Laser-plasma accelerator based femtosecond high-energy radiation chemistry and biology

International audienceRegarding the different protocols used for external cancer radiotherapy (X or. rays, electron, proton or ions beams) or radioimmunotherapy (Auger electron emitting radionuclides), the initial energy deposited in integrated biological systems (biomolecular and sub-cellular targets) represents a decisive parameter for the primary and more delayed radiation damage. A short-range energy distribution governs mainly (i)) the early survival probability of secondary electrons, (ii)) the spatio-temporal distribution of short-lived reactive radicals inside nascent tracks, (iii)) the primary biomolecular alterations triggered by low energy secondary electrons. The thorough understanding of these fundamental processes requires a real-time investigation of primary radiation events, typically in the temporal range 10(-14) - 10(-11) s. Laser-plasma accelerators based High Energy Radiation Femtochemistry (HERF) represents a newly emerging interdisciplinary field which can be driven in strong synergy with the generation of ultrashort particle beams in the MeV energy domain. The innovating developments of HERF would favour the investigation of prethermal radiation processes in aqueous and biochemically relevant environments. In this way, the quantum character of a very-short lived low-energy electron state (p-like configuration) represents a promising sub-nanometric probe to explore early radiation processes in native tracks. The specific properties of ultra-short electron beams accelerated by TW laser are very useful for future developments of spatio-temporal radiation biophysics in complex biological systems such as living cells

Synergy between low and high energy radical femtochemistry

International audienceThe deleterious effects of ionizing radiation on integrated biological targets being dependent on the spatio-temporal distribution of short-lived radical processes, a thorough knowledge of these early events requires a real-time probing in the range 10(-15) - 10(-10) s. This manuscript review is focused on the synergy that exists between low (1-10 eV) and high (MeV) energy radiation femtochemistry (LERF, HERE respectively). The synergy remains crucial for the investigation of primary radical processes that take place within the prethermal regime of low energy secondary electrons. The quantum character of very-short lived electron in a prehydrated configuration provides a unique sub-nanometric probe to spatially explore some early radiation-induced biomolecular damage. This approach would foreshadow the development of innovative applications for spatio-temporal radiation biology such as, (i)) a highly-selective pro-drug activation using well-defined quantum states of short-lived radicals, (ii)) the real-time nanodosimetry in biologically relevant environments, and (iii)) the ultrashort irradiation of living cell

Femtoradical events in aqueous molecular environments: the tenuous borderline between direct and indirect radiation damages

International audienceThe complex links existing between radiation physics and radiobiology concern the complete understanding of spatio-temporal events triggered by an initial energy deposition in confined spaces called spurs. Microscopic radiation effects (photons or relativistic particles) on integrated biological targets such as water 'the solvent of life' and biomolecular architectures (DNA, histones, enzymes) cannot be satisfactorily described from an absorbed dose delivery profile or a linear energy transfer (LET) approach. Primary radiation damages on biological targets being dependent on the survival probability of secondary electrons and short-lived radicals inside nascent nanometric clusters of ionisation, a thorough knowledge of these processes require the real-time probing of early events on sub-micrometric scale, in the temporal range 10^-15 - 10^-10 s. Major strides concern early water damages: primary water cation formation (H2O•+ or positive hole), concerted electron-proton couplings, attachment dynamics of p-like excited prehydrated electron on biomolecule, short-lived radical pairs involving water-bridged radical OH• and hydronium ion H3O+. The deactivation frequency of electron-radical pairs is comparable to an H-OH deactivation of excited water molecules (vH2O* ~ 0.33 × 10^13 s^-1). These short-lived events take place in the prethermal regime of delocalized secondary electrons and represent a tenuous borderline between direct and indirect molecular damages

Electron propagator study of the excitation spectrum of the solvated hydronium radical

In an ab initio MCSCF study, the hydronium H3O• radical has been hydrated within a spherical dielectric cavity of various radii, while using both the static and optical dielectric constants of water. The electronic excitation spectrum was computed with the help of a 2-particles propagator method. The effect of non specific solvation is an overall red shift, absorbing transitions at 1.8 eV and 2.4 eV are predicted. The resulting apparent absorption spectrum of the hydrated H3O• radical looks similar to the spectrum of the "hydrated electron" (e-)aq. However, an absorption band of the first excited state (H3O•)*aq is estimated to be centered around 1.50 eV. We perform a comparative analysis of this band with the 1.45 eV band observed within femtosecond spectroscopy experiments. An "itinerant radical model" of the hydrated electron is also briefly discussed

Elementary electron transfer processes in aqueous ionic solution

Primary steps of an electron transfer process in an aqueous sodium chloride solution are investigated by femtosecond absorption spectroscopy of the ionic solute. The influence of the molecular ratio (R = [H2O]/[NaCl]) on transient electronic configurations of Cl- and on the dynamics of multiple electron detachment channels is analyzed within the energy domain 3.44-0.99 eV (360 - 1250 nm). The results provide direct evidence for an electric field effect of the sodium ion on one specific electron transfer process. This channel involves transient electron-chlorine atom pairs ({(Cl:e-)pair:(Na+)hyd}) and leads to a relaxed polaron-like state ({e-:Na+}hyd)

FEMTOSECOND SPECTROSCOPY OF SOLVATED ELECTRON IN AQUEOUS MEDIA

The elucidation of detailed mechanisms of ultrafast events that occur in molecular charge transfer or reaction dynamics has been made possible by recent advances in spectroscopy techniques that use ultrashort laser pulse generation. Ultrashort laser pulses (100 femtoseconds duration, 1 fs = 10-15 s) allow to initiate selective photochemical processes (single charge transfer for instance) and to obtain unique informations on the dynamics of primary steps of radical reactions involving ultrafast electron or proton transfer : formation of the hydration cage around an electron, encounter pair formation, ion-molecule reaction. Recent investigations on the non-equilibrium states of electron in aqueous media are discussed