Interaction with Strong Fields

In the previous sections we assumed weak-field conditions where the probability of finding the atom in the initial state was not essentially changed by the interaction with the field. This means that the population in the initial state remains approximately constant during the interaction time. In the case of broadband radiation, this approximation results in a time-independent transition probability. Also the inclusion of weak-damping terms with yah coha did not affect the assumption of a constant population in the initial state. 

When intense laser beams are used for the excitation of atomic transitions, the weak-field approximation is no longer valid. In this section, we therefore consider the strong-field case. The corresponding theory, developed by Rabi, leads to a time-dependent probability of the atom being in either the upper or lower level. The representation outlined below follows that of [57]. 

We consider a monochromatic field of frequency w and start from the basic equations (2.96a, 2.96b) for the probability amplitudes in the rotating wave approximation with (Oba = —o ab 

Substituting this back into (2.113b) gives the relation 

This is a quadratic equation for the unknown quantity fx with the two solutions 

Substituting this back into (2.85b) gives the relation 2ix c0ba —co + fl) =.  

In case of broad-band radiation this approximation resulted in a time-independent transition probability. Also the inclusion of weak damping terms ab %a affect the assumption of constant population in the 

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Electro-optic The liquid crystal plastics exhibit some of the properties of crystalline solids and still flow easily as liquids (Chapter 6). One group of these materials is based on low polymers with strong field interacting side chains. Using these materials, there has developed a field of electro-optic devices whose characteristics can be changed sharply by the application of an electric field. 

We now turn to another important influence on ET dynamics, electric fields. It is reasonable that the presence of a strong electric field should influence the dynamics of charge separation processes because the dipole moment associated with the newly formed CS state will interact with the field. This interaction will modify the barrier height for the charge separation process. Indeed, it has been postulated that electric field effects might be the cause of the observed directionality of electron transfer in the photosynthetic reaction centre (see Figure 37). 

To determine an effective dressed Hamiltonian characterizing a molecule excited by strong laser fields, we have to apply the standard construction of the free effective Hamiltonian (such as the Born-Oppenheimer approximation), taking into account the interaction with the field nonperturbatively (if resonances occur). This leads to four different time scales in general (i) for the motion of the electrons, (ii) for the vibrations of the nuclei, (iii) for the rotation of the nuclei, and (iv) for the frequency of the interacting field. It is well known that it is a good strategy to take into account the time scales from the fastest to the slowest one. 

The CESE-SSA to the ab initio calculation of states interacting with strong electric static or ac-fields... 

Finally, if the gas molecule possesses a quadrupole moment Q—examples are CO, COj and Nj—this will interact strongly with the field gradient F to produce a further contribution fQ to the energy. ... 

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