Two-Quantum Volume Photoelectric Effect in Sodium

Two-photon photoelectric emission has been observed from a freshly deposited sodium surface. The two-quantum yield for the process, $\Lambda (\lambda , T)$, was determined to be $\Lambda (8450\mathrm{} \AA , 300°\mathrm{K})\sim 8\times {10}^{-16\mathrm{}}{I}_0$ A/W, where $I_0$ is the intensity of the incident radiation in W/${\mathrm{cm}}^2$. Previous work attributed the phenomenon to a surface effect. However, it now appears more likely that it arises from a volume effect. The two-quantum yields for various metals, semiconductors, and organic crystals are compared.