Session P2: Focus Session: Time Resolved Spectroscopy with HHG and FEL

Ultra-fast Dynamics: Pump-probe experiments at Free Electron Lasers

One of the most exciting opportunities opened by Free Electron Lasers (FEL) is the feasibility of performing, for the first time, pump-probe experiments in the VUV, EUV and X-ray wave-length regimes with femtosecond time resolution. Here, a first light pulse (IR, EUV, VUV or X-ray) initiates dynamics, like a chemical reaction, a phase transition, spin-, orbital-, or charge-density waves in solids and a second pulse, impinging at a variable but well-defined time delay, probes the motion. In the talk a first series of such experiments, performed at the VUV-FEL in Hamburg, FLASH, the SCSS test facility in Japan as well as pioneering measurements at the LCLS X-ray FEL will be presented. At FLASH and SCSS the VUV-pulse has been split by a back-reflecting mirror that is cut into two halves. One of the pulses can then be delayed by moving the two half-mirrors with respect to each other reaching sub-femtosecond accuracy. In a demonstration experiment the vibrational wave-packet motion in deuterium molecular ions with a round-trip time of about 22 fs could be traced indicating a time-resolution of better that 10 fs. Moreover, the isomerization time in VUV-excited acetylene evolving into vinylidene cations proceeding within about 50 fs was measured for the first time, ending a 20 years controversial debate. Autocorrelation measurements at FLASH and SCSS showed a sharp peak in the non-linear autocorrelation trace with a FWHM in the order of the coherence length of the radiation (4 fs at FLASH and 10 fs at at SCSS). This has been explained by the statistical nature and the coherence properties of the FEL pulses pointing towards exciting possibilities to perform attosecond X-ray - X-ray pump-probe experiments at the LCLS. Here, using the CAMP instrument, first optical pump - X-ray probe experiments have been performed on aligned molecules, clusters and biological nano-crystals highlighting the rich future potential of these methods.   

Femtosecond time-resolved imaging of Rydberg atom emission from electronically excited helium nanodroplets

The relaxation dynamics of electronically excited helium nanodroplets are investigated using femtosecond time resolved extreme ultraviolet photoelectron- and ion-imaging spectroscopy. The novel time-domain measurements provide new insight into one of the most prominent droplet relaxation channels in which the excitation energy is removed from the cluster by emission of Rydberg atoms. Droplets consisting of $\sim 2 \times 10^{6}$ helium atoms are excited with 23.6~eV pump-pulses from a high- order harmonic generation (HHG) light source. Photoelectrons and ions are produced by subsequent ionization using 1.6~eV probe-pulses. Strong indications for a very fast ($<$120~fs) emission of 1s4p Rydberg atoms and a delayed ($\sim$200~fs) emission of 1s3d Rydberg atoms with significantly different kinetic energies are revealed. The findings are interpreted within a description of excited droplet states by perturbed atomic Rydberg states, leading to a close correlation between the energy and the location of the excitation. The relatively simple model yields surprisingly accurate predictions of the droplet absorption band structure and the emission dynamics of Rydberg fragments, providing a detailed physical picture of a previously proposed intraband relaxation mechanism.   

Time-resolved photoemission by attosecond streaking: extraction of time information

Attosecond streaking is one of the most fundamental processes in attosecond science allowing for a mapping of temporal information to the energy domain. We study attosecond streaking setups for measuring the release time of electrons in atomic photoemission [cf.~M. Schultze et al, Science 328, 1658 (2010)]. We show that on the single-particle level, the extracted time delays (phase shifts) contain timing (or spectral phase) information associated with the Eisenbud-Wigner-Smith time delay matrix of quantum scattering. However, this is only accessible if distortion effects by the streaking infrared field on the emission process are properly accounted for. We show that the ``time shifts'' due to the interaction between the outgoing electron and the combined Coulomb and IR laser field can be described classically. By contrast, we also find a strong initial state dependence of the apparent time delay, which is of quantum mechanical origin.   

