Control systems electronic excitation

GAMESS is designed to have robust algorithms and give the user a fairly detailed level of control over those routines. This makes it better than many other codes at modeling technically difficult systems, such as transition metals and electronic excited states. 

Photocycloaddition of thiones to alkenes is the most popular and fruitful method for the preparation of the thietane system. In analogy to the formation of the oxetanes by cycloaddition of the electronic excited ( ,tc ) carbonyls, thietanes can be expected to arise photochemically from aromatic thioketones and substituted olefins as well as 1,2- and 1,3-dienes. ° Thiobenzophenone serves as a source of a sulfur atom and, because of its blue color, which disappears on photocycloaddition, permits exact control over the reaction time. A mixture of thiobenzophenone and a-phellandrene must be irradiated for 70 hr before the blue color disappears (Eq. 2) and... 

Summary. An effective scheme for the laser control of wavepacket dynamics applicable to systems with many degrees of freedom is discussed. It is demonstrated that specially designed quadratically chirped pulses can be used to achieve fast and near-complete excitation of the wavepacket without significantly distorting its shape. The parameters of the laser pulse can be estimated analytically from the Zhu-Nakamura (ZN) theory of nonadiabatic transitions. The scheme is applicable to various processes, such as simple electronic excitations, pump-dumps, and selective bond-breaking, and, taking diatomic and triatomic molecules as examples, it is actually shown to work well. 

As an example of an effective control scenario, consider the molecules ZJ and L in their ground electronic states and in vib-rotational states ED) and Ej)t of energy ED = EL. We choose E(0 so as to excite the system to two eigenstates E ) and 2s2) of the electronically excited potential surface G. The states i) and-IE2) are also coupled by an additional laser field (see Fig. 1). 

Recombination may also proceed via an electronically excited state if during the course of a bimolecular collision the system may transfer from the nonquantized part of the potential curve associated with one electronic state to a second state from which emission is allowed. This process is called preassociation or inverse predissociation, and the selection rules that control the probability of crossing in both directions are well known [109]. In such encounters total angular momentum must be conserved. For diatomic molecules, the system can pass only into the rotational level of the excited bound state which corresponds to the initial orbital angular momentum in the collision. 

Theoretical methods that combine ab initio MD on the fly with the Wigner distribution approach, which is based on classical treatment of nuclei and on quantum chemical treatment of electronic structure, represent an important theoretical tool for the analysis and control of ultrashort processes in complex systems. Moreover, the possibility to include, in principle, quantum effects for nuclear motion by introducing appropriate corrections makes this approach attractive for further developments. However, for this purpose, new proposals for improving the efficient inclusion of quantum effects for the motion of nuclei and fast but accurate calculations of MD on the fly in the electronic excited states are mandatory. Both aspects represent attractive and important theoretical research areas for the future. 

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