Two-Photon Capture Cross Sections of Pyrene and Benzpyrene from SCF–MO Calculations

Abstract

The two‐photon capture cross sections for pyrene and 3,4‐benzpyrene are calculated using SCF–CI pi‐electronic wavefunctions and the theory of Goeppert‐Mayer. The cross section is shown to be dependent on the product of the photon flux and a constant, δ, which may be calculated from suitable matrix elements and the line shape function. For randomly oriented molecules and polarized light the calculated values of δ for pyrene and 3,4‐benzpyrene are, respectively, 260×10^—51 and 1500×10^—51 cm⁴·sec atom^—1 photon^—1. These are to be compared with the experimental values of, respectively, 1×10^—51 and 50×10^—51 cm⁴·sec atom^—1·photon^—1. The effect of molecular symmetry on the two‐photon absorption of polarized light is illustrated for molecules of D_2h symmetry. For a two‐photon transition to a B_1g state, the maximum probability is shown to lie approximately 45° between the x and y axes of the molecule which are the axes of maximum probability for one‐photon absorption.