Supplementary information

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1 Supplementary information Binding energies, lifetimes, and implications of bulk and interface solvated electrons in water Katrin R. Siefermann 1, Yaxing Liu 1, Evgeny Lugovoy 2, Oliver Link 1, Manfred Faubel 3, Udo Buck 3, Bernd Winter 4 & Bernd Abel 1,2* 1) Institut für Physikalische Chemie der Universität Göttingen, Tammannstrasse 6, Göttingen, Germany 2) Ostwald-Institut für Physikalische und Theoretische Chemie der Universität Leipzig, Linné- Strasse 2, D Leipzig, Germany, 3) MPI für Dynamik und Selbstorganisation, Bunsenstrasse 10, Göttingen, Germany 4) Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, Berlin, Germany Schematics of ultrafast liquid microjet photoelectron spectroscopy Fig. S1 Details of the liquid microjet and the geometry of the UV-pump (267 nm) and extreme UV-probe (EUV) pulses. Also displayed is an enlarged section of the water beam illustrating the limited escape depth of photoelectrons and thus the reason for the techniques high surface sensitivity. nature chemistry 1

2 Fig. S1 schematically shows the arrangement of the liquid microjet in front of the time-of-flight photoelectron spectrometer. The UV-pump and EUV-probe pulses overlap in time and space on the liquid water jet in front of a 100 µm skimmer which is forming the entrance to the photoelectron spectrometer. The skimmer is differentially pumped, i.e. separating the microjet-chamber (with a pressure of ~10-5 mbar) from the spectrometer area with a pressure of about 10-7 mbar. The distance between the microjet and the skimmer typically amounts 500 µm. Also displayed is an enlarged section of the water beam illustrating the techniques high surface sensitivity. This surface sensitivity is due to the limited electron mean free path (IMFP) of photoelectrons in matter. Photoelectrons with kinetic energies of about 30 ev lie within the minimum of the IMFP curve and their escape depth corresponds to approximately 3-5 molecular layers. Experimental Setup Figure S2 shows the scheme of our experimental setup, which has been described previously (1, 2). The uncompressed output (800 nm, 1 khz, 1.45 W) of the commercial Ti:sapphire laser system Hurricane (SPECTRA PHYSICS) (1) is directly coupled into an external Ti:sapphire amplification unit pumped by a Nd:YAG-laser (2) (CLARK-MXR, Model ORC1000, 532 nm, 1 khz, 10W). Subsequent pulse compression leads to 100 fs short laser pulses with an energy of 2.5 mj/pulse. A beamsplitter (4) reflects about two thirds of this energy (1.7 mj) and a lens (5) focuses the reflected pulses into a laser drilled closed-end capillary (6) located in the first vacuum chamber. Argon at a pressure of mbar perfuses this capillary. Focusing the light (intensity in the focus of 2x10 14 W/cm 2 ) into the argon atmosphere results in nonlinear conversion of the fundamental radiation and thus in the generation of High Harmonic radiation (3-5). An aluminium filter (7) (LUXEL corporation, 150 nm thickness) blocks the fundamental radiation and simultaneously serves as a pressure shield between the High Harmonic generation chamber with a pressure of 10-2 mbar and the high vacuum (10-7 mbar) chamber containing the grating. nature chemistry 2

3 Fig. S2 Overview of the experimental setup employed in the present studies. (1) Ti:sappire laser, (2) Nd:YAGlaser, (3) multipass amplifier and pulse compressor, (4) beamsplitter, (5) focusing lens, (6) capillary with argon, (7) aluminium filter, (8) toroidal grating, (9) delay stage, (10) third harmonic generating unit, (11) parabolic mirror, (12) quartz nozzle, (13) time-of- flight photoelectron spectrometer. Pos. (6-12) are located in vacuum. We use a toroidal holographic grating (JOBIN YVON, working range from 34 ev to 103 ev (12-36 nm), 600 lines/mm) to disperse the different harmonics and to select one particular harmonic. In the present experiment the 25th harmonic of 800 nm with a photon energy of 38.7 ev (32.1 nm) was employed. The spacing of the grating grooves is computer optimized to obtain a point-to-point image with a minimum of astigmatism and coma. Consequently the grooves are neither equidistant nor parallel which allows to avoid the Rowlands circle rule (usual for classical concave dispersion elements) and to realize a very simple monochromator scheme where only the grating itself rotates around the axis. The grating focuses the 25th harmonic, which is used as the probe pulse in the present experiment, down to a spot size of about 150 m. The drawback of the simple grating unit is the length of the EUV pulses being on the order of 400 fs. A wavelength dispersion with a pair of gratings and/or dielectric mirrors is planned for the near future. The beam splitter (4) transmits a fraction of the fundamental radiation (~ 0.6 µj), which is frequency nature chemistry 3

