Skip to main content

Ultrafast dynamics of hydrogen bonded structures in condensed matter

Final Report Summary - ULTRADYNE (Ultrafast dynamics of hydrogen bonded structures in condensed matter)

Hydrogen bonding, a weak chemical interaction due to electric forces between polar molecular groups, determines the structure of biomolecules such as DNA, the basic carrier of genetic information, liquids such as water, and solids such ice or ionic materials containing hydrogen atoms. Physical properties and (bio)chemical function of hydrogen bonded systems are frequently linked to extremely fast processes in the femtosecond time domain (1 femtosecond = 10^-15 s). Mapping such processes on atomic length and time scales with most sophisticated photon probes, namely femtosecond pulses in a spectral range from the infrared to x-rays, has been the main goal of this project. A sequence of 'snapshots' taken with femtosecond resolution provides access to the momentary structure of the system and to the mechanisms governing functional processes at the atomic level.

The first system studied here are DNA double helices interacting with their fluctuating environment of water molecules. Using particular vibrations of DNA and water as local probes, interactions between different functional units, e.g. DNA base pairs or the DNA backbone, were determined. Moreover, processes of energy transfer within DNA and to the surrounding water shell were mapped directly. Within a particular base pair, NH stretching vibrators are coupled and exchange energy on a time scale of several hundreds of femtoseconds. In contrast, energy transfer between different base pairs in the double helix is negligible. Due to the strong interaction of phosphate groups in the backbone with the surrounding water, vibrational excess energy is transferred from DNA to the water shell within some 500 fs. The water shell in which DNA is embedded, displays dynamics on a multitude of time scales. The structure of the first and second water layer is comparably rigid. It shows limited fluctuations on a 300 fs time scale, leaving DNA-water hydrogen bonds and the structural disorder of the interface unaffected. In contrast, the outer parts of the water shell display pronounced subpicosecond (1 picosecond = 10^-12 s) fluctuations which are somewhat slower than in neat water.

The dynamics of hydrated phospholipids, the basic constituents of cell membranes, show pronounced similarities to DNA. Phosphate-water interactions in phospholipids are extremely efficient in dissipating energy and pools as small as 3 water molecules per phosphate function as very efficient heatsinks. Interfacial water again displays a relatively rigid structure which is stabilized by the strong electric fields at the surface of the phospholipids.

In a second research area of the project, x-ray diffraction with a 100 fs time resolution is applied to take snapshots of moving atoms and electrons in crystals. After launching a structure-changing process by optical excitation, the momentary atomic positions and distribution of electrons are derived from diffraction patterns taken at different times after excitation. Studies of hydrogen-bonded ionic materials gave basic new insight into the interplay of atomic vibrations and electron relocations. In ammonium sulfate, the combined motion of electrons and protons leads to a new transient crystal structure which was observed here for the first time. This structure is expected to modify the transport of charges in the crystal substantially. In potassium dihydrogen phosphate, a prototype material displaying ferroelectricity at low temperatures, outer electrons of the phosphate ions are moving over the distance of a chemical bond between the phosphorus and the oxygen atoms which is 100 times larger than the underlying vibrational elongation of the atoms. This behavior is caused by the electric forces the moving ions in the crystal structure exert on the highly polarizable outer electrons and highly relevant for the electric high-frequency response and phase transitions of ionic materials.

Charge relocations driven by strong external fields were mapped in a series of light ionic materials. Valence electrons are shifted in a fully reversible way between neighboring ions, a process influenced by the electric many-body interactions among electrons. This makes ultrafast x-ray diffraction a versatile probe of complex many-body physics and electron correlations in solids.

The project has generated a large scientific output of high quality. It led to a number of novel experimental methods for generating and applying extremely short pulses in the hard x-ray range. They demonstrate the potential of laser-driven x-ray sources for a broad range of applications in science and technology. In collaboration with a company, a prototype hard x-ray plasma source was developed into a commercial product and successfully introduced into the market for scientific equipment.