Sequential two-photon process

Figure 8.4 Schematic illustration of a sequential two-photon process involving a dissociative...

Finally, Chapter 8 deals with the evaluation of multidimensional Franck-Condon integrals. As an illustration of the complexity of the latter upon the normal mode rotation, a study of sequential two photon processes is presented. 

The intense laser beam at 2A8 nm is capable of dissociating SO2 by sequential two-photon absorption (60-62). Further excitation of the SO and S fragments was observed to occur by one and two photon processes, respectively (see Figure A) ... 

Since experimentally and theoretically optimized pulses obtained from OCT and CCL are available for NaK [84], first, it was possible to show under which conditions the shaped pulses are reproducible, and second, the connection between the forms of the shaped pulses and different ionization pathways was established. This allowed one determination of the mechanism for the maximization of the ionization yield under the participation of several excited states. The agreement between experimentally and theoretically optimized pulses, which was independent from the initial guess, showed that the shapes of the pulses can be used to deduce the mechanism of the processes underlying the optimal control. In the case of optimization of the ionization process in NaK, this involved a direct two-photon resonant process followed by a sequential one-photon processes at later times. These findings obtained for the simple system are promising for the use of shapes of tailored pulses to reveal the nature of processes involved in the optimal control of more complex systems. This will be addressed below. 

PRACTICE EXAMPLE B By using a two-photon process (that is, two sequential excitations), a chemist is able to excite the electron in a hydrogen atom to the 5d level. Not all excitations are possible they are governed by selection rules (see Are You Wondering 8-6). Use the selection rules to identify the possible intermediate levels (more than one are possible) involved, and calculate the frequencies of the two photons involved in each process. Identify the transitions allowed when a sample of hydrogen atoms excited to the 5d level exhibits an emission spectrum. When a sample of gaseous sodium atoms is similarly excited to the 5d level, what would be the difference in the emission spectrum observed ... 

FIGURE 6.14 Potential energy curves of Re(CO)3(4-phenylpyridine)2Cl. They account for the photophysical processes observed when the excited state is produced by the absorption of one photon (left) and the photochemical reaction induced when the excited state is produced by the sequential absorption of two photons (right). 

Upconversion lanthanide-containing nanophosphors, which emit higher-energy photons when excited by lower-energy photons have stirred increasing research interest in recent years. The predominant mechanisms of upconversion in nanophosphors are excited-state absorption (ESA), energy-transfer upconversion (ETU) and photon avalanche (PA) (Prasad, 2004 Auzel, 2005). In the ESA process, two photons are sequentially absorbed by the same ion,... 

Up-conversion is a process by which two photons of lower energy are subsequently converted into a luminescence photon of higher energy (typically, two IR photons giving rise to one visible photon, e.g. in Er111-containing compounds). This anti-Stokes process is usually observed for ions embedded in solids and is made possible by various mechanisms, such as the now classical excited state absorption mechanism (ESA), or sequential energy transfers (ETU for... 

What is not too surprising is that the one-photon LIF spectrum (Fig. 2.13a) and two-photon OODR spectra (Fig. 2.13b) are similar, since these spectra sample the same Si level structure. The major differences between these panels lies in the intensity relationships of the bands within the progressions. These differences can be understood by recognizing that the OODR is a sequential process where a substantial time delay is introduced between the pump and the probe photons. Thus the Franck-Condon factors for the S2<—Si <— So process is a... 

X. Zhang, Y. Xia, R.H. Friend, and C. Silva, Sequential absorption processes in two-photon-excitation transient absorption spectroscopy in a semiconductor polymer, Phys. Rev. B (Condens. Matter Mater. Phys.), 73, 245201 (2006). 

With photochromic systems, as with other areas of photochemistry, we are normally using monophotonic processes in which a molecule absorbs one photon. However, it is possible to have two-photon or multi-photon photochromic systems. These have certain attractive properties. Two possibilities exist [51]. In the first (sequential) case, a molecule absorbs one photon to form its excited state. This (or a subsequent species) may then absorb a second photon to give the product ... 

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