Electron ionization timescale

Most of these processes are very fast. Ionization happens on the low femtosecond timescale, direct bond cleavages require between some picoseconds to several tens of nanoseconds, and rearrangement fragmentations usually proceed in much less than a microsecond (Fig. 5.3 and Chap. 2.7). Finally, some fragment ions may even be formed after the excited species has left the ion source giving rise to metastable ion dissociation (Chap. 2.7). The ion residence time within an electron ionization ion source is about 1 ps. [9]... 

Equilibrium (i.e. local steady-state) ionization leads in this regime to solar-corona-like conditions where col-lisional ionization is balanced by recombination and the degree of ionization is fixed by the temperature alone, the electron density cancelling out. However, here departures from equilibrium occur because the time taken to establish ionization equilibrium is not negligible with respect to the timescale of expansion. 

Transfer of radiation-induced electrons and holes (H20 ) from the hydration layer of DNA has been of considerable recent interest. Results from ESR experiments at low temperatures suggest that ionization of hydration water (reaction 4) results in hole transfer to the DNA (reaction 5) [4, 24-28]. Since the proton transfer reaction (reaction 6) to form the hydroxyl radical likely occurs on the timescale of a few molecular vibrations [29], it is competitive with and limits hole transfer to DNA [27]. 

Comparison of the evolution of the transient absorption in pure water with the indole solution demonstrates that the dynamics of the generated elections depends on the donor molecule. The initial evolution in pure water is similar to that in indole (350 50 fs), but shows an additional contribution on a timescale of 1 - 2 ps (Fig. 2b)). The formation of electrons stemming from indole is similar to the ionization and solvation process in pure water, but the time constant corresponding to the dielectric relaxation is missing. It indicates that the electron is not completely separated from the indole cation and the interaction with the parent molecule disables the dielectric relaxation that occurs in pure water on the timescale of 1 - 2 ps. 

Interatomic Coulombic decay (ICD) is an electronic decay process that is particularly important for those inner-shell or inner-subshell vacancies that are not energetic enough to give rise to Auger decay. Typical examples include inner-valence-ionized states of rare gas atoms. In isolated systems, such vacancy states are bound to decay radiatively on the nanosecond timescale. A rather different scenario is realized whenever such a low-energy inner-shell-ionized species is let to interact with an environment, for example, in a cluster. In such a case, the existence of the doubly ionized states with positive charges residing on two different cluster units leads to an interatomic (or intermolecular) decay process in which the recombination part of the two-electron transition takes part on one unit, whereas the ionization occurs on another one. ICD [73-75] is mediated by electronic correlation between two atoms (or molecules). In clusters of various sizes and compositions, ICD occurs on the timescale from hundreds of femtoseconds [18] down to several femtoseconds [76-79]. 

The dynamics of strongly-driven electrons on ultrafast timescales also lies at the basis of several phenomena in molecules. Among these, we have investigated dynamic alignment [26] and enhanced ionization in Coulomb explosions [27]. 

In radiolysis, one of the most important reactions of solvated electrons is recombination with positive ions and radicals that are simultaneously produced in close proximity inside small volumes called spurs. These spurs are formed through further ionization and excitation of the solvent molecules. Thus, in competition with diffusion into the bulk, leading to a homogeneous solution, the solvated electron may react within the spurs. Geminate recombinations and spur reactions have been widely studied in water, both experimentally and theoretically, ° and also in a few other solvents. " Typically, recombinations occur on a timescale of tens to hundreds of picoseconds. In general, the primary cation undergoes a fast proton transfer reaction with a solvent molecule to produce the stable solvated proton and the free radical. Consequently, the... 

The reaction given in Eq. 3 represents ionization and electronic excitation of water molecules this occurs on the timescale of an electronic transition. The positive radical ion H20 + is known to undergo the ion-molecule reaction (Eq. 4) in the gas phase with a rate constant of 8 x 10 dm mol s [2], which sets the lifetime of the ion at less than 10 s in the liquid. The electronically excited states H2O are known to dissociate in the gas phase, as shown in Eq. 5, and the electron released in the ionization event is known to become thermalized and solvated in less than... 

The purpose of this chapter is to describe the technology that enables the investigation of radiation chemical phenomena at picosecond and femtosecond timescales. Several research groups have used femtosecond laser photoionization techniques to examine electron solvation dynamics and other processes relevant to early radiation chemistry events [1,2]. This chapter will focus on ultrafast studies using ionizing radiation, primarily electron beams, as the excitation source. 

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