Time scales electronic excitation

The time scales of excitation characterizing the violent cluster perturbations which we want to describe are so short that, to a very good approximation, ions can be considered as fixed during the excitation process and for an early stage of the electronic relaxation (typically up to 100 fs). 

Ultrafast injection of carriers into the UO2 substrate suggests that overall cell efficiency is not limited by this process but by intervening transfer mechanisms (e.g., trapped state populations reducing quantum yield) or longer time scale electron-dye recombination rates (typically taking microseconds). For example, it was found that the absorbed photon to current efficiency (APCE) is considerably reduced for V compared to IV under identical cell conditions (27,55). While V has a 350 mV lower electrochemical reduction potential than IV (and hence a red-shifted absorption spectrum), it is unclear why the redder absorbing dye (V) does not inject as efficiently (56). Recent visible excitation with broadband... 

Whether or not the primary excitation and ionization processes in the liquid and gas phases are the same, we do not know. As we shall see below, the yield of ionization in the liquid appears to be somewhat larger than in the gas phase, possibly at the expense of direct excited states. Excited states formed in the primary excitation may subsequently ionize, but, on the other hand, ion-electron recombination may take place at an extremely short time scale and excited states may be formed. The earliest processes are not very well understood at present. 

How must this theory be modified to describe the effect of the optical excitation The incident electric and magnetic X-ray fields are now pulses Ex(r, t) = Exo(t) exp[j(q r - Oxt)] and Hx(r, t) = Hxo(t) exp[/(q/r - Oxt)]. They still are plane waves with a carrier frequency Ctx, but their amphtudes Exo(t) and Hxo(t) vary with time. The same statement applies to the electron density n r, t), which also is time dependent. However, these variations are all slow with time scales on the order of 1/Ox, and one can neglect 5Exo(0/ 8Hxo(t)/8t as compared to iOxExo(t) and iTlxHxo(0- Detailed calculations then show that [17]... 

One would prefer to be able to calculate aU of them by molecular dynamics simulations, exclusively. This is unfortunately not possible at present. In fact, some indices p, v of Eq. (6) refer to electronically excited molecules, which decay through population relaxation on the pico- and nanosecond time scales. The other indices p, v denote molecules that remain in their electronic ground state, and hydrodynamic time scales beyond microseconds intervene. The presence of these long times precludes the exclusive use of molecular dynamics, and a recourse to hydrodynamics of continuous media is inevitable. This concession has a high price. Macroscopic hydrodynamics assume a local thermodynamic equilibrium, which does not exist at times prior to 100 ps. These times are thus excluded from these studies. 

The molecular time scale may be taken to start at 10 14 s following energy absorption (see Sect. 2.2.3). At this time, H atoms begin to vibrate and most OH in water radiolysis is formed through the ion-molecule reaction H20+ + H20 H30+ + OH. Dissociation of excited and superexcited states, including delayed ionization, also should occur in this time scale. The subexcitation electron has not yet thermalized, but it should have established a quasi-stationary spectrum its mean energy is expected to be around a few tenths of an eV. 

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