Essential-state Models for Two-photon Cross-section

Essential-state Models for Two-photon Cross-section 

As with [I and y, few-state models are useful for understanding structure-property relationships for the two-photon absorption (2PA) cross-section, d. In dipolar molecules, the peak cross-sections associated with 2 PA into an excited state e from the ground state, g, will have a two-level contribution according to 

For centrosymmetric molecules, however, A/ige = 0 and accordingly 52 state = 0. Moreover, 2 PA into one-photon-allowed states is forbidden according to the parity selection rule whereas one-photon transitions in centrosymmetric systems are accompanied by a change in state parity (g— u or u— g), 2PA is only possible between states of the same parity (g— g or u— u). Indeed, this feature has long been exploited in spectroscopy to obtain information complementary to that accessible from 1PA for example, see Refs. [122] and [123]. 

In centrosymmetric systems, it is only three-level terms (which can also contribute in dipolar systems, their significance depending on the relative magnitude of two- and three-level terms for the state in question) that are responsible for d. For absorption into a state, e  

The dependence of 5 upon /tge and /tee. tells us that absorption from the ground state to the intermediate state should be strongly one-photon allowed (which one 

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