Ultrafast photofragmentation dynamics of molecular iodine driven with timed XUV and near-infrared light pulses

Photofragmentation dynamics of molecular iodine was studied as a response to the joint illumination with femtosecond 800 nm near-infrared and 13 nm extreme ultraviolet (XUV) pulses delivered by the free-electron laser facility FLASH. The interaction of the molecular target with two light pulses of different wavelengths but comparable pulse energy elucidates a complex intertwined electronic and nuclear dynamics. To follow distinct pathways out of a multitude of reaction channels, the recoil of created ionic fragments is analyzed. The delayed XUV pulse provides a way of following molecular photodissociation of I(2) with a characteristic time-constant of (55 ± 10) fs after the laser-induced formation of antibonding states. A preceding XUV pulse, on the other hand, preferably creates a 4d(-1) inner-shell vacancy followed by the fast Auger cascade with a revealed characteristic time constant τ(A2)=(23±11) fs for the second Auger decay transition. Some fraction of molecular cationic states undergoes subsequent Coulomb explosion, and the evolution of the launched molecular wave packet on the repulsive Coulomb potential was accessed by the laser-induced postionization. A further unexpected photofragmentation channel, which relies on the collective action of XUV and laser fields, is attributed to a laser-promoted charge transfer transition in the exploding molecule

Time-Resolved Ion Spectrometry on Xenon with Jitter-Compensated Soft X-Ray Pulses of a Free-Electron Laser

Atomic inner-shell relaxation dynamics were measured at the free-electron laser in Hamburg, FLASH, delivering 92 eV pulses. The decay of 4d core holes created in xenon was followed by detection of ion charge states after illumination with delayed 400 nm laser pulses. A timing jitter of the order of several hundred femtoseconds between laser- and accelerator-pulses was compensated for by a simultaneous delay measurement in a single-shot x-ray/laser cross-correlator. After sorting of the tagged spectra according to the measured delays, a temporal resolution equivalent to the pulse duration of the optical laser could be established. While results on ion charge states up to Xe4+ are compatible with a previous study using a high-harmonic soft x-ray source, a new relaxation pathway is opened by the nonlinear excitation of xenon atoms in the intense free-electron laser light field, leading to the formation of Xe$^{5+}$

Time-to-space mapping in a gas medium for the temporal characterization of vacuum-ultraviolet pulses

Cunovic S, Müller N, Kalms R, et al. Time-to-space mapping in a gas medium for the temporal characterization of vacuum-ultraviolet pulses. APPLIED PHYSICS LETTERS. 2007;90(12):121112.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. (c) 2007 American Institute of Physics

Measurements and simulations of seeded electron microbunches with collective effects

Measurements of the longitudinal phase-space distributions of electron bunches seeded with an external laser were done in order to study the impact of collective effects on seeded microbunches in free-electron lasers. When the collective effects of Coulomb forces in a drift space and coherent synchrotron radiation in a chicane are considered, velocity bunching of a seeded microbunch appears to be a viable alternative to compression with a magnetic chicane under high-gain harmonic generation seeding conditions. Measurements of these effects on seeded electron microbunches were performed with a rf deflecting structure and a dipole magnet which streak out the electron bunch for single-shot images of the longitudinal phase-space distribution. Particle tracking simulations in 3D predicted the compression dynamics of the seeded microbunches with collective effects