Luca Longetti
Investigating molecular excitations with femtosecond time resolution is of pivotal importance to understand the out-of-equilibrium processes taking place in molecular systems upon light absorption. The photochemistry of solvated species is heavily determined by the early steps of the relaxation dynamics.
In this thesis, ultrafast electronic spectroscopies are exploited to study relaxation dynamics of photoexcited molecules in solution by probing the transient electronic structure.
Vacuum-ultraviolet (VUV) ultrashort monochromatized light pulses (HARMONIUM, 15-100 eV) are exploited for steady-state photoelectron spectroscopy (PES) and time-resolved PES in a pump-probe scheme. This is a powerful tool for investigating electronic structure and ultrafast processes of solvated molecules with high time (50fs) and energy (0.12eV) resolution.
PES retrieves binding energies of valence and core electronic states with elemental specificity and without the constraints of selection rules, and is here applied to liquids and solutions by means of a liquid micro-jet.
With the aim of extending this technique to a larger class of solutes, we demonstrate the capability to perform studies on volatile liquids such as organic solvents. PES spectra of gas phase and liquid phase aromatic compounds are presented and their valence orbitals are compared and identified. The solvation is discussed and the vertical ionization energies are reported.
This retrieves important parameters for electrochemistry and opens new perspectives towards PES studies of molecules in organic solvents.
Upon photoexcitation, molecules can undergo several relaxation processes such as nonradiative relaxation processes which include dissociation, internal conversion and intersystem crossing. Energy can also be released to the solvent environment by intermolecular processes such as vibrational energy transfer. All of these phenomena are reflected in a transient electronic configuration. PES can hence probe electronic and structural dynamics along the entire reaction coordinates.
The ferric trisoxalate complex represents an ideal coordination compound showing an intriguing cascade of processes upon photoexcitation. We have directly observed the metal photoreduction due to an instantaneous intramolecular charge transfer. We then suggest a branching between photodissociation and a back-electron transfer channels, which completes the picture of the compoundâs photochemistry. This experiment shows PES sensitivity to oxidation state changes, providing an interesting perspective for photoredox reaction studies in solution and for intra- and inter-molecular charge-transfer phenomena.
Vibrational wave packets (WP) are interesting observables to map non-adiabatic dynamics, relaxation pathways and coupling to solvent of the excited molecule. We have investigated molecular iodine in ethanol, which we have studied first by optical transient absorption spectroscopy using a white light continuum probe. The electronic absorption spectrum of I2 in ethanol is strongly modified compared to the case of other organic solvents, due to a strong coupling of covalent and ionic states. We have mapped the vibrational WP dynamics, in particular its trajectory and damping along the I-I bond axis. This study is intended to prepare for an ultrafast PES experiment aimed at linking the response of inner shells to the WP launched in the valence states. Preliminary steady-state PES spectra are reported and discussed.
EPFL2020
