Abstract
We study absolute cross section of simultaneous two-photon absorption (TPA) in a series of porphyrins and tetraazaporphyrins by 100-fs-duration pulses in two ranges of laser wavelength, from 1100 to 1500 nm and from 700 to 800 nm. The cross section in Q transition region is, sigma(2) ~ 1-10 GM, and is explained by partial lifting of the parity prohibition rule. In Soret transition region we find enhancement by about an order of magnitude due to Q transition, which acts as a near-resonance intermediate state, and also due to gerade energy levels, which we identify in this spectral region. Further enhancement (up to sigma(2) ~ 1600 GM) is achieved by symmetrical substitution in tetraazaporphyrins with strong electron acceptor groups.
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