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

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 D2h symmetry. For a two-photon transition to a B1g 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.