In the scientific field of radiation chemistry, the chemical effects of ionizing radiation of matter are studied to advance our knowledge both in fundamental atomic and molecular science and in applied radiation methods. In the latter case, research in radiation chemistry has strong connections to areas that study and employ radiation effects in living systems, with important medical applications in radiotherapy and radiation dosimetry, and also with areas dealing with the development of applied radiation processing methods, such as radiation sterilization, radiation sanitation, radiation polymerization, and food irradiation.

Pulse radiolysis is an experimental research method that has helped more than any other method in working toward an understanding of the underlying rapid chemical and physico-chemical processes that follow the absorption of ionizing radiation in matter and are ultimately responsible for the final observable effects. The basic principle of this method is that it uses a very short radiation pulse of ionizing radiation and then detects the resulting chemical changes with high time resolution. Since the characteristic time of most radiation-chemical processes is considerably shorter than 1 millisecond (10-3 s), human observation needs to rely on scientific instruments with high time resolution. Currently established methods for pulse radiolysis employ laser-photocathode radiofrequency linear accelerators (LINAC) to generate ultrashort pulses of electrons, routinely achieving time resolution of several picoseconds and in some cases resolving the sub-picosecond (< 10-12 s) temporal region.

The ELI-Beamlines facility will be capable of producing multiple kinds of pulsed ionizing radiation generated by direct laser-plasma interaction in matter (soft and hard X-rays, electrons, or protons and possibly even heavier ions). When compared to accelerators, these laser-driven radiation sources offer two principle advantages for applications in the area of pulse radiolysis:

  1. The short length of the duration of the pump radiation pulse (~ 10-14s),
  2. Near-perfect synchronization of the temporal overlap of the radiation pulse and the laser probe (low jitter).

The first advantage results from the original ultrashort duration of the laser pulses that is provided by the modern Ti:Sapphire laser oscillator technology coupled with optical parametric chirped-pulse amplification (OPCPA), which is capable of delivering high-power impulses measuring down to several femtoseconds. The second advantage stems from the use of one original laser impulse for driving the pump radiation pulse and operating the inspecting laser probe pulse; this greatly reduces the demanding problem of synchronization jitter that contributes to the challenges of overcoming the "picosecond barrier" in pulse radiolysis experiments using radiofrequency (RF) LINAC-based electron accelerators.

In principle, both advantages allow for achieving a truly sub-picosecond time resolution in pulse-radiolysis studies, and therefore promising to reveal a temporal landscape, which is still poorly charted, of the very early physical-chemical processes taking place in radiation chemistry (such as is radiolysis of water; see the image above).

Martin Přeček,