Binding Energy of the Excitonic Molecule

The binding energy of the excitonic molecule, a complex consisting of two electrons and two holes, has been determined as a function of $\sigma$, the ratio of the mass of the electron to that of the hole. We find the binding energy of the excitonic molecule in Si to be 0.63 meV, as compared with the experimental result of ≤2 meV. Our analysis also predicts that the excitonic molecules should also be observed in a wide variety of crystals for which $\sigma$ does not lie in the range $0.2\le \sigma \le 0.4$.