2014, McFarland, B.K., Berrah, N., Bostedt, C., Bozek, J., Bucksbaum, P.H., Castagna, J.C., Coffee, R.N., Cryan, J.P., Fang, L., Farrell, J.P., Feifel, R., Gaffney, K.J., Glownia, J.M., Martinez, T.J., Miyabe, S., Mucke, M., Murphy, B., Natan, A., Osipov, T., Petrovic, V.S., Schorb, S., Schultz, T., Spector, L.S., Swiggers, M., Tarantelli, F., Tenney, I., Wang, S., White, J.L., White, W., Gühr, M.

Free electron laser (FEL) based x-ray sources show great promise for use in ultrafast molecular studies due to the short pulse durations and site/element sensitivity in this spectral range. However, the self amplified spontaneous emission (SASE) process mostly used in FELs is intrinsically noisy resulting in highly fluctuating beam parameters. Additionally timing synchronization of optical and FEL sources adds delay jitter in pump-probe experiments. We show how we mitigate the effects of source noise for the case of ultrafast molecular spectroscopy of the nucleobase thymine. Using binning and resorting techniques allows us to increase time and spectral resolution. In addition, choosing observables independent of noisy beam parameters enhances the signal fidelity.

2017, Feifel, R., Eland, J.H.D., Carniato, S., Selles, P., Püttner, R., Koulentianos, D., Marchenko, T., Journel, L., Guillemin, R., Goldsztejn, G., Travnikova, O., Ismail, I., Miranda, B. Cunha de, Lago, A.F., Céolin, D., Lablanquie, P., Penent, F., Piancastelli, M.N., Simon, M.

Recent advances in X-ray instrumentation have made it possible to measure the spectra of an essentially unexplored class of electronic states associated with double inner-shell vacancies. Using the technique of single electron spectroscopy, spectra of states in CS2 and SF6 with a double hole in the K-shell and one electron exited to a normally unoccupied orbital have been obtained. The spectra are interpreted with the aid of a high-level theoretical model giving excellent agreement with the experiment. The results shed new light on the important distinction between direct and conjugate shake-up in a molecular context. In particular, systematic similarities and differences between pre-edge states near single core holes investigated in X-ray absorption spectra and the corresponding states near double core holes studied here are brought out.

2018-01-23, Busto, D., Barreau, L., Isinger, M., Turconi, M., Alexandridi, C., Harth, A., Zhong, S., Squibb, R.J., Kroon, D., Plogmaker, S., Miranda, M., Jiménez-Galán, Á., Argenti, L., Arnold, C.L., Feifel, R., Martín, F., Gisselbrecht, M., L’Huillier, A., Salières, P.

Autoionization, which results from the interference between direct photoionization and photoexcitation to a discrete state decaying to the continuum by configuration interaction, is a well known example of the important role of electron correlation in light–matter interaction. Information on this process can be obtained by studying the spectral, or equivalently, temporal complex amplitude of the ionized electron wave packet. Using an energy-resolved interferometric technique, we measure the spectral amplitude and phase of autoionized wave packets emitted via the sp2+ and sp3+ resonances in helium. These measurements allow us to reconstruct the corresponding temporal profiles by Fourier transform. In addition, applying various time–frequency representations, we observe the build-up of the wave packets in the continuum, monitor the instantaneous frequencies emitted at any time and disentangle the dynamics of the direct and resonant ionization channels.

2013, McFarland, B.K., Farrell, J.P., Berrah, N., Bostedt, C., Bozek, J., Bucksbaum, P.H., Coffee, R., Cryan, J., Fang, L., Feifel, R., Gaffney, K., Glownia, J., Martinez, T., Mucke, M., Murphy, B., Miyabe, S., Natan, A., Osipov, T., Petrovic, V., Schorb, S., Schultz, T., Spector, L., Tarantelli, F., Tenney, I., Wang, S., White, W., White, J., Gühr, M.

Nucleobases absorb strongly in the ultraviolet region, leading to molecular excitation into reactive states. The molecules avoid the photoreactions by funnelling the electronic energy into less reactive states on an ultrafast timescale via non-Born-Oppenheimer dynamics. Current theory on the nucleobase thymine discusses two conflicting pathways for the photoprotective dynamics. We present our first results of our free electron laser based UV-pump soft x-ray-probe study of the photoprotection mechanism of thymine. We use the high spatial sensitivity of the Auger electrons emitted after the soft x-ray pulse induced core ionization. Our transient spetra show two timescales on the order of 200 fs and 5 ps, in agreement with previous (all UV) ultrafast experiments. The timescales appear at different Auger kinetic energies which will help us to decipher the molecular dynamics.