Laser control nonadiabatic transition

Once the mechanisms of dynamic processes are understood, it becomes possible to think about controlling them so that we can make desirable processes to occur more efficiently. Especially when we use a laser field, nonadiabatic transitions are induced among the so-called dressed states and we can control the transitions among them by appropriately designing the laser parameters [33 1]. The dressed states mean molecular potential energy curves shifted up or down by the amount of photon energy. Even the ordinary type of photoexcitation can be... 

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 the excitation process in an external field can be regarded as being a nonadiabatic transition between dressed adiabatic states [32], effective laser control can be achieved by manipulating the parameters of these nonadiabatic transitions directly. Based on this idea, two control schemes have been proposed. The first one is a control scheme for the branching ratio during the molecular photodissociation, achieved by utilizing the phenomenon of complete reflection [24,43,44], The second is to control the population transfer by using a laser pulse with periodically swept parameters [24-29], In both cases the best parameters of the laser pulse can be easily estimated from the ZN theory of nonadiabatic transitions. 

Chapter 7 continues the presentation of nonadiabatic electron wavepacket d mamics as applied in various chemical reactions, mainly in electronically excited states. Quantization the branching paths (non-Born-Oppenheimer paths) will be also discussed. Likewise, in Chap. 8, the electron wavepacket dynamics is considered for molecules placed in laser fields. In addition to the ordinary nonadiabatic transitions due to the Born-Oppenheimer approximation, novel nonadiabatic transitions due to optical interactions appear to need special cares. This chapter is to be continued to future studies of laser design of electronic states and concomitant control of chemical reactions. 

As an example of the effect of an intense laser on chemical reactions, a possible control of nonadiabatic transition of Na - - Cl by modulating its avoided crossing is studied [425], Figure 8.3 shows the potential curves that cross each other between the lowest covalent and ionic adiabatic states. The calculation is in the level of RHF/CISD/6-31G, with 26 molecular orbitals, 91 CSFs, and 13 core orbitals. The target was set to the qualitatively correct description of only the sigma states, without careful attention to the two lowest Ft states. In doing so, we chose the core orbitals up to the second highest occupied molecular orbitals. 

---