1. BESSY-VSR – Unique science opportunities

BESSY-VSR, the „Variable Pulse Length Storage Ring” proposed by HZB, is aimed at the grand challenges of basic science of how to harvest, convert and store energy, how to transmit and archive information efficiently and how to govern rate and selectivity for clean chemistry. The key to these challenges is to move from observing and understanding to controlling function. BESSY-VSR will be the science-driven step from the observation and quantum mechanical description towards the control of materials and chemical function. This new light source will push the excellent properties of synchrotron radiation storage rings into the so far underexplored dimension of time and transient states and give users the free choice of observable time and length scales. User will thus be uniquely enabled to correlate chemical function with molecular dynamics on multidimensional potential energy surfaces and to attribute the active principles underlying functional materials to elementary structures and processes of condensed matter. This unprecedented ability to create and detect transient states and the associated dynamic pathways of excitations in matter with BESSY-VSR will form the basis for controlling materials’ function.

In the past, science with third generation synchrotron radiation has brought for materials science and physics as well as chemistry and biology a deep understanding and description of their electronic and structural properties on a fully quantum mechanical basis. Here photon science with soft X-rays has brought tremendous insight through its element specific, chemical and spin selective probes. As a consequence, materials characterization has been developing towards increasing complexity regarding dimensionality, concentration and extreme or ambient environments.

With the new synchrotron-light source BESSY-VSR users will be able to freely switch between the high precision determination of electronic and structural parameters in the ground state in order to prepare and detect low-energy excited states in a controlled way. BESSY-VSR will deliver ultra-stable radiation from the THz to the soft X-ray region with highest spatial and energy resolution in combination with short pulses for dynamic studies. Due to high average brilliance, it will allow for high-throughput measurements as users are accustomed to from storage rings.

BESSY-VSR will open up Unique science opportunities for quantum materials for energy and materials for future information technologies, in basic energy science and green chemistry and for unveiling structure and function in biological systems. It will provide a unique photon source as a MHz precision probe that allows switching between highest spectral, spatial resolution and temporal resolution.

Quantum materials for energy and materials for Future information technologies are the most intriguing and most promising materials for novel functionality where the coupling between different internal degrees of freedom and the coupling to the environment lead to complex energy landscapes. These intra- and intersystem interactions determine the material’s dynamic response to external fields and thermodynamic heat baths. Ultimately, all functionality is coupled to the dynamic response of the system and therefore determined by the respective energy landscape. Often, complex energy landscapes give rise to complex spatial patterns. Domain formation and phase separation on the nanoscale occur if several local minima in the energy landscape are accessible for the system. Material function can then be determined by segregation and percolation phenomena and spatially resolving probes in real and reciprocal space are required. An understanding of the shape and the dynamic pathways in these multidimensional energy landscapes allows controlling and ultimately tailoring materials’ functionality. This goes well beyond characterizing static properties of the constituents and thus provides a means to address the fundamental science underlying functional materials.

The relevant parameter space extends over many orders of magnitude in space and in time. These quantum materials have in common that their dynamic response cannot be described by a single degree of freedom alone. Energetically, the coupling between different degrees of freedom and to the environment leads to highly non-trivial low-energy excitation spectra, which require the combined use of various spectroscopic as well as structural probes over a wide energy range in order to provide complementary selective access to all relevant degrees of freedom. Time-resolved X-ray probes at BESSY-VSR will allow following directly the pathways through the multidimensional landscapes in real time, thus complementing techniques that identify the ground state and excitations based on an instantaneous response.

The deep correspondence and complementarity of such fundamental phenomena determine molecular properties and the functionality of materials. Basic energy science at BESSY-VSR  will address these fundamental questions. The transfer, stabilization and localization of charges, e.g., at functional centers in solution, at surfaces and interfaces and in solids are at the heart of technologically and elementary processes in nature. Understanding and the ability to control such elementary reaction steps therefore builds the basis for the development of alternative sustainable technologies. Electron excitation, transfer and solvation not only govern photochemical molecular reactions but also determine bio-molecular function. Understanding and the ability to control the elementary reaction steps with BESSY-VSR therefore holds the key to revealing the structure-function relationship in bio-molecules. Charge-transfer processes steer the primary steps in photovoltaic devices, in vision and they drive conformational changes in single molecules and of the optical or magnetic properties and conductance in functional materials. The effects of symmetry and bonds work similarly in molecular systems and solids. Screening, the effect of the ligand field on the orbital state, the coupling between orbital occupation and magnetic properties and geometric frustration can be found in simple molecules as well as in complex materials. Quite generally, a strong relation between electronic and structural degrees of freedom, between energetics and structure plays a crucial role in their dynamics. As a key to determining materials’ dynamical functionality, BESSY-VSR will thus allow addressing spatial complexity, the response time of the system and the relevance of long range interactions and low energy excitations with multidimensional X-ray probes.

Since low energy excitations on spin, orbital and electron degrees of freedom occur in the THz and far IR these controlled states of materials live on picosecond and sub-picosecond timescales. In addition, the typical timescale of phonon excitation of a picosecond per nanometer brings the dynamics of nanoscale objects and large amplitude motions in chemistry and bio-molecules well into the pico- and nanosecond domain. Also, spin-stabilized as well as chemically relevant metastable states can easily be detected with this temporal resolution. BESSY-VSR targets exactly at these time scales and, with the high MHz repetition rate of BESSY-VSR , intrinsic limitations like radiation damage or space charge to electron spectroscopy can be overcome. In addition to the picosecond and sub-picoseond timescale of correlated excitations and large amplitude motions, there is a scientific need to determine electron dynamics between a few hundred femtoseconds down to the attosecond domain. BESSY-VSR will be designed to produce picosecond pulses in its standard mode, with pulse lengths in the sub-picosecond range, possibly as low as 100 femtoseconds in special operating modes. Certainly, BESSY-VSR will also improve the existing time slicing by one order of magnitude due to the higher bunch compression which will create femtosecond pulses with 108 ph/s perfectly synchronized to driving pulses ranging from the THz to the UV region. Sub-femtosecond and femtosecond processes will be best studied at low repetition rate with complementary laboratory-based High Harmonic Laser Sources and large-scale X-ray Free Electron Lasers, once they reach their design parameters regarding pulse length and synchronization to external stimuli.

BESSY-VSR will merge experimental cutting-edge x-ray tools and cover a broad energy spectrum while addressing time scales from quasi-static to sub-picoseconds and length scales from Ångström to micrometers. With these tools, materials’ functionality and dynamics will be addressed in the full parameter space and, for the first time, ultimate spatial, momentum, spin and energy information can be combined with the relevant time resolution at a MHz repetition rate.