Time-resolved $KLL$-Auger decay via transient x-ray bleaching in O$_2$

Time domain measurements are directly sensitive to the dynamic correlations responsible for Auger decay. The Auger process in molecules is more complicated than in atoms since the nuclear degrees of freedom strongly influence the valence corellations responsible for the various Auger. For instance, the Auger decay of strongly dissociative states like the $\pi^*$ antibonding excitations of oxygen complicates the interpretation of broad Auger features; is the feature broad due to rapid decay or due to very steep molecular potentials? To demonstrate time-domain spectroscopy in the x-ray regime, we measured transient x-ray beaching of core-excited O$_2$ with two x-ray pulses of $\sim5$~fs duration and 5--20fs relative delay. Both pulses were tuned to the $1s\rightarrow2p\pi^*$ resonance in O$_2$ such that upon pump excitation, the transition became dark to the probe pulse. The molecule remains dark until it refills the core-vacancy primarily via $KLL$-Auger decay. The duration of the bleaching reflects $\mbox{O}_2$ Auger decay directly in the time-domain, pushing time-domain molecular spectroscopy into the $KLL$ regime.   

Probing Floquet State Dynamics of a Strong Field Dressed He Atom Using Attosecond Pulse Trains

Atoms in a strong laser field can be well described by Floquet states with different Fourier components. We perform an experiment to investigate the quantum interference between Fourier components of Floquet states in the He atom using attosecond extreme-ultraviolet (XUV) and strong infra-red (IR) laser pulses. We use a configuration with XUV and two IR pulses (probe and driver), with one of the IR pulses (driver) phase-locked to the XUV pulse train. We then measure the He$^{+}$ yield as a function of time delay between the XUV and probe pulses. This configuration allows us to measure the transition between Floquet states due to change in probe IR intensity as the delay is varied. In addition we can also measure the relative phase between the XUV pulse and the driver IR which generates the XUV pulse. Finally, we show that by tuning the energy of the high harmonics in the XUV pulse we can control ionization using this quantum interference process.   

Probing autoionization and AC Stark shift with attosecond transient absorption spectroscopy

The natural timescale for electronic motion in atoms and molecules is the atomic unit (1 a.u. = 24 as). Using synchronized isolated attosecond pulses from Double Optical Gating and intense few-cycle near-infrared lasers, we probe the autoionization process in argon atoms and the AC Stark shift in bound states of helium. In both cases, the infrared laser is used to modify the atomic states in a controlled manner and the probing attosecond pulse reveals dynamics evolving on both attosecond and femtosecond time scales. We experimentally demonstrate sub-laser-cycle dynamics in the AC stark shift for the first time.   

The physics of laser-based coherent x-ray generation-- attosecond science meets nonlinear optics meets nanotechnology

The generation of coherent soft x-ray light through high-order harmonic upconversion of fentosecond laser pulses has proven to be a topic of sustained and increasing interest, by virtual of the fact that it relates a relatively new regime of AMO physics to the prospect of broad application in science and technology. In the high harmonic generation (HHG) process, intense laser light interacting with an atom or molecule results in coherent light emission in the EUV and x-ray regions of the spectrum through the rescattering process. The basic physics of the emission process was understood relatively quickly after its experimental observation; more challenging has been understanding how to best take advantage of HHG as a nonlinear optical process where not-only is the nonlinear response of the conversion medium nonperturbative, but it is non-instantaneous, and the medium is strongly absorbing, dispersive, and dynamically changing. Remarkably, despite these issues the HHG process allows us for the first time to implement lab-scale ``x-ray laser'' sources that have broad application. In this talk I will present an overview of the nonlinear optics of HHG. I will also discuss some of the science that can be done with this source, in areas ranging from AMO science to nanoscale imaging and materials physics. \\[4pt] [1] T. Popmintchev et al., Nature Photonics 4, 822 (2010). \\[0pt] [2] T. Popmintchev et al., PNAS 106, 10516 (2009). \\[0pt] [3] O. Cohen et al., Physical Review Letters 99, 053902 (2007). \\[0pt] [4] X. H. Zhang et al., Nature Physics 3, 270 (2007). \\[0pt] [5] M. E. Siemens et al., Nature Materials 9, 26 (2010). \\[0pt] [6] C. La-O-Vorakiat et al., Physical Review Letters 103, 257402/1 (2009). \\[0pt] [7] R. L. Sandberg et al., PNAS 105, 24 (2008). \\[0pt] [8] L. Miaja-Avila et al., Physical Review Letters 101, 046101 (2008).