Emerging photon technologies for chemical dynamics

Efficient (∼10%) generation of vacuum ultraviolet femtosecond pulses via four-wave mixing in hollow-core fibers

We report the generation of the fifth harmonic of Ti:sapphire, at 160 nm, with more than 4 µJ of pulse energy and a pulse length of 37 fs with a 1 kHz repetition rate. The vacuum ultraviolet pulses are produced using four-wave difference frequency mixing in a He-filled stretched hollow-core fiber, driven by a pump at 267 nm and seeded at 800 nm. Guided by simulations using Luna.jl, we are able to optimize the process carefully. The result is a conversion efficiency of ∼10% from the 267 nm pump beam.

Toward time-resolved laser T-jump/X-ray probe spectroscopy in aqueous solutions

Most chemical and biochemical reactions in nature and in industrial processes are driven by thermal effects that bring the reactants above the energy barrier for reaction. In aqueous solutions, this process can also be triggered by the laser driven temperature jump (T-jump) method, in which the water vibrational (stretch, bend, or combination) modes are excited by a short laser pulse, leading to a temperature increase in the irradiated volume within a few picoseconds. The combination of the laser T-jump with X-ray spectroscopic probes would add element-specificity as well as sensitivity to the structure, the oxidation state, and the spin state of the intermediates of reactions. Here, we present preliminary results of a near infrared pump/X-ray absorption spectroscopy probe to study the ligand exchange of an octahedral aqueous Cobalt complex, which is known to pass through intermediate steps yielding tetrahedral chlorinated as final species. The structural changes of the chemical reaction are monitored with great sensitivity, even in the presence of a mild local increase in temperature. This work opens perspectives for the study of non-light-driven reactions using time-resolved X-ray spectroscopic methods.

Scientific instruments for soft X-ray photon-in/photon-out spectroscopy on the PAL-XFEL

An overview is given of the soft X-ray photon-in/photon-out instruments on the free-electron laser (FEL) beamline at the Pohang Accelerator Laboratory, and selected commissioning results are presented. The FEL beamline provides a photon energy of 270 to 1200 eV, with an energy bandwidth of 0.44%, an energy of 200 µJ per pulse and a pulse width of <50 fs (full width at half-maximum). The estimated total time resolution between optical laser and X-ray pulses is <100 fs. Instruments for X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) have been set up. X-ray magnetic circular dichroism spectra for a Co/Pt multilayer film and RIXS spectra for α-Fe₂O₃(100) have been obtained and the performance of the spectrometer has been evaluated.

Time-Resolved X-ray Absorption Spectroscopy in (Photo)Electrochemistry

This minireview aims at providing a complete survey concerning the use of X-ray absorption spectroscopy (XAS) for time-resolved studies of electrochemical and photoelectrochemical phenomena. We will see that time resolution can range from the femto-picosecond to the second (or more) scale and that this joins the valuable throughput typical of XAS, which allows for determining the oxidation state of the investigated element, together with its local structure. We will analyze four different techniques that use different approaches to exploit the in real time capabilities of XAS. These are quick-XAS, energy dispersive XAS, pump & probe XAS and fixed-energy X-ray absorption voltammetry. In the conclusions, we will analyze possible future perspectives for these techniques.

Perspective: Opportunities for ultrafast science at SwissFEL

We present the main specifications of the newly constructed Swiss Free Electron Laser, SwissFEL, and explore its potential impact on ultrafast science. In light of recent achievements at current X-ray free electron lasers, we discuss the potential territory for new scientific breakthroughs offered by SwissFEL in Chemistry, Biology, and Materials Science, as well as nonlinear X-ray science.

Time-resolved X-ray spectroscopies of chemical systems: New perspectives

The past 3-5 years have witnessed a dramatic increase in the number of time-resolved X-ray spectroscopic studies, mainly driven by novel technical and methodological developments. The latter include (i) the high repetition rate optical pump/X-ray probe studies, which have greatly boosted the signal-to-noise ratio for picosecond (ps) X-ray absorption spectroscopy studies, while enabling ps X-ray emission spectroscopy (XES) at synchrotrons; (ii) the X-ray free electron lasers (XFELs) are a game changer and have allowed the first femtosecond (fs) XES and resonant inelastic X-ray scattering experiments to be carried out; (iii) XFELs are also opening the road to the development of non-linear X-ray methods. In this perspective, I will mainly focus on the most recent technical developments and briefly address some examples of scientific questions that have been addressed thanks to them. I will look at the novel opportunities in the horizon.

