Excited-state populations fragmentation

In summary, we have chosen Na2 and Na3 to demonstrate coherent control of excited-state population, ionization yield, fragmentation, and ionic product formation using a variety of control parameters. 

Figure 13-9. Femtosecond timescale dynamics of the excited state population following excitation of guanine at 267 nm, as probed by photoionisation using a 400 nm light [77]. The photoion signal shown has been collected in the mass channels of guanine and of its fragments (due to fragmentation in the ionic state see [72] for details). The transient observed (hollow dots) fits well a sum of two exponential decays Tj and t2 (curve)...

Because of the rapidity with which the first-formed excited states are usually converted to the Sx or Tx state, most photochemical reactions start from these states. There are exceptions. An obvious one is when an upper dissociative excited state, in which the molecule immediately fragments, is populated.49 Other exceptions occur when a molecule contains two different chromophores. For example, 20 reacts analogously to franj-stilbene (21) from its state but also undergoes photoreduction, a process typical of an n,n state (see p. 719).60... 

The main features of the early kinetics could be reproduced using a five-level rate equation which included convolution with the pump and probe pulse shapes. These levels represent five locations, or time windows (L1-L5), describing five discrete time zones in the evolution of the Cr(CO)6 excited state and ultimate formation of Cr(CO)5 and Cr(CO)4 in the gas phase. These levels are consecutively populated and differ in the nature and ratio of the fragment ions they produce. Their populations are modeled by rate equations providing the lifetimes (x. for Li) and the ionization-dissociation cross section ("a. for Cr(CO)n+) for a particular fragment in Li. This five-level model is represented in Fig. 12 and Table 2 contains the optimized... 

At this point a few comments about the mechanism of the photochemical splitting of carbonate are appropriate. We suggest that in agreement with the excited state properties of Tp complexes in general (46,47) the photoexcitation of Cu Tp leads to the population of an IL excited state followed by electron transfer to the carbonate. A radical anion is thus formed as an intermediate which is subsequently reduced by a second Cu Tp complex forming CO. Simultaneously, two Cu Tp fragments are generated as oxidation products. Under these conditions... 

A possible way to detect strong coupling between the asymmetric and symmetric stretches of the 0- -H- -0 fragment is to compare the low-temperature INS spectrum of the crystal with that obtained at relatively high temperature, that is when the first excited state of the O -O stretch is sufficiently populated. To our knowledge, such a comparison has not been done yet. 

Figure 27. Successive interaction of two LCT fields where the first (Fi(r)) induces a population transfer and the second acting after 5 ps (E2(t)) triggers excited-state dissociation. These fields are shown in the lower panel, whereas the predissociation ( o(0) and excited-state fragmentation ( i(V)) yields are contained in the upper panel.

Figure 28. Simultaneous interaction of two LCT fields (E (t) + E2(t)) constructed to induce population transfer and excited-state dissociation, respectively. The two field components are displayed separately in the lower panels. The predissociation ( o(f)) and excited-state (B (t)) fragmentation yields are shown in the upper panel.

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