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Initiating the gain process in a free-electron laser (FEL) from an external highly coherent source of radiation is a promising way to improve the pulse properties such as temporal coherence and synchronization performance in time-resolved pump-probe experiments at FEL facilities, but this so-called "seeding" suffers from the lack of adequate sources at short wavelengths. We report on the first successful seeding at a wavelength as short as 38.2 nm, resulting in GW-level, coherent FEL radiation pulses at this wavelength as well as significant second harmonic emission at 19.1 nm. The external seed pulses are about 1 order of magnitude shorter compared to previous experiments allowing an ultimate time resolution for the investigation of dynamic processes enabling breakthroughs in ultrafast science with FELs. The seeding pulse is the 21st harmonic of an 800-nm, 15-fs (rms) laser pulse generated in an argon medium. Methods for finding the overlap of seed pulses with electron bunches in spatial, longitudinal, and spectral dimensions are discussed and results are presented. The experiment was conducted at FLASH, the FEL user facility at DESY in Hamburg, Germany.

This article discusses the results obtained from an extensive comparison set up between nine different European laboratories using different two-dimensional ͑2D͒ dopant profiling techniques ͑SCM, SSRM, KPFM, SEM, and electron holography͒. This study was done within the framework of a European project ͑HERCULAS͒, which is focused on the improvement of 2D-profiling tools. Different structures ͑staircase calibration samples, bipolar transistor, junctions͒ were used. By comparing the results for the different techniques, more insight is achieved into their strong and weak points and progress is made for each of these techniques concerning sample preparation, dynamic range, junction delineation, modeling, and quantification. Similar results were achieved for similar techniques. However, when comparing the results achieved with different techniques differences are noted.

The authors introduce a method for cross correlating vacuum-ultraviolet with near-infrared femtosecond light pulses in a perpendicular geometry. Photoelectrons generated in an atomic gas by laser-assisted photoionization are used to create a two-dimensional image of the cross-correlation volume, thereby mapping time onto a space coordinate. Thus, information about pulse duration and relative timing between the pulses can be obtained without the need to scan an optical delay line. First tests using vacuum-ultraviolet pulses from the free-electron laser at the Deutsches Elektronen Synchrotron set an upper limit for their temporal jitter with respect to external optical laser pulses. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2714999͔For measurement of the temporal properties of optical pulses in the visible, near-ultraviolet, and near-infrared ͑NIR͒ range the nonlinear response of an optical medium-usually a crystal-on two optical fields of different or the same color is often used to realize cross-or autocorrelation schemes. 1 The recent advent of sources of femtosecond-and even attosecond 2 -pulses of ionizing radiation, based on laser plasmas, 3 high harmonic generation, 4 and free-electron lasing calls for a transfer of the correlation principle to the vacuum-ultraviolet ͑vuv͒ and the x-ray range. Since, however, crystals are no longer transparent in the vuv and do not exhibit sufficient nonlinearity in the x-ray range, alternative approaches for pulse characterization are necessary. Freeelectron lasers ͑FELs͒ are the most powerful femtosecond laser sources at short wavelengths, currently operating in the vuv range ͓e.g., free-electron laser Hamburg ͑FLASH͔͒ 5 and down to x-rays in the future. 6,7 So far, the utilized mode of operation for short-wavelength FELs relies on the process of self-amplified spontaneous emission ͑SASE͒. 8 High photon energies combined with greatly enhanced pulse intensities as compared to other sources make FELs a promising source for time-resolved visible/vuv pump-probe studies. FELs, however, are based on linear accelerators and undulators, which together measure from a few hundred meters up to a few kilometers in length. This makes these instruments susceptible to path length variations on a micrometer scale. Moreover, the statistical nature of SASE comes with a frequently varying spectral and temporal profile for each individual shot. 9 As a result, the arrival time of the FEL pulses at the experiment fluctuates in the order of a few hundred femtoseconds. This jitter inhibits an exact synchronization to an external laser source needed for reliable pump-probe studies. Therefore, an adequate characterization method is needed to determine the pulse duration together with the actual delay with respect to an external laser on a shot-to-shot basis. State-of-the-art x-ray streak cameras have reached subpicosecond temporal resolution, 10 but the photocathode completely dissipates the beam. In addition, their readout rate is limited to a few hertz, while FLASH is operated at a...

Ultrafast dynamics of a molecular wave packet created by a strong 120-fs near-infrared (800 nm) laser pulse in iodine has been probed by synchronized 13.4-nm, 35-fs extreme-ultraviolet pulses delivered by the free-electron laser facility in Hamburg, FLASH. The kinetic energy release of the multiply charged ionic fragments reveals three essential steps of strong-field-induced molecular fragmentation dynamics: (i) The creation of I 2 2+ and (I 2 2+ ) * molecular ions proceeds within (75 ± 15) fs full-width-at-half-maximum. (ii) With the onset of the I 2 2+ fragmentation the probability to lose a further electron within the same optical laser pulse rises with increasing I + -I + internuclear separation and reaches its maximum after ∼30 fs with respect to the pulse maximum.(iii) Charge separation into the I 2 2+ → I 2+ + I dissociative channel with an asymmetric charge distribution is completed after (121 ± 22) fs.