Carbon K-edge x-ray emission spectroscopy of gas phase ethylenic molecules

We report on the C K-edge x-ray absorption spectra and the resonant (RXES) and non-resonant (NXES) x-ray emission spectra of ethylene, allene and butadiene in the gas phase. The RXES and NXES show clear differences for the different molecules. Overall both types of spectra are more structured for ethylene and allene, than for butadiene. Using density functional theory-restricted open shell configuration interaction single calculations, we simulate the spectra with remarkable agreement with the experiment. We identify the spectral features as being due to transitions involving localised 1s orbitals. For allene, there are distinct spectral bands that reflect transitions predominantly from either the central or terminal carbon atoms. These results are discussed in the context of ultrafast x-ray studies aimed at detecting the passage through conical intersections in polyatomic molecules.

Reduction of Mn19 Coordination Clusters on a Gold Surface

Harald Brune, Jan Gui-Hyon Dreiser, Stefano Rusponi, Christian Wäckerlin

The magnetic properties of a Mn-19 coordination cluster equipped with methylmercapto substituents on the organic ligands, [(Mn12Mn7II)-Mn-III(mu(4)-O)(8)(mu(3)-Cl)(7.7)(mu(3)-OMe)(0.3)(HLSMe)(12)(MeOH)(6)]Cl-2 center dot 27MeCN (Mn-19(SMe)) (H3LSMe = 2,6-bis(hydroxymethyl)-4-mercaptomethylphenol) deposited on Au(111) surfaces from solution, have been investigated by X-ray absorption spectroscopy and X-ray magnetic circular dichroism. The data reveal that in the submonolayer regime the molecules contain only divalent MnII in contrast to the presence of Mn-II and Mn-III ions in the powder sample. Brillouin function fits to the field-dependent magnetization suggest that the total spin ground state in the submonolayer is much lower than S-TOT = 83/2 of the pristine molecules. These findings suggest that significant changes of the electronic structure, molecular geometry, and intramolecular exchange coupling take place upon surface deposition. A sample with coverage of 2-3 monolayers shows the presence of Mn-III, suggesting that a decoupling layer could stabilize the Mn-19 core on a metallic surface.

Amer Chemical Soc2015

Competing Interactions in the Self-Assembly of NC-Ph-3-CN Molecules on Cu(111)

Harald Brune, Giulia Elisabetta Pacchioni, Marina Pivetta

We report on low-temperature scanning tunneling microscopy and spectroscopy measurements on NC-Ph-3-CN molecules adsorbed at 300 K on a Cu(111) surface. Upon cooling, the molecules form chain and honeycomb structures, incorporating Cu adatoms supplied by the substrate as metal linkers. In these assemblies, the molecules align along two main directions, with a relative abundance that depends on the coordination number and on the bond length. We show spectroscopic data about the unoccupied molecular orbitals and investigate the patterns obtained by depositing different amounts of molecules. Comparison of these results with the ones obtained for NC-Ph-5-CN molecules on the same substrate enables us to establish a hierarchy of the different interaction forces at work in the system.

American Chemical Society2015

Ultrafast Photoelectron and Optical Spectroscopies of Photoinduced Processes in Molecular Systems in Solution

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

Ultrafast Photoelectron and Optical Spectroscopies of Photoinduced Processes in Molecular Systems in Solution

Luca Longetti

EPFL2020