One- vs. Two-Photon Interference Symmetry Breaking

Ugh scenarios for interference between an JV-photon route and an M-photon -Where N, M are of the same parity, allow for control over both the differential -gral photodissociation cross sections, this is not the case when N and M are rent parity. In this case only control over the differential cross section is fe. [However, the control is such that it leads to the breaking of the usual StdMbrward symmetry, This is but one example of the breaking of svmmetrv 

To understand why control over the total cross section is lost and how the backward-forward symmetry is broken, we analyze in some detail the simplest case in this class, namely the interference between a one-photon and a two-photon absorption process [78], Consider irradiating a molecule by a field composed of two modes, o 2 and a)h with a)2 = 2coh for which the light-matter interaction is 

It follows from Eqs. (3.30) and (3,40) that the amplitude for the combined one4[ photon, two-photon absolution process is J)j 

Suppressing for the moment all channel indices m (which can be readily included), save for the final direction k, we square the amplitude to obtain Pq(E, It), the probability of photodissociation into channel q at angles k = (Bh, f k), 

The interference term P q2 E, k) is generally nonzero, so that control over the differential cross section is possible. 

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