Femtochemistry

A brief review of the developments leading to the advent of femtochemistry is presented, along with the major achievements that have marked the past fifteen years since the birth of the field

Ultrafast X-ray Spectroscopy of Haem Proteins

In this article we revisit our recent picosecond and femtosecond X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) experiments, probing the ultrafast electronic and geometric evolution of photoexcited haem proteins, namely ferrous Nitrosyl Myoglobin (MbNO) and ferric Cytochrome c (Cyt c). We show through these two examples, combined with results from ultrafast optical spectroscopy, the universal behavior of the excited state dynamics of ferric and ferrous complexes. Regardless of the type of ligand, its dissociation or lack thereof, or the metal oxidation state, the photoexcited system relaxes through a cascade of excited spin states leading to formation of a high spin state, which in the case of the haem is a domed porphyrin

Empirical rules of molecular photophysics in the light of ultrafast spectroscopy

The advent of ultrafast laser spectroscopy has allowed entirely new possibilities for the investigation of the ultrafast photophysics of inorganic metal-based molecular complexes. In this review we show different regimes where non-Kasha behavior shows up. We also demonstrate that while ultrafast intersystem crossing is a common observation in metal complexes, the ISC rates do not scale with the magnitude of the spin-orbit coupling constant. Structural dynamics and density of states play a crucial role in such ultrafast ISC processes, which are not limited to molecules containing heavy atoms

Exciton Control in a Room-Temperature Bulk Semiconductor with Coherent Strain Pulses

The coherent manipulation of excitons in bulk semiconductors via the lattice degrees of freedom is key to the development of acousto-optic and acousto-excitonic devices. Wide-bandgap transition metal oxides exhibit strongly bound excitons that are interesting for applications in the deep-ultraviolet, but their properties have remained elusive due to the lack of efficient generation and detection schemes in this spectral range. Here, we perform ultrafast broadband deep-ultraviolet spectroscopy on anatase TiO$_2$ single crystals at room temperature, and reveal a dramatic modulation of the exciton peak amplitude due to coherent acoustic phonons. This modulation is comparable to those of nanostructures where exciton-phonon coupling is enhanced by quantum confinement, and is accompanied by a giant exciton shift of 30-50 meV. We model these results by many-body perturbation theory and show that the deformation potential coupling within the nonlinear regime is the main mechanism for the generation and detection of the coherent acoustic phonons. Our findings pave the way to the design of exciton control schemes in the deep-ultraviolet with propagating strain pulses

Fluorescence and phosphorescence from individual C60_{60} molecules excited by local electron tunneling

Using the highly localized current of electrons tunneling through a double barrier Scanning Tunneling Microscope (STM) junction, we excite luminescence from a selected C$_{60}$ molecule in the surface layer of fullerene nanocrystals grown on an ultrathin NaCl film on Au(111). In the observed luminescence fluorescence and phosphorescence spectra, pure electronic as well as vibronically induced transitions of an individual C$_{60}$ molecule are identified, leading to unambiguous chemical recognition on the single-molecular scale

Giant exciton Mott density in anatase TiO2

Elucidating the carrier density at which strongly bound excitons dissociate into a plasma of uncorrelated electron-hole pairs is a central topic in the many-body physics of semiconductors. However, there is a lack of information on the high-density response of excitons absorbing in the near-to-mid ultraviolet, due to the absence of suitable experimental probes in this elusive spectral range. Here, we present a unique combination of many-body perturbation theory and state-of-the-art ultrafast broadband ultraviolet spectroscopy to unveil the interplay between the ultraviolet-absorbing two-dimensional excitons of anatase TiO$_2$ and a sea of electron-hole pairs. We discover that the critical density for the exciton Mott transition in this material is the highest ever reported in semiconductors. These results deepen our knowledge of the exciton Mott transition and pave the route toward the investigation of the exciton phase diagram in a variety of wide-gap insulators

Hydrophobicity with atomic resolution: Steady-state and ultrafast X-ray absorption and molecular dynamics studies

Static and time-resolved X-ray absorption spectroscopy (XAS) is used to probe the solvent shell structure around iodide and iodine. In particular, we characterize the changes observed upon electron abstraction of aqueous iodide, which reflects the transition from hydrophilic to hydrophobic solvation after impulsive electron abstraction from iodide. The static spectrum of aqueous iodide, which is analyzed using quantum mechanical/molecular mechanics (QM/MM) molecular dynamics (MD) simulations, indicates that the hydrogens of the closest water molecules point toward the iodide, as expected for hydrophilic solvation. In addition, these simulations demonstrate a small anisotropy in the solvent shell. Following electron abstraction, most of the water molecules move away from iodine, while one comes closer to form a complex with it that survives for 3-4 ps. This lifetime is governed by the reorganization of the main solvation shell, basically the time it takes for the water molecules to reform a hydrogen bond network in the hydrophobic solvation shel

Ultrafast X-ray spectroscopy of conical intersections

Ongoing developments in ultrafast X-ray sources offer powerful new means of probing the com- plex non-adiabatically coupled structural and electronic dynamics of photoexcited molecules. These non-Born-Oppenheimer effects are governed by general electronic degeneracies termed conical in- tersections which play a key role, analogous to that of a transition state, in the electronic-nuclear dynamics of excited molecules. Using high level ab initio quantum dynamics simulations, we studied time-resolved X-ray absorption and photoelectron spectroscopy (TRXAS and TRXPS, respectively) of the prototypical unsaturated organic chromophore, ethylene, following excitation to its S2 state. The TRXAS in particular is highly sensitive to all aspects of the ensuing dynamics. These X-ray spectroscopies provide a clear signature of the wavepacket dynamics near conical intersections, related to charge localization effects driven by the nuclear dynamics. Given the ubiquity of charge localization in excited state dynamics, we believe that ultrafast X-ray spectroscopies offer a unique and powerful route to the direct observation of dynamics around conical intersections.Comment: 5 pages, 4 figure