Optical rotation in LICS

Another way of considering the problem which is perhaps physically more meaningful is that in fact the two bound states 0 and 1 are coupled to each other by the two lasers, in one case via a bound virtual state (the Raman path) and in the other via the continuum (the autoionising path) as marked in fig. 8.4. It turns out that, if the Raman channel dominates, the resulting lineshapes tend to become symmetric, while if the autoionisation channel dominates, the characteristic interference asymmetries of Beutler-Fano resonances emerge.  

Since the ground state 0 and the excited state 1 are both to be coupled via the continuum by dipole-allowed transitions, it follows that they must have the same parity, and that they are coupled to each other by a two-photon matrix element which is called the two-photon Rabi frequency (for a discussion of the Rabi frequency, see section 9.10). 

If we define a resonance energy Eq somewhere in the continuum, and if we let uo take us from the ground state to Eq and u) take us from 1 to Eo then, when ujp is tuned into resonance uq — — ujp — u s 

We can thus define an energy variable e = z/Toi, while, as usual, q is defined as the ratio of the transition strength to the bound state to the transition strength to the continuum states viz. q = Mq /pocPic, where Moi is the two photon transition moment and the ps are single photon transition moments. 

In any real experiment with strong field lasers, the lasers will be pulsed, and so the time dependence of LICS will need to be considered (this is postponed to chapter 9). 

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