.

All matter exposed to intense femtosecond x-ray pulses from the Linac Coherent Light Source (LCLS) free-electron laser is strongly ionized on femtosecond time scales. On these time scales, the ionization is competing with the lifetimes of the created inner-shell vacancies. In the present work, it is shown that for nanoscale objects the environment, i.e., nanoparticle size, is an important parameter for the time-dependent ionization dynamics in intense x-ray pulses because it has an in uence on the inner-shell vacancy liftimes. As a sample system, argon atoms and clusters with sizes between ⟨N⟩ = 55 and ⟨N⟩ = 1600 were chosen. The clusters were irradiated with 480 eV x-ray pulses reaching power-densities of up to a few 10 17 W/cm 2 . At this photon energy dominantly the argon L-shell is ionized and the remaining vacancies are most likely lled via Auger decay. To investigate the electron dynamics, the x-ray pulse length was tuned between 30 fs and 85 fs, using the novel LCLS slotted spoiler technique. The ionization products were measured with an ion time-of-ight spectrometer with a special slit aperture. This aperture e ciently suppresses atomic background, which yields cluster spectra of unprecedented quality. Spectra for di erent pulse lengths and a range of pulse energies were collected for atoms and all cluster sizes. In atoms, as in clusters, longer x-ray pulses are absorbed more e ciently than shorter x-ray pulses with the same number of photons. To shed light on size-dependent e ects in clusters, an independent measure for the time-dependent component of the absorption for every cluster size was found by means of x-ray induced transparency increase (XITI). The XITI increases from atoms to clusters and shows a clear cluster size dependence. A rate equation model for the ionization of atomic systems has been developed to support the interpretation of the experimentally determined XITI as a function of the cluster size. As a result, the Auger lifetimes of large argon clusters are found to be longer than for small clusters and isolated atoms. This is due to delocalization of the valence electrons in the x-ray induced nanoplasma, resulting in a smaller overlap between valence electrons and core holes. As a consequence, large nanometer sized samples absorb intense femtosecond x-ray pulses less e ciently than small ones.