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.

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.

2014, Boll, R., Rouzée, A., Adolph, M., Anielski, D., Aquila, A., Bari, S., Bomme, C., Bostedt, C., Bozek, J.D., Chapman, H.N., Christensen, L., Coffee, R., Coppola, N., De, S., Decleva, P., Epp, S.W., Erk, B., Filsinger, F., Foucar, L., Gorkhover, T., Gumprecht, L., Hömke, A., Holmegaard, L., Johnsson, P., Kienitz, J.S., Kierspel, T., Krasniqi, F., Kühnel, K.-U., Maurer, J., Messerschmidt, M., Moshammer, R., Müller, N.L.M., Rudek, B., Savelyev, E., Schlichting, I., Schmidt, C., Scholz, F., Schorb, S., Schulz, J., Seltmann, J., Stener, M., Stern, S., Techert, S., Thøgersen, J., Trippel, S., Viefhaus, J., Vrakking, M., Stapelfeldt, H., Küpper, J., Ullrich, J., Rudenko, A., Rolles, D.

This paper gives an account of our progress towards performing femtosecond time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe setup combining optical lasers and an X-ray free-electron laser. We present results of two experiments aimed at measuring photoelectron angular distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C8H5F) and dissociating, laser-aligned 1,4-dibromobenzene (C6H4Br2) molecules and discuss them in the larger context of photoelectron diffraction on gas-phase molecules. We also show how the strong nanosecond laser pulse used for adiabatically laser-aligning the molecules influences the measured electron and ion spectra and angular distributions, and discuss how this may affect the outcome of future time-resolved photoelectron diffraction experiments.