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.

Solvation dynamics at liquid/metal-IV oxide interfaces

Erwin Portuondo Campa

This thesis presents a comparative study of the ultrafast solvation dynamics of liquids in bulk conditions, and at the interfaces of metal-IV oxides (specifically TiO2 and ZrO2), using ultrafast spectroscopy techniques. In the first part of the thesis we report on results of photon-echo peak-shift and pump super-continuum probe spectroscopy, that provided complementary ways for characterizing the solvation function of a dye, Eosin-Y, in aqueous solution. The solvation dynamics of the dye was studied both, dissolved in an aqueous buffer, and adsorbed to the surface of ZrO2 nanoparticles that were in turn, suspended in an aqueous medium. The results from both techniques indicate that only minor changes between the bulk and interfacial environments, are manifested in the solvation dynamics of Eosin-Y on the fastest timescales of the process (sub-picosecond). On the other hand, on the longest timescales ( >1 ps), we obtained consistent evidences for a slower solvation dynamics at the interface than in bulk water. From these results we concluded, that the presence of the ZrO2 surface affects the dynamics of librational motions and intermolecular vibrations of the hydrogen-bond network of water, only in a very narrow region of no more than 0.5 nm around the metal oxide. The long time behavior, on the other hand, was explained as due to hindered translational diffusion dynamics of the solvent molecules in the proximity of the interface. In order to overcome the limitations inherent to using dyes as probe targets of the solvation dynamics at interfaces, in the second part of this thesis we performed time-resolved Optical Kerr Effect experiments on liquids in the pores of nano-structured films of ZrO2. Three liquids were investigated: acetonitrile (an aprotic polar solvent), cyclohexane (apolar) and water (polar, strongly H-bond networked solvent). In all the cases, significantly slower dynamics were detected in the pores, as compared to the bulk behavior of each solvent. The most significant case was that of water, where the characteristic time constants of the dynamics in the bulk, were increased by a factor of 3 in the film. The results are discussed considering the possible physical models that determine the dynamics of solvation at the interface. In addition to this, using the same experimental setup, we carried out a detailed characterization of the non-resonant nonlinear optical response of nano-structured films of TiO2, by means of Transient Lensing, Cross Phase Modulation measurements, and Optical Kerr Effect spectroscopy. This investigation led us to define future experimental developments, that will allow the extension of our present investigations of solvent dynamics at the surface of ZrO2, to the interfaces of TiO2, of special relevance in several applications.

EPFL2006

Design and implementation of an optimal laser pulse front tilting scheme for ultrafast electron diffraction in reflection geometry with high temporal resolution

Fabrizio Carbone, Rajeswari Jayaraman, Giulia Fulvia Mancini, Malte Oppermann, Francesco Pennacchio, Giovanni Maria Vanacore

Ultrafast electron diffraction is a powerful technique to investigate out-of-equilibrium atomic dynamics in solids with high temporal resolution. When diffraction is performed in reflection geometry, the main limitation is the mismatch in group velocity between the overlapping pump light and the electron probe pulses, which affects the overall temporal resolution of the experiment. A solution already available in the literature involved pulse front tilt of the pump beam at the sample, providing a sub-picosecond time resolution. However, in the reported optical scheme, the tilted pulse is characterized by a temporal chirp of about 1 ps at 1mm away from the centre of the beam, which limits the investigation of surface dynamics in large crystals. In this paper, we propose an optimal tilting scheme designed for a radio-frequency-compressed ultrafast electron diffraction setup working in reflection geometry with 30 keV electron pulses containing up to 10(5) electrons/pulse. To characterize our scheme, we performed optical cross-correlation measurements, obtaining an average temporal width of the tilted pulse lower than 250 fs. The calibration of the electron-laser temporal overlap was obtained by monitoring the spatial profile of the electron beam when interacting with the plasma optically induced at the apex of a copper needle (plasma lensing effect). Finally, we report the first time-resolved results obtained on graphite, where the electron-phonon coupling dynamics is observed, showing an overall temporal resolution in the sub-500 fs regime. The successful implementation of this configuration opens the way to directly probe structural dynamics of low-dimensional systems in the sub-picosecond regime, with pulsed electrons. (C) 2017 Author(s).

Amer Inst Physics2017

Ultrafast structural dynamics in electronically excited many-body systems

Luigi Bonacina

This thesis reports on results of three different experiments of photo-induced structural dynamics in the condensed phase, investigated by time-resolved pump-probe spectroscopy with femtosecond time-resolution. In the first part, we address the ultrafast dynamics of a quantum solid : crystalline hydrogen. This is accomplished by optical excitation of a dopant molecule, Nitric Oxide (NO), to a large orbital Rydberg state, which leads to a bubble-like expansion of the species surrounding the impurity. The dynamics is directly inferred from the time-resolved data, and compared with the results of molecular dynamics simulations. We report the presence of three time-scales in the structural relaxation mechanism: the first 200 fs are associated with the ultrafast inertial expansion of the first shell of lattice neighbors of NO. During the successive 0.6 ps, as the interactions between the molecules of the first and of the successive shells increase, we observe a progressive slowing-down of the bubble expansion. The third timescale (~ 10 ps) is interpreted as a slow structural re-organization around the impurity center. No differences were observed between the dynamics of normal- and para-hydrogen crystals, justifying the simplified model we use to interpret the data, which ignores all internal degrees of freedom of the host molecules. The molecular dynamics simulations reproduce fairly well the static and dynamic features of the experiment. In line with the measurements, they indicate that the quantum nature of the host medium plays no role in the initial ultrafast expansion of the bubble. In the second part, we present the results of our study on the photo-physics of triangular-shaped silver nanoparticles upon intraband excitation of the conduction electrons. The picosecond dynamics is dominated by periodic shifts of the surface plasmon resonance, associated with the size oscillations of the particles, triggered by impulsive lattice heating by the laser pulse. The oscillation period compares very well with the lowest totally symmetric vibrational frequency of a triangular-plate, which we calculated improving an existing elastodynamic model. We propose an explanation for the unusual phase behavior of the oscillations, based upon the non-spherical shape, and size-inhomogeneity of the sample. Taking into account these effects, we are able to reproduce spectrally and temporally our data. In the last part, we present a comparative study of the ligand dynamics in heme proteins. We studied the photo-induced spectroscopic changes in the ferric CN complexes of Myoglobin and Hemoglobin I upon photo-excitation of the porphyrin ring to a low-lying electronic state (Soret), monitoring the UV-visible region of the Soret band, and the mid-infrared region of the fundamental C=N vibrational stretch. The transient response in the UV-visible spectral region does not depend on the heme pocket environment, and is very similar to that known for ferrous proteins. The infrared data on the MbC=N stretch vibration provides a direct measure for the return of population to the ligated electronic (and vibrational) ground state with a 3 ps time constant. In addition, the CN stretch frequency is sensitive to the excitation of low frequency heme modes, and yields independent information about vibrational cooling, which occurs on the same timescale. The similarity between ferrous and ferric hemes rules out the charge transfer processes commonly invoked to explain the ligand dissociation in the former.

EPFL2004