Dressed states, 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... 

Section 6.5.1), two avoided crossings arise in Figure 6.10c at t = +10 fs between states /) and 1 3), the first of which is marked by a gray circle. Due to the highly nonadiabatic time evolution, diabatic transitions between these dressed states are likely to occur. The Landau-Zener model [48, 104, 105] estimates the probability for a diabatic transition at the avoided crossings as... 

In order to switch the system into the upper target state 5) merely the sine-phase 0 has to be varied by half an optical cycle, that is, by A(p = n. In this case, the main pulse is phase-shifted by Af = -l- r/2 with respect to the pre-pulse and couples in antiphase to the induced charge oscillation. Hence, the interaction energy is maximized and the upper dressed state u) is populated selectively. Due to the energy increase, the system rapidly approaches the upper target state 5). The ensuing nonadiabatic transitions between the dressed states u) and 1 5) result in a complete population transfer from the resonant subsystem to the upper target state, which is selectively excited by the end of the pulse. 

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. 

An avoided crossing will mainly limit the application of the adiabatic theorem If the dynamics is not slow enough, dynamical transitions, so-called nonadiabatic transitions, will be induced between the dressed states forming the avoided crossing. A local Landau-Zener analysis can be invoked to determine... 

T, it may undergo rapid time evolution over many adiabatic states due to the interaction with the external field. Such behavior of the Floquet state is sometimes described as a field-dressed state. Its importance in analysis in the field-induced dynamics is analogous to that of the energy eigenstate under a time-independent Hamiltonian. We will further show that even a small non-periodic term can be incorporated in this framework in an analogous manner as the nonadiabatic transitions in the field-free dynamics. 

In order to treat the reaction in the laser field, we introduce an idea of "laser dressed" states [7]. Photoabsorption and photoemission processes are then modeled as the nonadiabatic transitions between the dressed ground and excited states. Under the assumptions of (1) two-state model, (2) single-photon excitation, and (3) rotating wave approximation, the laser dressed adiabatic PESs are given for two diabatic PESs W- and and the laser frequency as [7],... 

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