Infoscience

Thesis

Ultrafast Photoelectron Spectroscopy of Liquid Samples

This thesis focuses on the commissioning and characterization of HARMONIUM, an ultrafast tunable EUV source based on high-harmonic generation for performing photoelectron spectroscopy of liquids. In ultrafast photoelectron spectroscopy a short pump pulse induces an excited state in a molecular system while a time-delayed short pulse interrogates the electronic distribution of the transient species. This method is an important tool in photochemistry as it combines features like element specificity and surface sensitivity with the absence of forbidden transitions and the ability to retrieve absolute binding energies. Ultrafast photoelectron spectroscopy of liquid media became possible by combining efficient turbomolecular pumping with a liquid micro jet in vacuum from which collisionless evaporation takes place. In HARMONIUM, it is possible to choose between high energy (up to 0.16 eV) or high temporal (up to 40 fs) resolution modes by selecting one of the 4 different gratings in a pulse-preserving single-grating EUV monochromator. High photon flux ranging from $2.3\cdot10^{11}$ photons/s at 36 eV to $1.5\cdot10^{8}$ photons/s at 102 eV has been demonstrated. Tunability of the laser repetition rate allows distributing the laser power among the maximum number of pulses per second for a given EUV photon energy, thereby minimizing EUV-induced space charges without compromising the acquisition time of photoelectron spectra. An ellipsoidal mirror in a 4:1 configuration yields an EUV focal spot size similar to the $\sim$ 20 $\mu$m diameter of the liquid jet. Apart from its implementation on photoelectron spectroscopy of liquid samples, the versatility of this beamline makes it suitable to study different states of matter. To this end, HARMONIUM will be coupled to a molecular beam and an angle-resolved photoemission spectroscopy (ARPES) end stations. A photoelectron spectrum of a liquid sample is composed of electrons emitted from the liquid surface and those emitted from the surrounding evaporated molecules. In this thesis, a systematic approach for separating the gas contribution from the liquid+gas photoelectron spectrum is presented. This procedure opens the possibility to acquire photoelectron spectra of dilute aqueous media under conditions of moderate electrokinetic charging. In this regime, preliminary photoelectron measurements of protonated water in the absence of counterions suggest that changes in the water orbitals take place as a function of the hydrogen bond coordination. The laser assisted photoelectric effect in liquids is fully described for the first time. Convolution of its retrieved response function with the unpumped photoelectron spectrum reproduces the pumped photoelectron spectrum. This contribution is important as usually the laser assisted photoelectric effect masks photoinduced dynamics transients in time-resolved photoelectron spectra around the region of zero time delay. For the first time ultrafast photoelectron spectroscopy is used to temporally resolve photo induced dynamics in an aqueous metal complex. Impulsive photoreduction of the iron center followed by ultrafast oxidation in $\sim$ 550 fs was shown to take place upon excitation of the ligand-to-metal charge-transfer transition of aqueous ferricyanide solution.

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