Creating Excited States

The ability to populate specific energy levels using laser pulse sequences has I been realized in a number of ways. An example is the STIRAP approach discussed fi infection 9.1. A number of alternative approaches are discussed below. 

Whenever the lowest (TVth) order perturbation theory for the IV-photon problem i j valid, it is possible to generate a wave packet of bright states, assuming that suc r bright states exist. To see this, partition the excited state manifold into bright states f s) and dark states %m). The eigenstates E ) of the molecular Hamiltonian HM can therefore be written as.  

Consider then excitation with an optical pulse of the form fy 

This result can be generalized to the preparation of other types of zero-order  

states. For example, in accord with the objectives of mode-selective chemistry (Section 3.5), we may want to prepare a specific local mode vibrational state. To F do so, consider [455], an M-level oscillator that has the right anharmonicity, that is, a system whose energy levels behave like 

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An additional class of nonlinear optical effects is that of multi-photon absorption processes. Using these process, one can create excited states (and, therefore, their associated physical and chemical properties) with a high degree of three-dimensional (3D) spatial confinement, at depth in absorbing media. There are potential applications of multi-photon absorbing materials in 3D fluorescence imaging, photodynamic therapy, nonlinear optical transmission and 3D microfabrication. 

Large nonlinearities based on saturated absorption or bandfilling effects are reported for semiconductors. The response of these nonlinearities is fast but recovers only slowly due to the created excited state population. Decay times of the excited states on the order of some hundred picoseconds to nanoseconds are detrimental for all-optical switching with large repetition rates. 

Photodecomposition studies indicate that the initial step in the decomposition reaction is the absorption of light at 254 nm, which creates excited states of the azide ion whose lifetime is probably impurity controlled. Intensity data suggest that the decomposition reaction occurs by a bimolecular reaction between (N3) states, although details concerning energy transport and the nature of the decomposition site (lattice distortions, impurity centers, crystal surfaces, etc.) remain unanswered. Conjectures concerning probable reaction paths have been based on information about the intermediate products, identified by the optical and ESR measurements previously discussed. 

To explain the observed data, Groocock [231] assumed that electrons are excited to the conduction band, leaving holes in the valence band which become trapped at an impurity center decomposition occurs only when holes are trapped at neighboring impurity sites. Pai-Vernecker and Forsyth [119], on the other hand, assumed that photons create excited states of the azide ion, and the excitons become trapped on an impurity center decomposition occurs between the trapped excited molecule and a neighboring azide molecule in the ground state. In both cases it is assumed that electrons, freed from the azide anion, are responsible for neutralizing lead atoms which diffuse to form colloidal lead. 

Time-resolved emission spectra suggest that dissociation of excimers to create excited state monomer does occur in the copolymers studied including polymers containing styrene [72] in contrast to previous reports [24,74]. ... 

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