Beyond structure: ultrafast X-ray absorption spectroscopy as a probe of non-adiabatic wavepacket dynamics

The excited state non-adiabatic dynamics of polyatomic molecules, leading to the coupling of structural and electronic dynamics, is a fundamentally important yet challenging problem for both experiment and theory. Ongoing developments in ultrafast extreme vacuum ultraviolet (XUV) and soft X-ray sources present new probes of coupled electronic-structural dynamics because of their novel and desirable characteristics. As one example, inner-shell spectroscopy offers localized, atom-specific probes of evolving electronic structure and bonding (via chemical shifts). In this work, we present the first on-the-fly ultrafast X-ray time-resolved absorption spectrum simulations of excited state wavepacket dynamics: photo-excited ethylene. This was achieved by coupling the ab initio multiple spawning (AIMS) method, employing on-the-fly dynamics simulations, with high-level algebraic diagrammatic construction (ADC) X-ray absorption cross-section calculations. Using the excited state dynamics of ethylene as a test case, we assessed the ability of X-ray absorption spectroscopy to project out the electronic character of complex wavepacket dynamics, and evaluated the sensitivity of the calculated spectra to large amplitude nuclear motion. In particular, we demonstrate the pronounced sensitivity of the pre-edge region of the X-ray absorption spectrum to the electronic and structural evolution of the excited-state wavepacket. We conclude that ultrafast time-resolved X-ray absorption spectroscopy may become a powerful tool in the interrogation of excited state non-adiabatic molecular dynamics.

Quantitative prediction of photoluminescence quantum yields of phosphors from first principles

The first quantitative prediction of the photoluminescence quantum yields (PLQY) of a series of blue-to-green Ir(iii) complexes is presented.

Optimizing the photoluminescence quantum yields of Ir(iii) complexes is the key to their application as phosphors in organic light-emitting diodes (OLEDs). This work demonstrates for the first time that quantitative predictions of photoluminescence quantum yields (PLQY) in a series of blue-to-green Ir(iii) complexes can be derived exclusively from electronic structure calculations.

Ultrafast photophysics of transition metal complexes

The properties of transition metal complexes are interesting not only for their potential applications in solar energy conversion, OLEDs, molecular electronics, biology, photochemistry, etc. but also for their fascinating photophysical properties that call for a rethinking of fundamental concepts. With the advent of ultrafast spectroscopy over 25 years ago and, more particularly, with improvements in the past 10-15 years, a new area of study was opened that has led to insightful observations of the intramolecular relaxation processes such as internal conversion (IC), intersystem crossing (ISC), and intramolecular vibrational redistribution (IVR). Indeed, ultrafast optical spectroscopic tools, such as fluorescence up-conversion, show that in many cases, intramolecular relaxation processes can be extremely fast and even shorter than time scales of vibrations. In addition, more and more examples are appearing showing that ultrafast ISC rates do not scale with the magnitude of the metal spin-orbit coupling constant, that is, that there is no heavy-atom effect on ultrafast time scales. It appears that the structural dynamics of the system and the density of states play a crucial role therein. While optical spectroscopy delivers an insightful picture of electronic relaxation processes involving valence orbitals, the photophysics of metal complexes involves excitations that may be centered on the metal (called metal-centered or MC) or the ligand (called ligand-centered or LC) or involve a transition from one to the other or vice versa (called MLCT or LMCT). These excitations call for an element-specific probe of the photophysics, which is achieved by X-ray absorption spectroscopy. In this case, transitions from core orbitals to valence orbitals or higher allow probing the electronic structure changes induced by the optical excitation of the valence orbitals, while also delivering information about the geometrical rearrangement of the neighbor atoms around the atom of interest. With the emergence of new instruments such as X-ray free electron lasers (XFELs), it is now possible to perform ultrafast laser pump/X-ray emission probe experiments. In this case, one probes the density of occupied states. These core-level spectroscopies and other emerging ones, such as photoelectron spectroscopy of solutions, are delivering a hitherto unseen degree of detail into the photophysics of metal-based molecular complexes. In this Account, we will give examples of applications of the various methods listed above to address specific photophysical processes.

Computational insights into the photodeactivation dynamics of phosphors for OLEDs: a perspective

In this perspective we highlight recent computational efforts to unravel competing photodeactivation mechanisms of radiative and non-radiative nature of phosphors.