4 tripled and used as pump-pulses (a -barium borate (BBO) crystal for frequency doubling and a second BBO crystal for subsequent mixing with residual 800 nm is employed (10)). We control the pump-pulse intensities between 5-50 GW/cm 2 to match the conditions required for the present experiment. A parabolic mirror (11) weakly focuses the pump-pulses onto the liquid jet. The pump and probe pulses are nearly collinearly overlapped in space and their polarization vectors are parallel. The delay stage (9) allows for setting the delay between the pump and probe pulse. Temporal and spatial overlap of UV and EUV pulses We analyze the impact of the 267 nm pump pulse on the water beam and in turn on the photoelectron spectra probed by the EUV pulse to determine the exact overlap of the pump and probe pulses in space and time. The 267 nm pump pulse causes line shifts and deformations in the spectrum if applied with a pulse energy of about 10 µj or higher. We attribute these features to plasma formation and charging effects on the water beam. For the experiment itself, we decrease the the pump pulse energy below 3 µj in order to avoid these phenomena. Liquid microjet in vacuum The liquid microjet technique first introduced by Faubel is described in detail in our previous works (6, 7). We generate the microjet by pressing the solvent under investigation with an HPLC-pump at 20 bar (Economy 2/ED, TECHLAB INSTRUMENTS) through a quartz glass nozzle (12). The nozzles typically have diameters of m and the flux rate amounts 0.3 to 0.5 ml/min. Under carefully chosen conditions, the laminar jet has a temperature of 278 K and remains continuous for about 2-3 mm after the nozzle exit. It then breaks up into a stream of droplets which is frozen out in a liquid nitrogen cooling trap. We use an additional cooling trap and a powerful turbo-molecular pump (1500 l/s) to limit the pressure in the liquid beam chamber to 10-5 mbar. The liquid beam is stable and operating for several hours. nature chemistry 4

5 Time-of-flight photoelectron spectrometer A differentially pumped 100 µm skimmer at a distance of mm from the microjet samples the photoelectrons and the connected linear time-of-flight electron spectrometer (13) (KAESDORF) measures their kinetic energy. The drift tube length of the spectrometer is 0.87 m and we determined an energy resolution of 0.25 ev for our experimental setup. We record photoelectron spectra after each laser shot at a repetition rate of 1 khz and accumulate them over the time required to obtain a spectrum with good signal-to-noise ratio. In the present case, the acquisition times range from about 2000 s to s in order to account for the weak signals. We convert the time-of-flight data into kinetic energies of the photoelectrons and in turn into vertical binding energies. Relative photoemission line intensities Figure S3 gives an overview about the relative photoelectron signal intensities of the bulk-solvated electron and other photoemission lines of the system. The EUV-probe pulse with a photon energy of 38.7 ev (32 nm) is delayed by 100 ps with respect to the 267-nm pump pulse. We use here the term vertical binding energy (VBE) being the energy required to remove an electron from a bound state while maintaining the initial geometry of the system. Also the term vertical detachment energy (VDE) is widely used in this context. The spectrum displays the photoemission lines of gas-phase and liquid-phase water (VBE = 9-20 ev), of the [Fe(CN) 6 ] 3- (VBE = 7.6 ev) and the [Fe(CN) 6 ] 4- (VBE = 6.2 ev) complexes as well as the photoemission signal of the bulk solvated electron (VBE = 3.3 ev). The inset shows the spectrum in the VBE range between 0 and 5 ev in more detail (linear scale). Note that the photoemission signal between 2 and 4 ev - which we attribute to the bulk solvated electron - is only present if the 267-nm pump pulse precedes the EUV probe pulse. In addition, the signature of the [Fe(CN) 6 ] 3- complex at about 7.6 ev binding energy is also only present if the pump pulse precedes the probe pulse and is a direct consequence of the ejection of an electron from the [Fe(CN) 6 ] 4- complex after the absorption of a 267-nm photon. nature chemistry 5

6 Fig. S3 1-photon-pump/EUV-probe photoelectron spectrum (logarithmic scale) from an aqueous K 4 [Fe(CN) 6 ] solution (0.5 M). The EUV probe pulse is delayed by 100 ps with respect to the 267-nm pump pulse. The spectrum shows the photoemission lines of gas-phase and liquid-phase water molecules (VBE = 9-20 ev), of the [Fe(CN) 6 ] 3- (VBE = 7.6 ev) and the [Fe(CN) 6 ] 4- (VBE = 6.2 ev) complexes, as well as the photoemission signal of the bulk solvated electron (VBE = 3.3 ev). The inset shows the spectrum in the VBE range between 0 and 4.5 ev in more detail (linear scale). Note: This photoemission spectrum (in the range between 0 and 4.5 ev) is only present if the 267 nm generation pulse precedes the EUV analysis pulse. It does not change significantly for pump-probe delays between 100 ps and 750 ps. Sample preparation We prepare all samples with doubly destilled H 2 O. For the aqueous K 4 [Fe(CN) 6 ] solution (0.5 M) we use potassium hexacyanoferrate(ii) trihydrate from Sigma Aldrich (ReagentPlus TM, 99%). In the case of the pure water experiments, we use a 5 mm NaCl (Scharlau, 99,8 %) solution. The microjet technique requires a low concentration of ions to compensate for charging effects originating from the quartz nozzle (1, 2, 8). nature chemistry 6

7 References 1. O. Link et al., Faraday Discussions 141, 67 (2009). 2. O. Link et al., Applied Physics A, 96, (2009). 3. X. F. Li, A. Lhuillier, M. Ferray, L. A. Lompre, G. Mainfray, Physical Review A 39, 5751 (1989). 4. P. Agostini, L. F. DiMauro, Reports On Progress In Physics 67, 813 (2004). 5. E. Seres, J. Seres, F. Krausz, C. Spielmann, Physical Review Letters 92, (2004). 6. M. Faubel, S. Schlemmer, J. P. Toennies, Zeitschrift fü Physik D -Atoms Molecules And Clusters 10, 269 (1988). 7. M. Faubel, T. Kisters, Nature 339, 527 (1989). 8. B. Winter, M. Faubel, Chemical Reviews 106, 1176 (2006). nature chemistry 7