Solvation dynamics at liquid/metal-IV oxide interfaces

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.

Ultrafast Photoelectron Spectroscopy of Liquid Samples

José Javier Pancracio Ojeda Andara

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

\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.

EPFL2016

Effect of the Excitation Wavelength on the Ultrafast Charge Recombination Dynamics of Donor−Acceptor Complexes in Polar Solvents

Natalie Renuka Banerji, Eric Vauthey

The effect of the excitation wavelength on the charge recombination (CR) dynamics of several donor−acceptor complexes (DACs) composed of benzene derivatives as donors and of tetracyanoethylene or pyromellitic dianhydride as acceptors has been investigated in polar solvents using ultrafast time-resolved spectroscopy. Three different wavelength effects have been observed. (1) With complexes exhibiting two well-separated charge-transfer bands, the CR dynamics was found to be slower by a factor of about 1.5 upon excitation in the high-energy band. This effect was measured in both fast and slow relaxing solvents and was discussed in terms of different DAC geometries. (2) When the CR is faster than diffusive solvation, a slowing down of the CR with increasing excitation wavelength accompanied by an increase of the nonexponential character of the dynamics was measured. This effect appears only when exciting on the red edge of the charge-transfer absorption band. (3) When the driving force for CR is small, both nonequilibrium (hot) and thermally activated CR pathways can be operative. The results obtained with such a complex indicate that the relative contribution of these two paths depends on the excitation wavelength.

2005

Real-time observation of the charge transfer to solvent dynamics

Andrea Cannizzo, Majed Chergui

Intermolecular electron-transfer reactions have a crucial role in biology, solution chemistry and electrochemistry. The first step of such reactions is the expulsion of the electron to the solvent, whose mechanism is determined by the structure and dynamical response of the latter. Here we visualize the electron transfer to water using ultrafast fluorescence spectroscopy with polychromatic detection from the ultraviolet to the visible region, upon photo-excitation of the so-called charge transfer to solvent states of aqueous iodide. The initial emission is short lived (similar to 60 fs) and it relaxes to a broad distribution of lower-energy charge transfer to solvent states upon rearrangement of the solvent cage. This distribution reflects the inhomogeneous character of the solvent cage around iodide. Electron ejection occurs from the relaxed charge transfer to solvent states with lifetimes of 100-400 fs that increase with decreasing emission energy.

Nature Publishing Group2013

Ultrafast Photoelectron Spectroscopy of Liquid Samples

José Javier Pancracio Ojeda Andara

2.3\cdot10^{11}

photons/s at 36 eV to

1.5\cdot10^{8}

\sim

\mu

\sim

550 fs was shown to take place upon excitation of the ligand-to-metal charge-transfer transition of aqueous ferricyanide solution.

EPFL2016