Electron deexcitation process

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Let us turn our attention to the dominant recombination or deexcitation processes that follow the excitation of electrons from the inner shell or from the valence shell (Fig. 13). The first mode of deexcitation is the Auger process, which leads to further electron emission. The second mode of deexcitation may result in the emission of electromagnetic radiation and is commonly called X-ray fluorescence. In the Auger transition, the electron vacancy in an inner shell is filled by an electron from an outer band. The energy released by this transition is transferred to another electron in any... 

Blythe, Grosser, and Bernstein [151 ] have used crossed molecular beams to observe the J = 2 - 0 rotational deexcitation process in D2. A velocity-selected atomic beam of potassium was made to impinge on a modulated Da beam from an effusive (T = I8PK) source. The scattered K atoms were detected by surface ionization on a hot Pt-W ribbon, from which the ions were drawn into an electron multiplier equipped with lock-in amplification. 

Hie lifetimes for the various excitation and deexcitation processes hence vary over many orders of magnitude (Appendix III, Section A)—femtoseconds for light absorption, picoseconds for radiationless transitions between excited electronic states, nano- to microseconds for fluorescence, and milliseconds to seconds for phosphorescence. 

The next deexcitation processes that we consider are the radiationless transitions by which S(W)K-) eventually dissipates its excess electronic energy as heat. As for fluorescence, radiationless transitions generally obey first-order kinetics. Two different states can be reached by radiationless transitions from S(W Jt.) ... 

As another type of deexcitation process, S -) can take part in a photochemical reaction. For example, the excited n electron can be donated to a suitable acceptor ... 

Figure 5-7. Energy level diagram including vibrational sublevels, indicating the principal electronic states and some of the transitions for carotenoids. The three straight vertical lines represent the three absorption bands observed in absorption spectra, the wavy lines indicate possible radiationless transitions, and the broad arrows indicate deexcitation processes (see Fig. 4-9 for an analogous diagram for chlorophyll).

As we indicated in Chapter 4 (Section 4.3B), the emission of fluorescence means that the excitation caused by the absorption of light cannot be used for photochemistry. In particular, the excited trap of Photosystem II, Pg80, can become deexcited by photochemistry involving the electron transport chain (rate constant = /tph0tochem)> by fluorescence ( F), or by various other deexcitation processes (/fcothei. = sum of the rate constants for all such... 

In the absence of an electric field, the final step of the deexcitation process is the recombination of electrons and holes and the return of the crystal to its neutral state. 

Due to the simple electronic stmcture of Yb ", Yb -doped ceramic lasers have no problem of de-excitation processes like the concentration-dependent selfquenching by downconversion or upconversion cross-relaxation. However, a reduction in emission lifetime and emission quantum efficiency has been observed. In addition, cooperative processes could occur, due to the interaction between two excited Yb " ions, when the concentrations of Yb " are too high. The parasitic deexcitation processes are usually accompanied by the generation of heat that is up to twice as much as that due to the quantum defect [202]. Therefore, it is important to have a good matching between the pump and mode volume in the laser materials. 

AFS excitation and deexcitation processes involve changes in the energy of valence electrons and are called electronic transitions. Analytical applications of AFS employ transitions in the ultraviolet (UV)-visible region of the electromagnetic spectrum (between 200 and 800 nm). AFS transitions may involve a combination of absorption, fluorescence, and non-radiative processes. 

This process is often called internal conversion. However, we shall limit use of the latter term to describe radiationless singlet-singlet electronic deexcitation. 

Fig. 3.7 Deexcitation processes for atomic cote holes, a emission of X-ray radiation, b emission of an Auger electron [80]. G. Ertl and J. Kiippers Low Energy Electrons arul Surface Chemistry, page 29. 1985. Copyright Wiley-VCH Verlag GmbH Sc Co. KGaA. Reproduced with permission...

---