Direct Two-Photon Creation of Excitonic Molecule in CuCl

The probability of direct excitation of excitonic molecule in the two-photon absorption process is calculated with an Hylleraas-Ore type biexciton envelope, which is variationally optimized. In the second order of perturbation, because of the resonance effect, only the exciton Γ$\text{}_{5}$ intermediate state is taken into account. The band structure at band extrema due to spin-orbit interaction is assumed. The transition probability of two-photon absorption is expressed by matrix element between the free exciton and the biexciton envelopes. The obtained probability of two-photon absorption in CuCl is about three orders of magnitude smaller when compared to that obtained by Hanamura.